Image Credit: GE
More Musings of an Energy Nerd
UPDATED on January 8, 2018
Most homes have several exhaust appliances. These typically include a bathroom fan (40-200 cfm), a clothes dryer (100-225 cfm), and perhaps a power-vented water heater (50 cfm), a wood stove (30-50 cfm), or a central vacuum cleaning system (100-200 cfm). But the most powerful exhaust appliance in most homes is the kitchen range-hood fan (100-1,200 cfm).
Every time an exhaust fan removes air from your house, an equal volume of air must enter. The air that enters cracks in a home’s envelope to replace air that is exhausted is called “makeup air.” Two trends affecting makeup air are causing increasing problems for homeowners: homes are getting tighter, and range-hood fans are getting more powerful.
So where does a powerful range-hood fan get its makeup air? If the house doesn’t have enough random air leaks around windows, doors, and mudsills, the makeup air is often pulled backwards through water-heater flues or down wood-burning chimneys — a phenomenon called backdrafting. Since the flue gases of some combustion appliances can include carbon monoxide, backdrafting is dangerous. In some cases, it can be life-threatening.
Range hood manufacturers can’t answer homeowners’ questions
Most residential Q&A columns get regular questions from homeowners asking how to solve backdrafting problems or how to provide makeup air for exhaust fans. Yet range hood manufacturers are notorious for failing to provide installers with any guidance on makeup air. Several years ago, I looked into the problem and reported what I found in an article published in the August 2006 issue of Energy Design Update.
The genesis of my research was a homeowner query posted on Breaktime, the Web forum maintained by Fine Homebuilding magazine. Cheryl Morris described problems with her expensive new 1,200-cfm GE Monogram range hood. “It will pull a negative in the house,”…
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165 Comments
sealed combustion fallacy
Just a minor note to a good post. Sealed combustion appliances aren't always sealed 100%
For example... Triangle Tube Prestige boiler. When you take off the front panel it is breathing from the room. And if one looks at the cabinet it is not fully sealed. Yes the exhaust is sealed so no CO is going to be emitted, but no the intake is not fully sealed. Air pressure changes do effect the start up of some appliances, tighter homes may cause more problems then just moisture, rot and mold.
Thanks!
Wow, wonderful timing! Thank you very much Martin, adding a kitchen vent is on the list for early winter, and I was worried about the time I'd spend spinning my wheels trying to find this very info.
Thanks!
Great Stuff!!
Thanks Martin. What an important item to be discussing and exposing! I live in a retirement/second home community where a Viking stove and range hood seems essential for quality home status. The ducting of these are more times than not, afterthoughts, and not to your surprise, not one of them have ever thought of make-up air. The IRC's new code is a great addition, but getting code officials to enforce the issue and compliance is another battle all unto itself.
Experimentation?
"•Some Passivhaus builders are experimenting with range-hood fans that don’t exhaust air to the exterior"
Are these the ones that didn't fit a commercially-available recirculating range hood?
Response to Interested Onlooker
Interested Onlooker,
Here are three links for further discussion of kitchen exhaust options for Passivhaus buildings:
https://www.greenbuildingadvisor.com/community/forum/general-questions/15959/kitchencook-top-ventilation-passivhausleed-home
http://www.passivehouseca.org/does-required-kitchen-air-change-rate-ashrae622-seem-high
http://www.passivehouse.us/bulletinBoard/viewtopic.php?f=6&t=298
Like a glass works hood?
We recently did a teaching studio for a client who works lamp-work glass jewelry and glass art work and needed an efficient studio with localized ventilation for open flame working of colored glass involving associated heavy metals and toxic fumes. We basically created a hood within a hood that was in someways similar to a heat recovery ventilator and would work for a person needing a large volume hood in an air tight house for cooking as well, though the version we use had 200 CFM out in a 7" duct with and 200 CFM in in a 10" duct wrapped around the out-going duct and a hood that distributed the incoming air around the edge of the hood. the key was that we ran the air concentrically to a point very near the exterior of the house to maximize heat exchange and then split the streams by using a 10" wye with an end cap and a 7" A-collar to pull the outflow up through the roof through an inline fan and the inflow just below the soffit so the roof overhang isolated the air streams. I do remember that my hands and wrists were quite bloody by the end of the assembly process, concentric duct work is a bear to assemble and seal, We used galvanized stock for this one and painted it with a metallic oil rubbed bronze finish after completion (it also had wiring for lights that was very specific due to the need for good color rendering on the flames associated with the glass work)
We make a lot of copper hoods for island kitchens and also do the big tile, stucco and even timber hoods (See p 88 of this falls Fine HomeBuilding Kitchen and Bath special) for wall applications that would be adaptable to this concentric venting system but for wall applications it's easier to use a cape hood damper behind the range and talk the folks into using a smaller fan. (We've even pulled the blower out of one unit and replaced it with a smaller blower)
Fantech makes in-line duct fans and flex duct that is rated for kitchen exhaust. (check out the 980 CFM FKD10) and i find that giving the client the option to use a fan speed controller helps to keep the CFM at a more reasonable level. Once you get over four amps aggregate for the two fans you are locked into using the fairly industrial looking Varifan MVS-1. Below 4 amps the Lutron speed controller works well.
PS jump two duct sizes on concentric venting
If your client is fixed on a 1,000 CFM exhaust you need a 10" out (78 sq in) and 14" in (154 sq in) to get the right cross-sectional area and you will still get more volume going out in the ten than in through the 14 cut down to a 2" ring w/ 76 sq in NFA you could make a strong arguement for jumping the outer duct to a 16" but aesthetically that's sort of glommy-looking for even a 48x36 island hood.
Make-up Air
I’m glad to see a more dedicated article on this topic. When I raised the issue back in February, there wasn’t much discussion about it. To be fair, it’s no just GE, it applies from Amana to Zero (or Subzero). Back then I called every major appliance and hood manufacturer, several distributors and a couple of engineers, and to my surprise, no a single entity had a clue about it, and more scary than that is that no one entity had any plans in the near future to address this code change.
Here is another site you may consider looking at: http://www.electromn.com/gen/makeup_air.htm
Response to Armando Cobo
Armando,
Thanks for the link to the page about units from Electro Industries.
These units are electric resistance heaters. If you buy the unit, it doesn't include a blower or a motorized damper -- you've got to buy those separately.
One model to consider is model EM-WM2035L. It draws a whopping 20,000 watts.
It can supply 790 cfm of makeup air at an 80-degree temperature rise, or 1,580 cfm at a 40-degree temperature rise. The list price is $870. But remember, you still have to buy all the other necessary components.
Great work, thanks
Martin,
Very good discussion. One point. As a woodworker I have spent years working with dust collection. A very important part of effective dust collection is to get your "source of suck" very close to the "source of dust" (sorry for the technical jargon). This principle applies to kitchens too. When I see range fan inlets several feet away from the burner or frying pan I shudder. They are moving about 90% clean air and only a bit of fumes with all the consequent energy loss. A much lower CFM fan can work much better with close collection than a huge fan further away. Cheers and thanks for your good work. john the toymaker
Lowering the "source of suck"
John,
Thanks for your comment. You're right, of course.
However, there's no easy way to lower the "source of suck." Ideally, you want a range hood that doesn't obstruct lines of sight for the cook, and doesn't get in the way of a tall stockpot, and doesn't interfere with lighting. A tall order.
Maybe range manufacturers will come up with a new design -- perhaps a range hood with dangly octopus arms that are adjustable, so you can move a flexible stainless-steel octopus arm near the steak you are grilling.
MA Supply Location
A couple of ways we’ve done it lately was to place 3 supply vents 10’ away in the ceiling, and a second way 2 supply vents under the cabinets by the toe-kick. Installing 2 or 3 supply vents minimizes noise pollution, plus you could insulate the equipment or cavity as well. Also, you need distance between the supply vents and the hood to create an air loop close to the range, but far enough to not draw from the house make-up air. The equipment can be installed in the attic, floor trusses above, pantry or cabinets.
Ducted Articulated Arm Fume Hood Extractor
Martin,
Here's a fun way to lower the source of suck from a scientific supply house:
http://www.triadsci.com/index.asp?page=catalogue&noprod=1006
My octopus arm!
My octopus arm! Somebody already invented it!
Darn -- I was already spending the anticipated proceeds from my patent license agreements. There goes another six-digit idea...
balanced fans
I notice that FanTech has inline fans that are rated for vent or supply -- Could these be wired to balance one another (no, the supply wouldn't be conditiioned, but ...)
Just a thought.
Joe W
Response to Joe Wilson
Joe,
You've got the right idea: to supply makeup air, you need to wire a supply fan to come on simultaneously with the range hood. In a post on this page, Armando Cobo has provided good suggestions for locating the grilles for delivering this supply air.
Whether or not you want to condition the supply air is up to you. If you are a builder in Minnesota, and a client likes to cook with the 1,200 cfm fan running for four hours on Thanksgiving, I imagine that 1,200 cfm of cold makeup air will lead to complaints. On Christmas day, when the makeup air is 20 degrees below zero, you've got real problems.
Balanced venting hood
Joe that is what we were doing in the hood I discussed above. by using concentric duct work and a fairly modest pair of 200 CFM blowers on a single fan speed controller we were able to gain some of the benefit of an HRV as well. But mainly the benefit was that the make-up air was integral and concentric in the hood. (even at 200 CFM X 2 it's strong enough to blow out a propane cigarette lighter)
We haven't had any condensation issues with that one but I can imagine that in certain situations that would be a possible problem.
Plan B
The problem is partly the arms race of bigger commercial-style stoves and bigger more powerful hoods to go with 'em, but it's also in the layout. By virtually universal request almost all the kitchens we design these days are integral with the main living area. This is fine for heating up a ready meal, but you can't saute a pan full of garlic shrimp on a stove top in the living room and expect not to smell it, no matter how many cfm you're pulling.
A lot of folks around here just cook the smelly stuff outside. If this doesn't work for you and your cooking style is too stinky to live with, just accept you'll need to plan a traditional separated kitchen area where you can close the doors to the rest of the house and open a window. Good gracious, why do we insist on making our lives so complicated?
I don't get it either
I live in a simple home with a simple range and a simple not-so-sucky venthood.
When you guys start talking about these big honkin venthoods..
might as well be talking about your Polo Ponies
This is not my world
Vent-A-Hood
The duct free recirculating model in this link
http://www.ventahood.com/ductfree2.jsp
looks like it has some potential for regular sized cooktops. They have a video explaining how it works. Anyone here tried this one yet?
Hot air rises, so why downdraft?
Re comment by James Morgan.... my inlaws have one of those arrangements with the stove top in an island looking over the breakfast bar into the entertaining area. To keep the cook from losing sight line to family and guests, the stove has a built in downdraft exhaust.... and WOW does it suck. All other things being equal, do downdrafts have to pull more air than an identical kitchen setup using a hood over the stovetop?
Response to Steve El
Steve,
Q. "Do downdrafts have to pull more air than an identical kitchen setup using a hood over the stovetop?"
A. Yes. You're not going to find any 150 cfm or 200 cfm downdraft fans. Most such fans pull between 500 and 600 cfm -- enough to cause backdrafting problems.
The reason is simple: the heat output of the stove burners and the high temperature of the cooking pots heat the air above the stove, causing a column of hot air to rise from the stovetop toward the ceiling, and pulling cooler air toward the burners from either side of the stove. A downdraft fan has to fight this convection current, while a range hood fan can take advantage of it.
Energy Star downdrafts..... oxymoron?
Thanks Martin,
I'm assuming some downdraft models can get Energy Star labels, and I'm finding it rather hard to get my brain around that, since fighting against physics seems to make them inherently wasteful no matter how efficient the motor might be. Can you make sense of that?
Second response to Steve El
Steve,
Are you sure that Energy Star gives labels to downdraft fans? I just did a quick Google search and didn't find any -- so I'm skeptical.
Sex and Advertisements and Profits
Are people interested in performance, or image? Are they interested in cooking gourmet food, or in owning a "gormet looking" kitchen?
Viking was the first company to start producing a commercial style range for the residential market. They started the trend toward larger and more powerful ranges and hoods.
Now, regardless of the company---Wolf, Monogram, Viking---the product development managers are simply following that market trend, and the business managers are looking at the bottom line. Premium products, and especially BRANDED premium products, usually have a significantly higher profit margin. So these upscale 1,200 CFM hoods are a combination of sexy advertisements and increased profit margins.
I'm not sure it's the demand or supply side of the market that got us here---but here we are . . .
James noted that kitchens used to be kept separate from the main living areas. They were too smelly to be part of the public place. Working exhaust fans were part of the reason kitchens moved inside the house (well, that, and a lot of other technology like refrigeration and running water).
As Martin noted, competing trends---tighter houses and more-powerful hoods---are causing a new problem. I think that, if we're going to build tighter houses, then what we need are BIGGER hoods with SMALLER fans.
Heated air rises vertically, but the much of an exhaust hood's fan energy is working to pull the air laterally (following the path of least resistance - from around the open edges of the hood). And even the big CFM hoods don't necessarily overcome the vertical lift and volumetric expansion from gas stove tops and steaming pots.
Most hoods are only 20"- 24" deep, but ranges and stovetops are 36" deep. Therein lies the problem---and bigger fans are not the solution.
Custom hoods, set above head height, and built with a surface area LARGER than the stovetop, can exhaust cooking fumes very effectively---and they can do it with a small fan.
Maybe the kitchen exhaust hood manufacturers need to follow a modified version of ASHRAE 110-95??? Ha!
"Specifies a quantitative test procedure for evaluation of a laboratory fume hood. A tracer gas is released at prescribed rates and positions in the hood and monitored in the breathing zone of a mannequin at the face of the hood. Based on the release rate of the tracer gas and average exposure rate to the mannequin, a performance rating is acheived."
Martin, My bad, I was assuming
No Martin, I have no such knowledge.... just assuming, since so many other products that to me seem rather whacko do get the labels. But..... I was wrong. I apologize for sending you on a wild goose chase.
Island Range Hood
I had previously posted about Icynene and the Ventilation choices/requirements with Iycnene. We now have an island range hood on our house plan ( 715 CFM ). Reading the postings and blog above, I am in complete violation with these two competing goals. Or is it possilble to achieve the tight envelope along with the ventilation options that Martin had discussed with me. Woud I still need a make up air unit. Or do I simply need to pick one goal.
Response to Rahul Baijal
Rahul Baijal,
I strongly urge you not to abandon your goal of achieving a tight envelope. That's always a good idea.
My advice concerning your range hood is the same advice as I gave in the list of bullet points at the end of my article. Here are your choices:
1. Choose a range hood with a low-flow fan.
2. If you want a 715 cfm fan, provide a powered makeup air unit.
3. If you want to imitate Passivhaus builders, use a range hood with a recirculating charcoal filter, and pull your exhaust air from the kitchen ceiling, far from the stove, using a low-cfm fan.
Question for Martin
In point #3 above, when you refer to "exhaust air from the kitchen" are you talking about this as part of the whole house ventilation system?
Response to Garth Sproule
Garth,
Yes. Most Passivhaus buildings have an HRV with dedicated ventilation ductwork. Typically, exhaust air is pulled from bathrooms and the laundry room, and fresh air is delivered to bedrooms and the living room. In some cases -- the cases I'm referring to -- Passivhaus builders put a ceiling grille in the kitchen, far from the stove, and connect the grille to the exhaust ductwork for their HRV.
HRV and ERV manufacturers warn installers that a range hood should never be connected directly to an HRV.
PHIUS HRV Switch
One of the other recommendations in the PHIUS strategy is to consider using a humidistat switch in the Kitchen close to the cooking appliance so that the HRV (ERV) is automatically kicked into high speed once cooking begins. This may not work in the case of say "stinky garlic" which doesn't necessarily generate enough steam to trigger the humidistat, so a manual switch would also be needed. Another suggestion I got from one of the more prominent HRV manufacturers was to hard wire a relay switch into the non-vented hood so that the HRV was automatically kicked into high-speed as soon as the hood was turned on.
I asked them if both of these options didn't create a double energy penalty since you were running two fans to perform the work that's usually accomplished by one. They responded that this strategy allowed for one less direct penetration in the building envelope and thus one less source of uncontrolled and unconditioned air entry. So then I countered with the idea of using a gasketed controlled damper on a dedicated range hood exhaust duct. They still felt that since the range hood might be used for an hour per day at most and then the duct would sit there the other 23 hours per day, that the passive loss of the duct was worse than the "momentary" double penalty of the dual-fan power draw.
This was for a LEED Platinum Habitat for Humanity project so we had a LEED Rater available to run the modeling for us as well as a PHIUS Architect who could run it through their complex spreadsheet. My understanding is that they both ran it through their respective software programs and both came up with the same results. Extra duct and penetration bad. Short-term dual-fan power draw more energy friendly (in a mixed humid mid-Atlantic climate).
The project's still under construction, so we don't have any final results yet to compare against the modeling. But I'm very interested to see the final results and get feedback from the future homeowners after periods of use (say 6 mos and 1 yr post). I remain skeptical how well the system will perform, but the "modeling" says it's the way to go. Stay tuned ...
Passivhaus, HRV, makeup air
So I'm wondering if you go the way of a Passivhaus ventilation as described by Martin could you add a 4" vent or two behind the fridge or stove for makeup air...might be good for the fridge (draw off some heat) and keep the draft off your feet (under counter venting). Would you still need dampers?
squibt
More island range hood questions
Martin,
We are a family who cooks daily with strong South Asian cuisine, so we need a reasonably fuctioning kitchen exhaust system. Our current home has a recirculating down-draft that fails miserably, so given our desire for an island range hood. After reading this discussion and the discussion on the internet, what is an ideal location for a make up air unit. My only concern is that if i punch a hole into the building envelope, have i not defeated what i am trying to accomplish. Being in Houston, let's assume, I cool this hot, humid air, then I am not defeating the goal of energy efficiency. Where I am more confused is, let's say I achieve a tight-buiding envelope and then appropriately ventilate the house with the following suggestions you had recommended: 1. fresh air intake with damper plus air cycler 2. ERV, how does this independent system affect that ventilation system. Should I be worried about an independent system bringing in this hot , humid houston air into my home. How would that impact the rest of my system?
Thanks,
Rahul
Response to Daniel Ernst:
"Most hoods are only 20"- 24" deep, but ranges and stovetops are 36" deep. Therein lies the problem---and bigger fans are not the solution."
I agree with the general point, (though most ranges and stovetops are 24 - 26" deep, not 36"): to exhaust even moderately effectively a hood should be not only at least as deep and wide as the stovetop but also held down close to the heat/smell/vapor source. Unfortunately this arrangement would make the processes of cooking difficult if not impossible, which is why it is never set up this way, or only temporarily until the homeowner gets tired of hitting their head against the hood. Not that a conscientious inspector would sign off on an installation lower than the manufacturer's recommended minimum height, which is generally 24" - 30" above the cooking surface and far too high for comprehensive vapor capture without a fan large enough to power a hovercraft. The problem is fundamentally one of multiple incompatible goals: to be able to cook as if in a commercial kitchen; do this in the main living room and not expose it to odor and vapor; and to be to be energy-efficient. Where are we headed: a completely enclosed lab-style fume cupboard? That would sure impress the neighbors.
Response to SquibT
SquibT,
You asked, "If you go the way of a Passivhaus ventilation as described by Martin could you add a 4" vent or two behind the fridge or stove for makeup air...might be good for the fridge (draw off some heat) and keep the draft off your feet (under counter venting). Would you still need dampers?"
I don't recommend this method of providing makeup air, because you end up with a wall penetration that leaks air, even when not in use. It's possible to include a motorized damper with such an installation; that would be better than a backdraft damper, but it would still leak.
I've posted an illustration above (as Figure 2); the drawing comes from the April 2003 issue of Energy Design Update. The article accompanying the illustration noted, "Glenn Mitchell, a designer in Comox, British Columbia, has come up with a clever solution: he ducts exterior makeup air from an intake in the rim joist to a grille on the wall behind the refrigerator. That way, the incoming cold air improves the efficiency of the refrigerator, while the refrigerator coils preheat the air. Mitchell, who posted the tip on a Journal of Light Construction Web forum, calls his system 'a poor man’s HRV.' ”
As I noted, this solution increases air leakage, and is not recommended. Moreover, it should be noted that a 4-inch duct cannot balance the exhaust of a 1,200-cfm fan, since the fan requires a 10-inch duct.
why 1200 cfm?
Why would they select a range hood that is probably 3 times the capacity required?
Second response to Rahul Baijal
Rahul Baijal,
First of all, when your house is complete, I hope you invite me over for a home-cooked South Asian meal. I don't know who does the cooking in your family, but it sounds like you're lucky and you eat very well.
There is no need to have two types of mechanical ventilation systems in your home. You need to choose either a central-fan-integrated supply ventilation system (that's the one you describe as "fresh air intake with damper plus Air Cycler") or an ERV with dedicated ventilation ductwork. You need one or the other -- not both.
If you choose a supply ventilation system, it can't be used to help exhaust your kitchen.
If you choose a balanced ventilation system with an ERV, it can be used to help exhaust your kitchen, as long as the exhaust grille isn't too close to the stove. (You don't want to introduce grease into an ERV.)
You are correct that a 700-cfm fan will do a better job removing grease, smoke, and cooking odors from your home than a 200-cfm fan. But there is no getting around the energy penalty. If you operate a 700-cfm fan during the summer in Houston, you will inevitably be pulling in 700 cfm of hot, humid outdoor air into your house. One way or another, that outdoor air will need to be cooled by your air conditioner every time you operate your exhaust fan.
Response to Travis T
Travis T,
You asked, "Why would they select a range hood that is probably 3 times the capacity required?"
There are several possible reasons:
1. To impress the neighbors, or
2. Because they saw the range hood in a catalog and it looked slick, or
3. Because they can afford it and they always want to buy the most powerful available option, or
4. Because they do commercial-style cooking on a huge stove and are convinced they need a lot of exhaust.
Travis, I agree with your point. That's why I wrote, "Since most residential kitchens are adequately served by a 150-cfm or 250-cfm range hood, it comes as no surprise that a 1,200-cfm range hood can cause depressurization and backdrafting problems."
Response to Andy Ault
Andy,
I would be very surprised if the folks at PHIUS were recommending a dehumidistat located in the kitchen and wired to an ERV/HRV. They know very well that this setup is not a good one for most climates. If the moisture content of the outdoor air is greater than the indoor air (a good part of the year in MD + VA), you'll wind up bringing in extra moisture INTO the house. Perhaps it was a recommendation from an independent Certified Passive House Consultant? I'm not sure what a PHIUS Architect is.
What about TWO humidistats and a chip?
John, is anybody marketing a system that also has a sensor at the inlet for the replacement air? Seems like a computer chip should be able to help automate what to do with moisture or BTUs in the house (ie, add more, move some outside, or redistribute within building). Such a system would need a gizmo to sniff at the incoming air, though.
ERV Range Hood?
So... If the high end range hood manufacturers are so into 1200cfm hoods, why can't the next generation be erv's? What's another wall penetration if it pulls in fresh air (albeit warmer)? I just brought up the 2009 code req. here at my office, and no one knew of it. I had inadvertanly stumbled accross it a couple weeks ago, and they've been enforcing 2009 IRC here since July. Thanks for the post. I only wonder when the field inspectors start enforcing it.
Response to Steve El
Steve,
I'm not sure what you are suggesting. The fact is, if a homeowner turns on a 1,200 cfm exhaust fan, the homeowner is also bringing in 1,200 cfm of outdoor air into the house, whether the homeowner wants to or not. One way or another, that outdoor air has to be conditioned -- heated in winter, and, if we are talking about an air-conditioned home, cooled in summer.
That air is coming in, like it or not -- whatever your humidistat recommends. The only way it won't come in is if the fan isn't capable of exhausting 1,200 cfm.
Response to Matt Dirksen
Matt Dirksen,
Q. "If the high end range hood manufacturers are so into 1,200-cfm hoods, why can't the next generation be ERVs?"
A. In theory, it's possible to design a 1,200-cfm HRV. (I wouldn't suggest using an ERV for this application). There are several problems with your suggestion, however:
-- Grease would gum it up.
-- It would be huge and expensive. Most residential HRVs are rated at 60 to 120 cfm.
In the meantime, Michael Chandler is experimenting with his homemade HRVs. They work, but not if you have to handle grease.
Strategic location of makeup air vents
It might reduce the energy penalty to locate makeup air vents below the range, as suggested above. This way some of the exhaust air takes a short trip past the range and right up the hood, never getting conditioned conditioned indoors. An air inlet above the range could compromise ventilation effectiveness, so ideally the incoming air would flow across the range toward the hood.
Response to John Semmelhack
Picky Picky, Picky ... A PHIUS Architect is simply shorthand for an AIA Registered Architect who also happens to be a Certified Passive House Consultant. I was trying to keep it simple for the sake of expediency.
And no one recommended a DE-humidistat. The suggestion was for a simple humidistat relay. This is completely unrelated to any outdoor RH values. It is a simple relay which would go in the Kitchen and be pre-set by the installer / homeowner for a specific indoor RH in the Kitchen area only. Just like in a bath, when there is a user triggered event which causes the indoor RH to rise above the setting, it would kick the fan into boost mode until the RH drops back below the setting ( a common PHIUS recommendation for dealing with bath settings to satisfy local inspectors who would otherwise require a separate, dedicated, direct-ducted bath fan per IRC 2009).
Yes, in theory this could bring in overly humid outside air, but if you A) use an ERV instead of an HRV this would be partially mitigated and B) the set point would presumably be one that would be high enough that you would want the interior air to be exhausted regardless and C) The original post was about make-up air, so any make-up system would create this issue. But at least an ERV would attempt to give you a fighting chance.
Guess I was going beyond just hoods, Martin
Sorry I wasn't more clear, Martin. And this bit of babble takes a little bigger view of ventilation than JUST range hoods....
I'm unclear about it myself, so I'll try to explain this way. First, take three variables:
X = some indoor activity
Y = some set of outdoor conditions
Z = what's happening with open or closed windows and shades, and HVAC of all stripes
In my simple house, if I'm thinking about it when I do X, I might manually fine tune Z in response to Y. If Y changes I might tweak Z, but that means being home and thinking about it.
That got me to thinking about high tech super tight homes. Could that be automated? Maybe depending on temp it makes sense to open the window instead of run the AC. But maybe depending on RH it makes more sense to close the window. Similarly, maybe there are different times where you might fine tune the range hood exhaust, or maybe not, I don't know. But that was the gist I was getting at. To give the computer brain data to make these decisions, the chip needs data on outdoor conditions.
Hope that is more clear.... I'm fuzzy about the idea myself, which is unsurprising since I'm just an interested homeowner that knows almost nothing about HVAC.
Steve El
Response to James Morgan
Thanks for catching my mistake. I meant most ranges are 26" deep, not 36".
Also, I didn't mean to imply that we should head toward laboratory fume hoods in the kitchen. ;-)
My point was that hoods are not rated, tested, or measured in any possible way. Nobody knows how one performs against the other. So how does a consumer purchase a hood? They pick the most powerful one, the one with the highest CFM rating (logically, the best performing, right?!).
I think mfg. restrict the hood installation height to 24" - 30" because they are trying to make up for a basic design flaw. Their hoods are just too small. Larger hoods, set higher (above eye level and the head bump caution area) can do the job with less horsepower.
Response to Daniel Ernst
Daniel,
I'm not sure that I agree that "larger hoods, set higher, can do the job with less horsepower."
I'm most skeptical of the "set higher" part. Evidence? Data? Test results?
Chasing rainbows
I watch water vapor rising from pans on my range and the higher it goes the more it disperses sideways at a visible angle which seems to average at about 20° from the vertical. If a walk-under hood is set at 80" that would suggest a hood at least 16" wider all round than the cooktop. Though I'd hate to have to integrate that into a cabinetry run with any degree of elegance it might stand a chance of capturing a significant portion of the vapor load, even with a low speed fan - but only if we completely ignore turbulence and vapor dispersal effects. Taking those into account would surely take some fairly extensive aerodynamic studies and hood-makers seem more interested in cranking up the brute force than in nuanced engineering.
Odor capture is another and still more demanding issue altogether. Odor spreads through a room by completely different mechanisms which have little to do with the 'hot air rises' scenario, which is why I mention fume cupboards. Labs use them for a reason. Bigger fans, bigger hoods - all chasing a rainbow.
Response to Andy Ault
Andy,
I'm going to be picky picky again...I stand by calling the switch you describe a dehumidistat. A dehumidistat is used to turn on a machine when the RH% exceeds a certain set point. The machine will then run until the RH% drops below the setpoint. A humidistat is used to turn on a machine (such as a humidifier) when the RH% drops below a certain setpoint. The machine will run until the RH% rises above the setpoint. Yes, they're essentially the same device with different control logic.
In my opinion, a dehumidistat has no place in a home in a mixed-humid climate unless its wired to a machine that can actually dehumidify. I do agree with you that using an ERV-only strategy makes sense. I just don't agree with boosting it via a dehumidistat control.
Humidistat or dehumidistat?
John,
Oh boy -- a vocabulary dispute! I love these.
You're right that many HVAC installers and equipment manufacturers use the term "dehumidistat." However, just because a word is gradually adopted and becomes common usage does not mean it makes sense. (For example, consider how the phrase "I could care less" is gradually displacing "I couldn't care less.")
Consider the analogy with a thermostat: a thermostat can control heating equipment, cooling equipment, or both. Most thermostats have contacts that allow them to operate electrical equipment as the air temperature drops below or rises above the set point.
I'm going to side with those who use the term "humidistat" to describe a control that operates a switch in response to a change in humidity. I hold that a humidistat can control any appliance you like: a dehumidifier, a humidifier, a bathroom fan, or even a light bulb. Just because a humidistat is wired to control a light bulb, doesn't mean you can't call the control a humidistat.
There is no need to invent the word "dehumidistat."
Vocabulary
Some folks could care less about quibbles like this but a humidistat controls humidity, irregardless of what it's connected to. ;-)
Response to James Morgan
James,
Sorry, I have to disagree. A humidistat opens or closes an electrical connection in response to humidity changes, but there is no need for a humidistat to control humidity.
For example, if I connect a humidistat to an alarm or a buzzer, the alarm or buzzer is not controlling the humidity. But the alarm could be a useful signal that something is wrong with my crawl space, alerting me to the need to investigate.
a simple solution to a big downdraft fan
We have a Dacor downdraft fan on our island. Not sure it's capacity but it has a 10" duct, and when turned on it causes immediate backflow though the family room fireplace, which is in the next room. As Martin pointed out, if you expel 1200 cfm, you inhaul 1200 cfm. So, I open the window on the far side of the kitchen. It's not energy efficient but minimizes smells from the fireplace, and only the kitchen gets cool , and the stove is hot. We cook outside mostly in the summer, so it's less of an issue then. Low tech and not right, but it's cheap and it works.
more Vocabulary quible
James,
water vapor is invisible ;-)
I mistspelt quibble on porpuse
oops
Larger Hoods
James / Martin,
I've no data or studies to present on my hypothesis, just personal experience.
I worked in a Class 100 cleanroom environment / sterile processing facility. Much of the work revolved around laminar flow cabinets, smoke stick studies, particulate counters, negative hoods (fumes / chemicals), positive hoods (microbial), etc. Airflow---directional airflow---was an everyday topic and concern.
Negative hoods are qualified based on their face velocity, which is dependent on both the fan speed and the open area of the hood. With kitchen exhaust fans, the large open area reduces their face velocity to a very low number. There's very little directional airflow---the hoods are missing those smart arrows ;-).
Your observed 20° angle is exactly why a 20" deep hood will never do the job. Set a stockpot on the front burner of a standard range. If this hood is sitting 30" above the range, then it misses the outer edge of that steam column by nearly 12" (horizontal distance).
Now, place a 36" deep hood at your 80" height---and you've captured the entire steam column---without any brute force. Side cabinets or recessed cook centers don't hurt, they help.
I'm not arguing for MASSIVE hoods. I'm saying that the natural ebullience of steam and hot air can be captured with a judiciously designed hood. And yes, it needs to be deeper than 20". Yes, there's a limit to the height. The higher it sets, the more turbulence influences the airstream. It's a balancing act . . . but the industry has this knowledge.
I don't think smart design is reaching for rainbows. Maybe someday the hood manufacturers will start testing their hoods and rating them against a performance metric. Until then, this is probably a moot argument. Until then, the world will keep buying exhaust fans based on style and sexy advertisements and CFM ratings, not performance.
Vocabulary
Martin,
I love a good vocabulary dispute, too. I'm surprised this one didn't come up in your post a couple of months back!
To my knowledge, most manufacturers are making two different kinds of these controls - a) ones that close a circuit when the RH% goes above a setpoint, and b) ones that close a circuit when RH% goes below a setpoint. If the manufacturers made ones that could be switched back and forth between "a" and "b", then I would be happy calling them "humidistats". However, since this isn't the case, I prefer to call these controls by different names for practical reasons, namely reducing the risk of an HVAC installer installing an "a" when I want a "b".
Also, thanks for your original post. It's always an interesting topic, and quite timely for me with respect to a couple of projects I'm working on.
More on humidistat / dehumidistat
John,
Thanks for your response. I'll have to take your word for it that manufacturers don't make controls that can be wired either way, since I don't have time to verify it.
From an electrician's standpoint, however, there is no need for two devices. You can always wire either a humdistat or a dehumidistat to a relay that opens or closes -- your choice -- when it is energized.
My old Honeywell round thermostat with the mercury switch will close a circuit either way -- when the temperature drops or when the temperature rises.
Finally, the only problem with your suggested distinction is, what do you call a device that can be wired either way? Even if no one makes one yet, it would be trivial to make one.
Honeywell makes one
John,
It didn't take long to find a humidistat that closes a circuit either way, when indoor RH rises or falls. Honeywelll makes it -- Honeywell H1008D1002.
Science?
Martin - point taken. My focus was a pitiful attempt at humor, hoping to yank someone's chain with 'irregardless' - apparently nobody noticed. John B: OK, steam. John S: Laminar flow studies - now we're talking. Hundreds of years of experience of chimney construction have given us some fairly simple rules for the design of systems that effectively remove smoke from an open fireplace within an enclosed space. It'd be nice to see some actual science doing a similar job for kitchen exhaust, instead of just adding horsepower and hoping for the best. Hood manufacturers only list cfm and dB in their performance data, neither of which tells me what I really need to know, i.e. how effective the product is in any particular installation.
BTW I've always assumed, intuitively, that downdraft, backdraft and overhead represent in general terms the least to most effective exhaust systems, with open installations being less effective than those against a wall. Intuition is often a poor guide. Anyone have any actual data on that?
I still believe that the ultimate goal is unachievable, if that goal is defined as being able to cook whatever we like, whenever we like, wherever we like, with no odor, humidity or energy consequences. But I'd still like to give the best advice possible to my kitchen clients to achieve the best BALANCE of all these factors within a reasonable financial and energy budget.
Not Steam Either
Steam is invisible too
picky picky
What you are "seeing" is liquid (condensed) water
Water vapor, steam, or...
John,
It's a cloud, otherwise known as fog.
Water droplets.
Got your humor
James, I got your humor and liked it
I also like your thoughts about a "chimney"
Cloud, Fog, water droplets ..agreed
Martin I agree
Vapor or Cloud
Even though there is a difference
I suspect they are following a very similar path
the Steam(vapor) and the Visible Water Droplets are rising "together"
Makeup Air for Range Hoods
I have designed sealed houses for a long time.
Naturally we use heat exchangers for the bathroom fans. But the presence of grease in the kitchen exhaust prevents their use. So we tried several solutions after pulling the ashes and smell from the fireplace on the first house in 1986.
In 1990 we tried the push pull method (two 800 CFM fans activated at the same time, one pushes in, one exhausts) with inlets in the countertop. It worked too well: at full power it formed a visible tornado that could lift wooden spoons.
I 1991 we tried the single 800 CFM fan with atmosferic intake ducted to two inlets located in the bacsplash, a couple of feet on each side of the cooktop. It worked very well in summer, but in winter on a windy day food would freeze on the coutertop next to the inlets, not to mention the cook.
Since then we solved the problem by sizing and engineering appropriately the makeup air of the HVAC system. The HVAC system fan comes on with the hood and mixes the makeup air with the return air.
Response to Mario Grigni
Mario Grigni,
Anyone see a pattern here? For at least 30 years, architects and builders have been encountering this problem. Range hood manufacturers have happily promoted these exhaust fans, and have steadfastly refused to provide guidance on makeup air. Instead, they have deliberately obfuscated the issue by having their lawyers add gobbledy-gook to their installation instructions.
So all over North America, builders and architects have been trying to solve the problem on a piecemeal basis, using trial-and-error methods and a succession of experiments. The expense for these experiments, many of which failed, fell on the architects, builders, and homeowners -- never on the range-hood manufacturers.
It's 2010, and the installation instructions still haven't changed.
Makeup air
I certainly don't have any background in hvac work but this is what I have been thinking about for my remodel in progress.
I have an inline fan in the ductwork in the attic connecting to a custom hood above the range (located in the middle of a "peninsula"). It's an electric range/convection oven because I wanted to limit the number of combustion based appliances. There really isn't a comfortable solution to extending the hood out to occupy more than the area of the range top. The hood fan is on a variable speed control and I do have an ERV as part of the strategy to make the remodeled structure as airtight as is reasonable to accomplish.
So my thought is to try and introduce makeup air from underneath via ducting built into the range cabinet that delivers the air around the outside edges of the glass range top. I imagine having a column of air rising up around the vapors, smoke (and droplets!) and helping to direct them all into the exhaust port. I get that this would not provide enough area to provide sufficient makeup air, especially since I imagined it being a passive vent. So perhaps a secondary intake in the ceiling as was suggested and/or some switching interaction with the ERV. I assume a gravity operated damper in the makeup air duct.
I had noticed that makeup air design running air past the refrigerator coils earlier. The refrigerator is located between the range and the nearest exterior wall . . .
I have not gone so far as to try and design a way of doing this that doesn't pose a significant cleaning challenge but I suspect it could be done and will at least make a stab at it since I am building all the cabinetry.
Any thoughts as the crazy vs. viable quotient on this idea?
Building Code Requirements
It is unfortunate that the issue of depressurization is not even addressed for new construction, (in Canada anyway). The only time protection from depressurization is required to be checked is if the house is over 4 bedrooms or when a natural draft appliance is present. Otherwise, you throw in a couple bathroom fans, along with the central vac, clothes dryer, kitchen hood and hope that the builder doesn't do that good of a job sealing it up.
GE and Technology
Martin: The saga of your interaction with GE was immensely entertaining (although dispiriting)! GE can engineer enormous jet engines and locomotives, but can't handle a decent kitchen exhaust fan?
Commercial kitchen hoods and laboratory hoods entrain the makup air stream with the contaminated air so that the fresh air and the contaminated air can be exhausted together.
Our residential solution is even simpler: No garlic or frying allowed in the house!
How many CFM, and how do you know?
You guys sure are credulous when it comes to thinking the CFM out depends only on the number printed on its specs. For example I have a downdraft hood on a kitchen island, and although I think the blower is "rated" 600 CFM there is no way it is actuallyl moving that much air given the ductwork it flows through.
Getting better acquainted with the basics of Building Science would be very helpful here. For example you have talked and talked about the subject without ever mentioning the *possibility* of measuring the house pressure relative to outdoors. This concept is very important to understanding whether there is a likelihood of dangerous backdrafting of combustion appliances. If the negative pressure is below about 3 Pascals one does not have a big problem.
For the Houston kitchen problem, I submit there is much value in using a supply-only fresh (outside) air ventilation system, with the inflow approximately equal to the outflow. Using a ERV will not address the problem directly, because of bringing in outside air at 70-75F dew point in the summer -- you will want to run it unbalanced since you are primarily looking at an air balancing problem, and that will do harm to the ERV's ability to transfer humidity. Not that the ERV was more than about 50% effective to begin with. If you need to address humidity, a ventilating dehumidifier is applicable here, or it may be sufficient to just have that fresh-air going to your AC return plenum, in sufficient quantity. In some cases it may be good to just pull a kitchen window open a couple inches.
This is about more than just being an energy nerd, it is about having an indoor climate which is safe and healthy for people too. I suggest toward that end, there are many writings by Joseph Lstiburek and his peers which teach what one needs to know.
Hope this helps.
Venting ranges
It seems like the leakiness of the house is part of the equation. In my case, I live in a fairly leaky 100 year old house (originally 9ach50), done lots of weatherstripping, windows, hopefully now down to 2-3ach50 and live in a very windy climate. I'm considering switching to a gas range when the electric dies, partially cause I don't like coal electric. I'm installing a 110cfm Panasonic spot fan in the kitchen and would like to run it as follows: Normally off, low speed with motion detector, high speed by wall switch. I'll keep it away from the range and usually cook meat outside, so mostly trying to clear out oders, excess moisture and any CO from the gas flames.
I'd like to eliminate the range hood altogether. What do you think?
Response to Jim Baerg
Jim - IRC 2009 requires direct ventilation above the cooking appliance. Indirect venting at another location would not fly with the inspector. You could possibly try to submit for a variance, but I'd be amazed if they'd grant one without some fairly extensive (and expensive) engineered documentation to prove your theory out. Besides that, you're playing a dangerous game by expecting an indirect solution to sufficiently capture the CO which would be created by a gas appliance.
Kitchen venting requirements
I am pretty sure 2009 IRC doesn't require any ventilation above a cooking appliance. A listed recirculating range hood is accepted in M1503. These are a totally stupid idea on a number of levels, but as far as I know they are perfectly legal.
If you want to add an exhaust in the kitchen ceiling, it would seem to be allowed per code.
I wanted to mention that "professional" ranges say in their instructions that you have to use a high-volume range hood. We install a lot of 1200 CFM hoods because the cooktop under it puts out 240000 BTU/hr and the manufacturer requires that much air flow, right in the instructions.
That doesn't mean any of it is a great idea, and it certainly doesn't mean that planned makeup air isn't a good idea and a good requirement. But it's not all about how shiny and big they are.
Response to M. Johnson
M. Johnson,
Thanks for your reminder about the need to commission exhaust equipment, the need to measure air flow, and the need to measure depressurization with respect to the outdoors.
While your reminder is certainly useful, many GBA readers are familiar with the basic concept, and I've been reminding our readers of these concepts regularly. One of my regular drumbeats is, "Every ventilation appliance needs to be commissioned, and that means measuring its airflow."
For example, in my June 2009 blog, Designing a Good Ventilation System, I wrote, "It’s disheartening to learn that many green homes waste energy because of poorly designed ventilation systems that were improperly commissioned. ... The big downside to central-fan-integrated supply ventilation is that the installer needs to understand how to design and commission the system. HVAC contractors capable of this task are rare. ... Anyone who commissions a ventilation system needs to learn how to measure airflow."
Similarly, in a January 2010 post, I wrote, "Every ventilation system needs to be well designed, properly installed, and commissioned. Commissioning must include flow rate verification."
Similarly, this is what I wrote about commissioning for an April 2009 blog, Simplicity versus Complexity:
"Builders often underestimate the importance of commissioning all HVAC equipment after installation. (“Commissioning” simply means making final adjustments and tuning up equipment to verify that it functions properly.)
Unfortunately, most new homes are imperfectly commissioned, leading to one or more of the following errors:
- The air conditioner has the wrong refrigerant charge.
- The airflow over the cooling coils has not been verified.
- Duct systems haven’t been checked for leaks.
- The airflow through forced-air registers hasn’t been adjusted to meet specifications.
- Pressure imbalances between bedrooms with closed doors and adjacent hallways have not been remedied.
- Exhaust fan airflow hasn’t been tested.
- Heat-recovery ventilators have not been balanced.
- Atmospherically vented appliances haven’t been checked for backdrafting during exhaust fan -operation.
- The temperature set-points on the solar hot water system controller are improperly set.
HVAC commissioning errors almost always result in needless increases in energy costs. Many HVAC specialists can share horror stories about commissioning errors, running the gamut from irritating to outrageous (for example, air-source heat pumps with electric resistance elements that operate for most of the winter)."
Anyway, thanks for the reminder. The bottom line is, to know the actual airflow of an exhaust fan, you'll need to some measurements or calculations. Unfortunately, measuring very large airflows (for example, 1,200 cfm) isn't easy. But that's a topic for another blog.
Building Science, and hot-humid subset of that
Why has nobody yet mentioned a speed control on this large exhaust fan? That still leaves house depressurization in the hands of the homeowner, which is not ideal but where it was before.
Point 2: Really you must know the leakiness of your house to understand whether a problem will result from depressurization. This is more elementary than actually placing a micro-manometer in the room with the fan running. The house I am in measures 5.3 air changes per hour at 50 Pascals depressurization (ACH50). A regular blower door test will deliver this measurement for the house in question. Based on this number one can estimate closely how much airflow is required to create 2-3 Pa depressurization and therefore potential backdrafting problems.
I understand the Northeast has a great amount of knowledge and thinking people, however that often results in considering only those problems that result in that kind of climate. The hot-humid South is prone to an entirely different set of problems from infiltration and I wish there were regular mention of that side of things. For example a huge exhaust fan is likely to result in indoor humidity problems as 70-75F dew point air is infiltrated in large quantity.
The laws of physics are the same wherever we go, but it is only human nature to focus on the problems one actually experiences.
Regards -- M. Johnson
Hot-Humid Stepchild
Amen
M.Johnson
Open a window
WOW...I bought a 4 ft by 3 foot back-lit stainless steel hood at a bankrupt burger joint for $ 100 and have been pondering how to supply adequate back-up air for the 500cfm fan that i got with it. It sits on top of my commercial restaurant unit ( 2 ovens, 6 burnerd, flat top griddle and broiler) I think after reading all the long and involved solutions, I will just be opening the window across the kitchen whenever I operate the hood-fan ( christmas, thanksgiving, pizza day, and about another half dozen occasions per year. Now I just need to find a tin-basher to plumb the exhaust hole thru the wall to the outside.
Speed controls
M. Johnson,
Why haven't speed controls been discussed? (Actually they were, briefly, in the sixth comment by Michael Chandler.) Speaking personally, I forget things that seem obvious to me, such as the fact that speed controls are included with every high-CFM manufactured range hood I've ever seen.
But, I would say the real reason they haven't played a big role in the discussion: they are not especially relevant to the makeup air issue.
Even though every owner I've ever discussed this with says they rarely use the high speed (too noisy), that doesn't mean we can design & build houses that don't work right when they do.
So while using a speed switch reduces the potential problems, I would say it doesn't really address the real worst-case scenario.
Heat recovery for commercial kitchens
From the GreenSpec top 10 products guide: http://www.buildinggreen.com/auth/productDetail.cfm?ProductID=4835
It's a large heat recovery exhaust system for commercial kitchens. Some day the concept should trickle down to a version at residential scale. Or for those homeowners with 1,200 cfm hoods, this may be about the right size.
Range hood venting solved
I am a contractor who regularly puts in high end kitchens. Our customers have 1200 cfm hood fans and wood burning fireplaces in the same room.
What we do is this - We use the air from the HRV system to make up the air for the hood fan. We put the vents for this into the kick of the cabinets - as they have to be quite large.
The system is designed so that when the hood fan is turned on the make up air is turned on.
We haven't had any problems with our situations - however every situation is different and you would want to talk to your mechanical contractor.
Cooking and Venting...
I have to vehemently disagree with all of you here...do you cook? There is no way a 150-250cfm hood is going to do much of anything if you do more than boil water (and even then...)
The recommendations for a hood are:
6" wider than the cooktop/rangetop/range installed against a wall; even wider for an island or peninsula installation. (E.g., 30" range/cooktop/rangetop against a wall --> 36" wide hood)
24" deep for wall installation; 27" for island or peninsula installation [Note that the front of the front burners of a cooking surface are not at the very edge, they're more like 20" from the back wall, so 24" is plenty for a wall installation.]
350 - 600 cfms for "normal" residential cooking (close to the higher # if you do much more than boil water); even higher cfms if (1) the hood is installed higher than the manufacturer's recommendation or (2) you do a lot of high-smoke/high-heat cooking
The extra width helps capture smoke,steam, grease, etc. because they do begin to disperse as they rise.
Island and peninsulas are subject to more and stronger air currents, hence the need for a wider hood or stronger cfms when a range/cooktop/rangehood is installed in these locations. Additionally, people tend to install these hoods too high because they do not want to block their line-of-sight (one of the many disadvantages to island and peninsula ranges/cooktops/rangetops).
As to downdrafts, they are not very effective unless pots and pans are right next to the downdraft and are shorter than the downdraft (for telescoping downdrafts).
I am not disputing the need for makeup air, but I am disputing the recommendation for low-power hoods. If makeup air is a current issue, then cracking a window or two will provide it in the short-term. A permanent solution is preferred, however, if you live in a climate with cold winters or very hot summers.
Make-up Air
I worked on a make-up air design for use with my 1200CFM hood forover two years before starting the remodel of my kitchen . The hood is over a WOLF rangetop capable of putting out around 90,000 btu (it includes a rangetop grill). I found the Broan engineers very helpful, working with them at the International Builders Show in Orlando over three shows.
My solution was to use 12" ducting to a grill on an outside porch ceiling. We used a grill with a built-in filter, too. Within the intake duct we installed a servo damper that opens whenever the range hood is turned on and closes when it is powered off. The ducting leads to two grills built into the backsplash on both sides of the rangetop. When operating, the intake air enters - drawn from the outside - and forms a vortex above the rangetop which is sucked up and out by the overhead fan unit. It works very well.
Broan engineers have been working on this issue for at least three years and this year will offer products to address the make-up air issue for the large, high volume exhaust vent hoods. These products are likely to be on display for the first time at the International Builders Show in Orlando this January.
make up air
I installed a 60" wolf range with the grill in our house, had to remove a wall to do it. Many people complain about their grills and end up not using them because of inadequate hoods. I decided to use a 66" island hood against the wall for the greater capture area. I installed a 1500CFM remote blower on the roof and had a sheet metal shop make a plenum that was 10" square where it met the hood, this necessitated widening the opening in the top of the hood, probably doubled the stock opening where the hood mated with the plenum. If I had to do it again I probably would go with a 10" x 16" shape to better transition fron the hood to the plenum for noise control. With this setup we can grill fatty salmon with flames jumping a foot above the fish and smoke so dense you can barely see through it and standing 6" away you can't smell a thing. This has blown all the loose gravel off the roof and the bushes in front of the house get bent over. Sounds like a jet engine for the few minutes the fan is on high but you kinda get use to it. We're lucky we live in a temperate area, Berkeley, Ca. so there's no great concern regarding heating or cooling. We live in a leaky 1950's house and we just open up a window when the fan is on high. For us in our area, type of house and type of cooking the high cfm works very well. I don't see how this can work during those times of the year in a area with high cooling or heating needs and the house is well sealed.
Simplest solution is the best
We have a Viking hood with an external, roof-mounted motor, and the instructions said to open a window when we turn it on, so that's what we do - actually we open up the sliding door a bit. Works great. We also have a Wolf range with a grill in the middle, which we use often, so we need a powerful hood to get the smoke from that out of the house. We love our setup.
make up air
Would it be possible/effective to install a 10" run directly from outside (thru rim joist with grille) through a 12 merv filter to catch the crap then right into the existing intake line in front of the hvac unit.. When the exhaust fan is running just turn the hvac unit on to condition the air. Too simple?
1200 CFm Hood Vents
I've only read about half the replies on high air volume hood vents but have not seen the possibility of looking at the point where the hood starts pulling a vacuum in the house. Of course the vacuum would also be at the exhaust point registered in inches of water thus also causing the fan motor amps to increase. Seems to me one could put in a vacuum releif valve on the exhaust stack or high amp switch on the motor operating openings either on the exhaust vent or other area points (garage during winter etc) to help equalize a closed loop system (which is what a house is unless you live in an old house like me that is anything but tight).
Makeup Air for Range Hoods
Fantastic article. I always see these monster vents for 4,000 sqft upscale houses, but, never thought about makeup air.
Mr. Holladay:
I suggest you
Mr. Holladay:
I suggest you consult the National Building Code of Canada, Article 9.32.3.8, Protection Against Depressurization.
By the way, in addition to back-drafting, depressurization can also cause a condition called "combustion spillage" in which the flow in chimneys and vents is not reversed but some portion of the products of combustion spill into the living space rather than exiting through the chimney or vent. This is equally dangerous.
Makeup Air in the modern home
This article hits the high discussion points, including the experts don't have a clue. The "makeup air" questions is the one that I have been asking for the past 15 years, with out a good answer. I designed and built a 4400 sq. ft. Insulated concrete form house using hydronic heating and Energy Recovery Ventilation. Fresh air comes from the ERV system as it exhaust the stale air. The wood fireplace is a closed air system. I am searching for a atmospheric pressure controlled makeup air component to provide replacement air depending on what exhaust fans are operating (Range vent, bathroom fans, clothes dryer exhaust and central vacuum system. Opening doors and windows is not a design option for me. Seems the solution would be simple, apparently not. I have sent request to companies for a system using a differential pressure sensor to control fan speed. I have not heard a reply. I see a intake air vent boost control for my ERV system - problem solved, if someone was interested. So, I'll keep looking. Thanks for the article.
kitchen venting design
Some kitchen ranges have venting of some kind just above the burner platform. Why not install a vent "sucking scoop" near the back and top of the range. You can have a decorative hood above or no hood.
With forced air heating and cooling systems I would think that the potential volume of air from these units that may draw air from outside the house when the inside air pressure is negative could be increased when the vents are in use.
With modern computer controlled systems a system should be able to be designed that would coordinate temperature, air pressure, flow rates,and fan vent and intake volumes. I wonder how they would be doing that on a submarine.
In my house I don't have these problems. I use a greenhouse-sun room and a franklin wood stove to heat and windows to cool.
Response John Semmelhack
"A dehumidistat is used to turn on a machine when the RH% exceeds a certain set point."
So, the thing that controls my A/C is a "dethermostat"???
Response to ACraigi
ACraigi,
You wrote, "We use the air from the HRV system to make up the air for the hood fan."
HRVs are balanced ventilation systems that exhaust the same amount of air as they supply. Every instruction manual for an HRV that I have ever seen warns installers that an HRV is not designed to provide makeup air.
Because grease will quickly gum up an HRV core, the exhaust air from a range hood should never be connected to an HRV.
Response to Anonymous
Anonymous,
Q. "I have to vehemently disagree with all of you here...do you cook?"
A. Yes, I have a local reputation as an excellent cook. I'm a single father who is a strong believer in family meals.I grow my own vegetables, bake my own bread, and make my sauces and soups from scratch.
Q. "I am not disputing the need for makeup air, but I am disputing the recommendation for low-power hoods."
A. There is no doubt that powerful exhaust fans are effective at removing smoke and odors. The only drawback to your suggestion is the energy penalty.
Response to Jim Trout
Jim Trout,
I'm glad that Broan is addressing the issue and is prepared to offer makeup air equipment soon to residential buyers. The next hurdle: tackling the energy penalty.
Response to Rick
Rick,
Thanks for your vivid description of what happens when a powerful exhaust fan is operating: "This has blown all the loose gravel off the roof and the bushes in front of the house get bent over. Sounds like a jet engine for the few minutes the fan is on high but you kinda get use to it."
Indeed, it's good you live in Berkeley.
Response to Anonymous
Anonymous,
Q. "Would it be possible/effective to install a 10" run directly from outside (thru rim joist with grille) through a 12 MERV filter to catch the crap then right into the existing intake line in front of the hvac unit? When the exhaust fan is running just turn the HVAC unit on to condition the air."
A. If I understand your suggestion correctly, you want to install a 10-in.-diameter fresh-air duct from your rim joist (that's a big chunk of wood to remove from a rim joist, by the way) to the return-air plenum of your furnace or air handler. You can do that. I would suggest a few controls -- a motorized damper on the fresh air duct, and some type of interlock between with the range hood fan, the motorized damper, and the HVAC blower.
You should be aware that most air conditioners cannot cool 1,200 cfm of hot humid air. But the HVAC equipment will eventually catch up.
The biggest drawback is the air leakage through the motorized damper when the unit is not in use. The HVAC blower will tend to depressurize the fresh-air duct, pulling air around the edges of the closed damper.
Response to John Haysom
John Haysom,
Thanks for your comments.
You're right, of course. "Combustion spillage" is a type of backdrafting. To have problems, it isn't necessary for 100% of the flue gases to enter the home; if 50% of the flue gases enter the home, you still have a problem.
Response to Fred Sawyer
Fred,
You wrote, "I am searching for a atmospheric pressure controlled makeup air component to provide replacement air depending on what exhaust fans are operating (range vent, bathroom fans, clothes dryer exhaust and central vacuum system)."
The control exists, but it isn't cheap. You want an Exhausto ECB 14, a control that lists for $1,500. Here is the link:
http://us.exhausto.com/library/3912017.pdf
In a product review published in the January 2008 issue of Energy Design Update, this is what I wrote about the Exhausto ECB 14:
"The EBC 14 consists of three components: a control box, an indoor pressure sensor, and an outdoor pressure sensor. The control box includes an LCD display showing the indoor room pressure. The control compares the indoor air pressure to the outdoor air pressure; if the indoor space becomes depressurized, the control energizes a variable-speed supply fan. The control modulates the fan speed in order to supply just enough fresh air to the house to correct the depressurization. (The EBC 14 can control a single-phase fan directly, or a three-phase fan indirectly, by means of a variable frequency drive.)"
Response to Lewis A. Saxton
Lewis,
You wrote, "With forced air heating and cooling systems I would think that the potential volume of air from these units that may draw air from outside the house when the inside air pressure is negative could be increased when the vents [exhaust fans] are in use."
Essentially, you are describing a powered makeup air system, which is the most common solution to the problem described in my article.
The problem illustrated
If you look at the first picture on this page http://www.buildingdiagnosticsnh.com/photos.html you will see an illustration of the potential problem. When I walked into this kitchen (a school cafeteria kitchen) I couldn't get my camera out fast enough.
The propane water heater is atmospherically vented, the device on the top is a (non functioning) damper meant to prevent heat loss when the burner is off. The hood next to it is a ten foot commercial hood, with makeup air as most commercial hoods have.
As mentioned the building was so incredibly leaky (1990 construction BTW) that this thing did not back draft. I prefaced every relevant section of my report with lots of "thou shalt not air seal until you buy a sealed combustion water heater" statements but in every subsequent conversation the customer asked if it was really necessary. I ended up sending a certified letter just to make sure.
Response to Bill Smith
Bill,
Great photo -- and the other photos on the page are great too. It reminds me of my days working as a home inspector.
Can I reproduce the first photo on your page for this blog? I'd be glad to provide credit to you in the caption.
HIgh efficiency
I don't see why someone could come up with a duct system similiar to that of a high efficiency gas furnace where there is a pipe in a pipe bringing air in and out in the same run. Let me know your thoughts if anything is out there?
Response to Kevin
Kevin,
If you read through the posted comments, you'll see that Michael Chandler describes this solution (see Michael's post of Nov 19, 2010, at 9:14 AM EST). I call it a "homemade HRV."
Here are the disadvantages:
1. It's hard to do if the duct path requires elbows.
2. For efficient heat-recovery, you need a long length of pipe.
3. The pipe diameters get big fast. If you require a 10-inch-diameter makeup air duct, then the duct-within-a-duct solution requires a 14-in. or 16-in. duct. It's hard to find the room for ductwork like that in a residence.
4. Like all range-hood exhaust ducts, the duct will eventually get greasy and need cleaning. The incoming cold air will tend to cool the exhaust duct, causing the grease to congeal and the moisture to condense. This cooling effect will tend to make the duct get dirty much faster than a warm exhaust duct.
My 2 cents
I'm happy to see makeup air is finally being discussed! There are other appliances that also need makeup air, specifically clothes dryers. I'm a professional mechanical engineer, and I designed my home with separate makeup air ducts on two floors of the home to balance the air losses from the bathroom fans, clothes dryer, range hood, and wood stove. I have a sealed combustion gas boiler and it also provides domestic hot water, so I don't have fears of combustion exhaust backdrafting, but it is a real issue for those that have older systems that obtain combustion air from the room. They will need makeup air too.
All this is coming to light because of the increased air sealing of contemporary homes, which does save energy but introduces the problem of not having enough leaks around windows and doors to provide sufficient makeup air for modern appliances. Plus, air quality can suffer because we need fresh air to breathe, and that fresh air has to come from somewhere.
Ideally, for energy saving, all exhaust air from all venting appliances would be sent through a heat recovery ventilator or energy recovery system, but in practice this is difficult and quite expensive to accomplish. Also, if you have a gas-fired cooktop your vent hood must meet gas code, and your local inspector will likely NOT approve an HRV or any such device to be installed in the vent. Plus, as others have said, grease could eventually clog a range hood HRV if not properly filtered at the source.
Another subject that has not been discussed is the stack effect, or the effect of air buoyancy as a function of temperature. Hot air rises in your home, cold air sinks. If you design a separate "chimney" for your range hood, and provide a damper, the stack effect can drive your vent for free (and silently) when you open the damper, and since the convection is a natural effect, it will be less likely to cause backdraft problems at other appliances. However, natural draft is probably not going to be enough flow for 5-burner cooking or strong odors, so there is a drawback and compromise with this method. The stack effect can cause problems as well. If you have a two-story home, your interior space is a “stack” and warm air will want to leak upward and thus pull cold air in from below. If you vent an appliance out on a first floor wall, it could end up backdrafting if you have an open window or vent on the second floor or attic.
If you have a fireplace or woodstove in a well sealed and insulated home, you should address the makeup air needed for these appliances -- called outside air (OSA) in that industry. Some modern units include a duct connector for OSA on their units, and this is ideal because you are bringing cold outdoor air into your woodstove, rather than using up warm room air for combustion.
Here are my recommendations:
- If you are designing a new home, design the makeup air ducts before you build. Make sure you get approval from your local building, gas, and fire department inspectors.
- Use modern construction methods to air-seal and insulate your home
- Use sealed combustion appliances whenever possible
- Provide some makeup air for all appliances that cannot be sealed, such as the range hood, clothes dryer, bathroom fan, and woodstove.
- Bring the makeup air into the home in places where it will cause the least drafty effects felt by you. One person mentioned behind the refrigerator – that’s a good idea because it warms makeup air as it passes across the warm coils.
- Use the stack effect to get free venting where you can, in other words pipe the hot fumes up a separate chimney and they will go up on their own without a fan. A damper is required to avoid losses when you are not running the appliance.
- If heating or cooling your home costs you a bundle, consider improving your insulation and installing an HRV or ERV. They are expensive and payback periods can be long.
If this all sounds complicated, that’s because it is!
Ventilation versus inspectors
Martin,
As a kitchen designer/installer, I have to deal with the 2 ends of ventilation. On the one hand you have clients who want, need, or have to have (to impress) big, beautiful hoods. On the other -- as an installer, the inspectors have no guidelines for ventilation.
The problem arises when you install a Viking, Wolf, Dacor or others. The first question from the inspector is, "Let me see the specification for the stove." Hence all manufacturers usually require 900 - 1,200 cfm ventilation.
Rock and a hard place.
Response to Aich
Aich,
I don't believe you are between a rock and a hard place. You actually have two choices:
Choice one: explain to your clients why a 48-in. Viking stove with a 1,200-cfm range hood will not perform as well (in a residential setting) as a conventional 30-inch range with a 350-cfm range hood.
Choice two: if your clients insist on the huge range and the huge exhaust fan, it is your responsibility to also provide a system to deliver makeup air to the kitchen.
I concur with James Morgan ("Plan B")
I completely concur with James Morgan's comments in his post, "Plan B"...fer cryin' out loud, open a window (you wouldn't have to throw it wide open, I would think) or go outside and cook whatever offends one's delicate senses. The idea of a summer kitchen has great merit, actually, and if I could ever build my perfect house, I think that the kitchen ought to be a "cookhouse" separate from the other living spaces...
But thank you for shining light on something I hadn't really thought about until I read this...as a household manager, it's important for me to learn all I can about these issues, and apply that new knowledge in my workplace (my employer's high-end home).
Picture rights
Martin, feel free, let me know if you need a larger size.
I think that sharing photos amongst the professionals is important. I wonder how many of us still visualize the stack effect from Dave Keefe's iconic photo from many years ago. I'm sure that given the number of uses he wishes he was getting royalties.
Ventilation
If you can afford a high end hood of this type then you might want to look into adding/integrating (if possible) the economizer into your hvac system. The economizer draws from outside air until it reaches a certain temp. If regulated the right temp, the make up air could possibly be drawn through the return air grill. I do like the vent behind a refrigerator. It dont have to be dryer duct. you could easily (if 2x4 wall) could make a 3 1/2" x 14" plenum and vent to the attic/roof/eave vent or through the floor similar to a down draft system.
Thanks, Bill
Bill,
Thanks for providing permission. I've added the photo to the blog as Figure 3. I included your Web address with the photo credits at the bottom of the blog.
Motorized dampers from Broan
While HVAC suppliers have been selling motorized dampers for years, Broan is now selling marketing several sizes of motorized dampers specifically to residential contractors looking for a way to provide makeup air for exhaust fans.
Broan should be applauded for providing instructions on the installation of these dampers in passive makeup air ducts. Let's hope that other manufacturers take a cue from Broan and follow suit.
I have added information on Broan's products and instructions to the bulleted list of advice at the end of my blog.
Dehumidistat feedback loop
John.
You bring up a god point which many people who own preinstalled HRVs and ERVs often don't understand well if at all.
When the outdoor RH far exceeds the indoor RH and a dehumidistat is set to anything below 90 (or preferably higher, even), ushering in damp outdoor air will then make the indoors air still more humid, leading to the dehumidistat staying ON more.. sigh.
In the middle zone, humidity wise, this isnt an issue, but on really humid days it can lead to trouble.
Dehumidistats should have a little label on them spelling out that they need to be "adjusted" off- i.e. set to their upper limit- during super humid weather. My HRV control can be set to not trigger unless the RH is (I think) 95%.
Computer geeks call something like that a "race condition" as its a feedback loop that is temporarily operating erroneously leading to a waste of system resources accomplishing the opposite of whats intended.
cheap and reliable differential pressure switching
Does anybody know of a manufacturer who makes an economical adjustable pressure-activated microswitch or better yet, a sensor that can be used to translate a modest pressure differential to a variable voltage or current?
Basically, my application would be setting limits to trigger makeup air control.
Increased radon levels from large range hoods
I suggest that anyone who installs large range hoods, such as described in this great article, should consider having their home tested for radon after it has become operational. Radon is a soil based gas that is the second leading cause of lung cancer. It is usually drawn into the home's living space by the slight negative pressure inside the home's envelope by warm air rising & leaving the building via air leakage (Stack Effect), devices which consume oxygen for combustion such as low efficiency furnaces, hot water heaters, wood stoves, and devices which exhaust air to the outside of the house ( clothes dryer, exhaust fans). The typical negative pressure inside the house I've seen is usually between 0.002" to 0.006" WC ( Inches of Water Column). Operating a 1200 cfm device without makeup air will add significantly to the potential radon levels in the house if it is present in the soil below the home floor slab or basement floor.
I suggest performing a short-term radon test ( 3-4 days) using a "Continuous Radon Monitor" which records the radon levels hourly during the testing period & then averages the results. ( FYI, do not use a carbon test kit or the Home Depot/Lowe's off the shelf tests because the only give you the average radon reading during the test period). During the test period operate the range hood. If you have radon in your soil I predict a large spike during the range hood operation.
For more radon info. every State has a Department of Public Health, they will have an person who can discuss local radon issues or direct you a certified radon tester/mitigator in your area.
'Duct in duct' is interesting old technology
This is the solution for make-up air in old-style steam powered ships. The funnels are/were double walled; furnace gases exited via the central space, and makeup air came through duct created by the double wall.
The scale is really different, of course; imagine a funnel 12 feet in diameter with an annular space 8" all around.
Air makeup at the stove
After reading the above posts, I feel that the suggestions that keep trying solving the air make up for the cooking stove, combustion and cooking by product elimination via the hood by tying the solution of that problem to other systems in the house have strong potential to compromise or complicated those systems unduly. It’s one thing to devise a complex interconnected system that only you understand, but is beyond any other user’s ability or desire to want to understand. The solution for the hood/stove should happen at that location, partly because as with the other systems in the house, it is used at different times and in different capacities. With tighter houses, air make up will require new solutions. All stove/hood combinations come with a built in energy penalty. Originally, the penalty was hidden by having a leaky building, but it was there, never the less and won’t disappear until the stove/hood disappears. Gas stoves have a larger penalty because they are an open flame combustion device in the middle of you house (not much different than a campfire sitting in the middle of your kitchen.) So it seems that an air make up system needs to be tied to the stove/hood directly by using some of the following concepts:
1. All stoves would have an outlet that is activated when any of the stove’s controls are used (think igniters).
2. This outlet supplies power to the hood and starts a low speed/volume fan that would exhaust basic combustion products. Simultaneously, an electrically controlled damper would open in a supply air duct that supplies make up air directly near the stove. In this make up duct is a heat coil (here is your penalty) for cold climates governed by a thermostat. In warm, humid climates (I am not really familiar with these, having never lived in one) because the makeup air is not circulating though out the house, and is being immediately vented via the hood, there may not be any big issues with increase humidly in the rest of the house.
3. As the hood fan is turned up to higher speeds and volumes, more air is pulled in through the makeup air duct.
4. When not in use, the makeup air duct damper is electrically closed.
As with all the other mechanical system that are now being used, and an integrated air make up, exhaust system is build into each device: sealed combustion forced air systems, water heaters, tankless water heaters, gas fireplace/heaters. The kitchen stove/ hood now needs to be designed as its own system. Until this happens, the solution is still similar but user invented: to have the air makeup and hood operate in a connected way at the stove.
For commercial loads, look at commercial solutions
We design and furnish LEED qualifying, energy efficient commercial hood systems, some of which end up in high-end homes. In our world, a 1200 cfm hood easily ventilate an 8' commmercial cookline (no chargrille). Its way more exhaust than is necessary for any residential application.
Sizing the range hood starts with the applicance under it, calculating the volume of the heat plume, and providing an efficient collector (the hood) to remove it. The plume is stupid; it will rise on its own. The hood is not a vacuum cleaner, just a collector. Ours happen to be very efficient.
There is no free lunch. In the commercial world, make up air is required by code, and must be tempered. In residential, the codes, for now, are more forgiving; but it doesn't change the physics. The air has to come from somewhere. Matching the hood to an ERV in another area doesn't change anything, unless the ERV can bring in more than it is taking out, and is operated whenever the hood is on. But by minimizing the exhaust, the make up air requirement can be brought under reasonable control.
There are residential furnaces with outdoor air dampers, which can bring in outdoor air thru the HVAC system when the hood is on. We have also provided outdoor air fans with electric heat, interlocked with the hood. There are gas fired units that do the same thing, but are cost prohibitive on a residential project.
Control systems already exist that use infra-red technology to sense the heat load under the hood, and modulate the exhaust fan and make up air system to effectively exhaust the appliance and maintain building balance. $$$.
When my phone rings with a residential application , my first question is, " Are you going to cook, or boil water". A cook's kitchen needs a well designed system, for safety and comfort.
A commercial solution in a residential kitchen may not be the most beautiful, but it can provide the most energy efficient design that works. Most kitchens can get by with a small hood and a small fan. Many of the 1200 cfm hoods could also be smaller, based on the use of the applicance. But for a serious kitchen where actual cooking is going on, a commercial system is a viable way to go.
Solution
Holey moley. I'm eating out.
Motorized dampers? Really?
Martin,
Great article. This topic will continue to grow as regulation spreads. That said, I have a big problem with your suggestion that range hood manufacturers should provide generic solutions for make-up air needs (Broan motorized dampers). Such product recommendations encourage flawed solutions in more instances than not. Large hood customers are spending large sums of money and want the end result to be right. Where make-up air is concerned, every home has a unique set of circumstances that should be evaluated by an HVAC engineer or mechanical contractor (like me). In most cases, the solution requires much less cfm than the name plate rating of the hood. The idea of an appliance salesperson or range hood installer recommending poking a large hole in my house to the steaming or freezing outdoors based on universal range hood installation instructions is scary indeed. You said it yourself; motorized dampers have been readily available for some time. Let the local pros who are familiar with regional conditions evaluate the unique dwelling and engineer an appropriate solution. In my opinion, the range hood manufactures should ONLY publish considerations and guidelines and recommend seeking local professional assistance.
Response to Mike Hunt
Mike,
Just because I think that motorized dampers are potentially useful and could be incorporated into a makeup air system doesn't mean I think that they should be installed willy-nilly without a "whole systems" approach.
I agree with you: any solution must be designed properly, installed properly, and commissioned properly.
However, that does not absolve range-hood manufacturers from the responsibility to inform contractors about the need for makeup air systems. It does not justify their head-in-the-sand approach, nor does it justify the deliberately obfuscating language that they now include in their installation manuals.
motorized dampers
Martin,
Irregardless (sic), I think we are on the same page. I appreciate your enlightened input on this overlooked (or suppressed) subject.
Yes, I agree
Mike,
We're definitely on the same page. We need makeup air systems to be designed by people who know what they're doing.
Thanks for taking the time to post your comment.
Typical corporate runaround
Obviously, GE will sell far fewer high powered range hoods if they are accompanied by dire warnings about need for makeup air. Therefore they perceive that it is in there best interest to duck and hide from the issue.
The only way to get their attention would be using a bigger stick. The stick I would suggest in this case is the CPSC. Shoot some video showing ashes (or worse) coming out of a fireplace when one of these Monogram Mammoths is turned on.
GE may be able to happily blow off a nosy blogger, but that strategy WON'T work on the feds...
Are you kidding?
"Select a modest, 30-inch-wide residential range. It will perform better than a 48-in.-wide Viking or Wolf range, since these larger commercial-style ranges require very powerful (and problematic) exhaust fans."
Good research and interesting article.. but your first line of advice is pretty lame! (:
No, Scott, I'm not kidding
Scott,
I cook on a 30-in. wide range with 4 burners, Scott. So -- no, I am not kidding.
Whoops!
My house is so tight...
1. The kitchen exhaust damper flies open when I shut the front door
2. When I had a blower door test done, they thought their machine was broken
3. I thought a tree fell on my house the first time the ice maker dropped ice (I actually walked around the house to look for the tree.)
Since I designed and built the house myself with Taunton"s "Green From The Ground Up" as my bible, I was all too aware of the dangers of excess indoor humidity. Naturally, I installed humidity sensing exhausts in the bathrooms, installed a Recoupairator ERV in separate ducting, and installed a Fantech RVF 8 exterior mounted kitchen exhaust fan. The kitchen exhaust however doesn't really get used yet because I'm still designing the custom hood. After reading this article and the subsequent comments, I turned the fan on and opened the casement window a smidgeon and much to my surprise, a blast of air came in so hard the window tried to shut on its own! Apparently I'm green--real GREEN!
This all has me wondering now: forgetting the kitchen vent--what about my 2 bathroom exhausts? What if they are both on at the same time? Where is the air coming from? Fortunately my only ignition source in the house is a direct vented propane boiler--and it hasn't shown any signs of malfunctioning. But this seems to be a HUGE blind spot in the industry. No-one talks about make-up air. Its all about getting rid of moisture--exhaust, exhaust! Does anyone have any suggestions on how to determine if--and how much--return air I need? Or is all this over-blown?
Thanks
Response to David Metzger
David,
Although you are right that range hood manufacturers don't talk about makeup air and backdrafting, I can assure you that building scientists and home performance contractors have been lecturing on these issues for 20 or 30 years. Fortunately, builders and homeowners are finally beginning to pay attention to these issues.
If you don't have any atmospherically vented combustion appliances in your home -- for example, an atmospherically vented water heater, a wood stove, or a conventional fireplace -- then you don't have to worry about backdrafting.
In almost all cases, homes have enough cracks and leakage to allow bathroom exhaust fans to function. If you want, you can hire a home-performance contractor to measure the air flow of your exhaust fan. (Here's the deal: if the fan is blowing 50 cfm out of your house, you can be assured that 50 cfm is finding its way in.) Or you can use a common-sense test:
1. Will the bath fan hold a piece of toilet paper tight to the grille when it is on?
2. Is the outdoor flapper pushed outward when the fan is on?
3. Can you feel air blowing out the outdoor flapper?
If your fan passes these simple tests, I wouldn't worry. However, if you want to go further, you can use the credit-card test developed by Terry Brennan. This test has been calibrated, unlike the famous toilet-paper test. Here is the description of Brennan's test, courtesy of NYSERDA's Homeowner's Guide to Ventilation:
"Find a cardboard box with an opening big enough to fit over the exhaust fan grille. If the fan is mounted in the wall, cut a hole slightly smaller than a credit card in the bottom of the box, or, if the fan is mounted on the ceiling, in the side of the box. Using any kind of tape, attach a credit card inside the box over the hole. Make sure the card can swing back and forth in the box. Turn the fan on and put the box over the exhaust grille. If the fan is working, the credit card will swing into the box. The greater the air flow, the more the credit card will swing open. If it swings open 11⁄2 inches or more, the fan is moving at least 25 cubic feet of air per minute, which is a reasonable amount for a bathroom. If the card swings open less than 11⁄2 inches, you should consider repairing or replacing the exhaust fan. (Tip: use a pencil instead of a ruler to measure how far the card swings open, because a ruler will block the air flow.)"
The test set-up is shown in the illustration below.
Other Back Draft Effects
In our new house we just moved into. We noticed a bad smell coming from the kitchen sink when the exhaust fan was running (1200 CFM). I have never heard of it before, but apparently we were pulling air back through the traps from the drains. Cracking open a window stopped the problem. Guess I'll be installing a make-up air system in the future. I was worried about this issue, when we were building, but found a complete dearth of information on the subject. Especially from the local HVAC contractors.
Response to Tom Hutchison
Tom,
If you just bought a new house, and you just moved in, the person to contact is your builder. You also might want to check the building codes in effect in your community. If your house is defective -- and it sounds like it is -- the responsibility for correcting the problem lies with your builder, not you.
Re: Response to Tom Hutchison
Well...... the builder would be me. I designed and built the house myself and am the GC. Any clue what might be defective? The drains were done by a professional plumber and passed by the building inspector and myself. Since we have hydronic heat and the both the water heater/radiant heater and the gas fireplace are direct vent we don't have back drafting through them. So the only other air entrance, other than leaks, would be from the drain vents, back through the plumbing traps. That's my best SWAG :)
Response to Tom Hutchison
Tom,
You asked, "What might be defective?" You know the answer, of course: you forgot to include a makeup air system for your 1,200 cfm exhaust fan. If the house were built today in a jurisdiction where the 2009 IRC applied, then the lack of a makeup air system would be a code violation.
Energy Efficient Damper
Hi All,
Excellent discussion. I've not read the entire MILE of the thread - just the first 2 or 3 pages. One thing I've not seen is a link to an energy-efficient, (insulated) and airtight damper - and one that preferably also does not create a large pressure drop - ie is aerodynamically efficient as well. We need something like this in both the make-up air stream and another one (albeit more fire and spark-rated) in the grease exhaust stream. Seems this is not a crazy request - perhaps from another industry - Without this, I will stick with the recirculating range hood - Something like the Vent-a-hood item for our passive house.
Airtight Damper
Here is an interesting item - a damper that is really airtight: All steel construction looks like it might be good for the Kitchen Exhaust ducting - and perhaps for the make-up air duct as well. No insulation - but perhaps that can be an installation detail.
http://www.ebaircontrol.com/eba_industrial_positive.html
The cunundrum of ventilation
I have been dealing with the conundrum of kitchen ventilation for over 10 years as I am in a state that has had make-up air requirements for some time. I think people get frustrated that a hood manufacturer should have all of the answers in their installation manuals but you must understand that make-up air requirements not only vary from state to state, they also vary from house to house and then throw in the inspector and how he will interpret and apply all of the information. It simply isn't that simple. In our state there is a 'formula" that is to be used that takes into account the square footage of the home, the size of the rooms, the amount of all fans removing air from the home, and then the size of the furnace room and finally the HVAC system. Now we can start determining what if any make up air is needed. Simply, not so simple.
Reply to Michael Mahin
Michael,
You're right that the issue of makeup air for range hoods is not simple. However, that fact does not absolve the manufacturers of range hoods from providing useful installation and safety advice.
Many pieces of equipment installed in homes require complicated installation instructions. In other industries, manufacturers have risen to the challenge. It's time for range hood manufacturers to do the same.
At a bare minimum, they could provide advice similar to the bullet points I provide at the end of my article. That would be far better than the current situation.
Discussions with GE on this subject
The article describes calling GE Answer Line about make up air and flow through / back drafting. They haven't got any better, so apparently didn't learn the first time. We had EXACTLY the same conversation in 2010 early 2011, when trying to size GE Profile equipment for our 3500 sf house - totally CLUELESS. We have to open windows . . . but since we have a ground source heat pump and direct vent water heater, and sealed fireplaces we are not concerned about backdrafting.
This overall issue needs to be addressed seriously by theappliance manufacturers for high performance homes.
Response to Richard and Miranda Menzies
Richard and Miranda,
Thanks for the anecdote that backs up my experience. It's astonishing, isn't it, that the people who answer the phone on this question are so clueless.
Actual CFM needed is far less than 1200 CFM for 48" ranges
I just ran across a 48" Dacor hood that claims a 6600 CFM capacity! Totally ridiculous! Take a look at: http://www.greenheck.com/media/pdf/otherinfo/KVSApplDesign_catalog.pdf
It's a technical design guide for commercial kitchen hoods and make up air. Using their recommendation of 50 feet per minute air velocity in the updraft hood a 22" X 48" (7.22 sq ft) hood only needs 367 CFM. If frying on the whole surafce and you use 85 fpm it's still only 623 CFM.
My guess is the ridiculous 900 to 1200 CFM capacities being offered are largely marketing hype. If 300 is good 900 is better and if 900 is better then 1200 is even better. That kind of ridiculous logic can influence a marketing centric company to force their engineers to do stupid things.
I know this first hand. My company produces products to correct lip-sync in home theater systems and even though scientific research over 20 years ago verified no human can hear sound above 22 KHz (for which a sampling rate of only 44 KHz is required) the industry is driven by snake-oil marketing types and the new HDMI audio format is "8 channels of 192 KHz uncompressed audio". This requires 4 times the bandwidth necessary to reproduce the maximum frequency anyone can hear!
It all started when DTS in an effort to gain advantage over Dolby doubled the sampling rate form Dolby's 48 KHz to 96 KHz and consumers swallowed it "hook line and sinker." So if they believe 96KHz is better they should believe 192 KHZ is even better, right? So the marketing goes and our new products must now have 8 times the memory to provide the same audio delay to correct lip-sync as our DD740. And for what?
No one can hear the difference in these delivery formats. The Boston Audio Society conducted a double blind study proving no one could detect the difference in a 16 bit 44KHz audio (CD standard) delivery format and 24 bit 192 KHZ and the prestigious AES (Audio Engineering Society) found the study credible and published it under their name! But consumers simply bought into this 192 KHz hype and want HDMI's 192KHz formats.
I think the 6600 CFM Dacor hood I just saw on eBay confirms this folly. If you pull 6600 CFM conditioned air out of a building 6600 CFM must be replaced and it would take over 15 tons of air conditioning to cool and dehumidify Summer outside air where I live (SC).
Ha--a bit of a tangent, Nick, but as an audio engineer I agree wholeheartedly. I might add the irony of the move towards higher bit and sampling rates happening in conjunction with MP3 and other radical compression schemes! It's been seven years since your post and no surprise to me that these high bandwidth audio formats (with your apt comparison to range hoods) are a commercial flop.
> proving no one could detect the difference in a 16 bit 44KHz audio (CD standard) delivery format and 24 bit 192 KHZ
>> I agree wholeheartedly.
I generally agree too. But I can set up a test involving poor gain settings where you will clearly hear the difference.
Relevance to green building - be careful about what test data is really proving and don't form overly broad conclusions.
Response to Nick Johnson
Nick,
Of course you are right -- no residential range needs a 1,200 cfm exhaust fan. Energy experts have been battling this insanity for years, but it's an uphill fight.
Thanks for the link to the design guide for commercial exhaust hoods.
As for the ads on CraigsList for "6600-cfm range hoods", I think the sellers are probably strung out on meth and accidentally hitting some of their keyboard keys twice. I think they were trying to type "600 cfm."
More thoughts and Wolf's response
I've read every post in this thread and will likely go back and read it over again. I'm in the planning stages for my retirement home and I'm looking at installing a Wolf 48" rangetop. Based on my research a 54" hood (with 3" to spare on each side of the range) is what I should use with this unit. And yes, the suggested rate of air movement is 1200 CFM. However, I believe this rate is based on having all burners on at once, including the grill. I expect this to be a rare scenario in my house, but I want the ability when needed. In most cases I would expect that I may not need to run the hood any higher than 200 or 300 CFM. I’m mostly concerned about having a spot solution to pull the CO2 out of the house. If I miss a little of it I figure my ERV will eventually take care of the rest. (I’m not operating restaurant and cooking all day long!)
I see this blog is two years old and in that time some positive things have taken place. I recently emailed Wolf to ask a few questions about the hood design (by the way they have a pretty good ventilation guide). My first question to them was on noise and their response has me leaning towards a design with a remote blower. My second question was on duct silencers, which they replied that they didn’t have a position on because they have not tested them and didn’t know if a silencer would affect the required CFM. At this point I came along this blog and so I thought I would hit them up with one more question. Here’s my post: “What is your recommendation on a good system for make-up air? 1200 CFM is a lot of air to try to pull out of a tight house a make-up air system is definitely called for. I was wondering if Wolf had a recommended system.” To my pleasant surprise they sent me a pdf file (I’d be happy to share it but not sure how on this forum.) with their AUTOMATIC MAKE-UP AIR DAMPER WITH TRANSFORMER & PRESSURE SWITCH. It has a flow chart to help you decide which type of installation is called for.
Knowing that I will rarely run this hood at 1200 CFM helps me too breathe a sigh of relief concerning energy conservation. (The hood I’m looking at has a variable speed control.) I like the idea of having the make-up vents near the hood/range. I also like the positively sealing dampers that someone else mentioned earlier. Being a typical engineer (electrical) I love trying to solve problems and I’m formulating an idea for a motorized damper with insulation on the damper itself that won’t interfere with the airflow when open. However, I speculate that the small energy loss at the damper may not be worth the effort.
I’ve got plenty of time before I finalize my whole house system design (a few years), so I can wait and see what great ideas others come up with. Keep posting!
range exhaust make up air via HRV
Hi Martin,
Could a 200 CFM HRV be relied upon to provide adequate make up air for a 325 CFM range exhaust? If the sole HRV inlet was in the kitchen, would the range exhaust exerting negative pressure on the kitchen pull extra air through the HRV inlet beyond its' rated 200 CFM?
My client is experiencing backdrafting even mid-burn with their Tulikivi masonry heater!
Thank you for all of your work and for your brain..
Andrew Mckay
Response to Andrew Mckay
Andrew,
As I explained in my article titled HRV or ERV?, "The purpose of an HRV or an ERV is to deliver fresh air to a home’s interior. Neither appliance is designed to provide makeup air for combustion appliances or kitchen exhaust fans."
An HRV is designed to provide balanced ventilation; air out = air in. Most HRVs have controls that attempt to address pressure imbalances by adjusting the airflow rates of their fans to seek balance.
That said, it is still possible to overwhelm an HRV and put it out of balance. That doesn't mean it's a good idea, however. No one should depend on an HRV to provide makeup air.
Information On Residential Make Up Air System
Tightly sealed, energy efficient homes are great for conservation, but bring a new challenge to home builders – namely, finding a way to replenish the air exhausted by the kitchen vent hood. When the new IRC M1503.4 building code requiring make-up air (MUA) for range hoods in excess of 400 CFM was introduced, builders had few cost-effective compliance options – everything required re-engineering, special tools or methodologies that slowed productivity down.
Now, there’s a better solution: The Residential Make-up Air System (RMAS) (patent pending) by CCB Innovations. This universal fresh air infiltration system takes the complications, worries and high costs out of meeting the IRC M1503.4 make-up air code compliance.
Our solution:
A Universal Make-up Air System that works with any brand of kitchen vent hood.
Air flow sensor is pre-mounted inside an exhaust system coupling for quick convenient installation.
Does not require cabinet dimension modifications or speciality trade contractors.
Works for new construction or retro-fit jobs.
Is made in the USA. RMAS’ universal design reduces builder costs, while improving air quality for the homeowner.
5 year warranty
Makeup air.
When we moved here, we opened the window to provide make up air - then winter came and my wife said I am freezing! So I fitted another exhaust fan with heat exchanger close to the ceiling in our kitchen this allowed warmed air to drop from the ceiling - no more complaints.
When the electric oven was on we used the exhaust fan with heat exchanger. When the hob exhaust was running we left the heat exchanger open to provide make up air - but testing found air was still being pulled from other parts of our home.
Very simple solution. I cut a hole in the wall in the wall below our gas hob. I enlarged the hole until all the required air was coming through the hole, no drafts round the door. Then I made a draw with a plate on the back that seals the hole when the draw is closed. Now, when my wife is cooking, she merely opens the draw that allows air to flow into the space under the hob, which then rises through slots on both side of the hob.
Problem solved, fan doesn't work so hard, water vapor passes up and out though the extractor fan, our expensive heat stays indoors, wife stays warm and happy.
Perfect result, at minimum cost.
Response to Roger Anthony
Roger,
Since I am American, I find it necessary to adjust some of your vocabulary. I assume a "hob" is a stove or range, and I assume that a "draw" is either a damper or a grille -- not sure which.
I'm not sure what you mean by an "exhaust fan with heat exchanger." Presumably you are referring to a heat-recovery ventilator (HRV).
If I have properly deciphered your description, it sounds like you cut a hole in the wall behind the stove, and fitted the hole with a register with louvers that can be closed. The louvers are opened when you want to cook, and closed when you don't.
This solution isn't very expensive, but there are disadvantages. In a cold climate, this hole in the wall will leak a lot of air -- even if the louvers are closed, and even if the hole is equipped with a damper. The second problem is that it only works when the cook remembers to open the louvers; because of this requirement, your solution can't be used by most builders (since they would be liable for any backdrafting problems, and since this owner-operated mechanism can't be depended on).
Commercial ranges - other requirements?
Learning lots of great information from the blog post and from the comments that I can use on the job as I sell appliances.
With commercial style ranges, are there any other requirements necessary to keep a kitchen safe and efficient? I've heard that there should be some fire retardant backsplash installed against the back wall.
Response to Eric K
Eric,
Installing a commercial range in a residence will void the manufacturer's warranty. Moreover, such an installation is a violation of many homeowners' insurance policies.
Even if the installation is allowed by the local building code, you'll have to maintain a gap between the commercial range and adjacent cabinets, countertops, and walls -- gaps that many homeowners would find unacceptable.
Here are links two three online articles that list all of the reasons why you don't want to install a commercial range in a residence:
What to consider before purchasing a commercial range for a residential kitchen
Does a commercial range belong in a home?
Commercial ranges in homes
Study on how to design makeup airflow for max. efficiency
In the course of building an addition and remodeling our kitchen I am researching the problem of makeup air for the range hood. As we renovated our fixer-upper room by room over the years we insulated and airsealed nearly all outer walls, and the addition will make that job complete. I'm really hoping that our house will end up very airtight, which in turn might cause problems with a range hood so I am trying to be prepared. If we should have to do MUA, I really don't want to heat that air (we're in NH) but ideally introduce it near the range so that much of it would be sucked by the hood. Obviously, one solution is to just go with a regular size range and a <400cfm hood, but in case anybody is interested, i came across this very detailed study about introduction of mua commercial kitchens. think the general airflow physics also apply to residential hoods even though some solutions might be unacceptable from an aesthetic point view.
http://www.fishnick.com/ventilation/designguides/CKV_Design_Guide_2_031504.pdf
Check out the perimeter supply as well as the backwall supply for introduction near the range. Side panels allow you to reduce the capacity of the hood. I'll report back in a year how the project went ;-)
Effect of the operator on ventilation devices
When it comes to range vent hoods, my observations parallel those of Mr. John Brooks. After all the calculations of size and placement of a range exhaust system and the hardware is installed, it's still up to the cook to decide when to use the system. It's not like an industrial furnace where excess combustion air and adequate exhaust must be proven before a flame is allowed. Most of our cooking (with natural gas) uses no ventilation other than whatever house leakage we have (no heat exchanger fresh air supply). When we DO use power ventilation, it comes in big doses with our downdraft unit.
How's it work? Well, we observe and we breathe. If it LOOKS like the fumes are out of control, we turn on the vent. If we can't stand the SMELL of the usually desirable odors, likewise. But mostly, the vent is on standby. In most cases, therefore, our vent system is oversized. If our range were in a laboratory type enclosed chamber with controlled inlet and outlet air, we could vent only what is minimally required. That's not our case, however. In January when it's below 0 F, we even put a pot of water on the range to boost the living space humidity when desired. We don't use the vent at these times. Our CO/CO2 monitor has shown no issues.
In reading comments about ventilation, there's a lot of talk about what conditions should be. The elephant in the room is how the conditions actually are. I'm guessing that's because of code requirements and potential health consequences from breathing other than pristine quality air. That's probably why GE is so vague and why advice in this area seems so guarded. (The word "liability" comes to mind.) But even a perfectly sized vent unit is out of spec if it's not turned on. I'm OK with the way it is. Size the vent reasonably and let the occupants decide when and how to use it.
Size of pipe per CFM
I plan on adding a PVC pipe for make up air with a ball valve in my pantry for range hood. I cant see why I would need a hood more than 150 CFM. My thought is to run this under my covered porch foundation next to my house foundation wall, in nice dry sand, sealed, at a depth of about six feet down, for about 24 feet, then enter through a penetration in foundation. Then the pipe will travel several feet in the basement and up through an interior wall into the pantry about eight feet from the range. Hopefully this will temper the air somewhat before it enters. I plan on being able to manually control it when the range fan is on. So i have two questions: how big a pipe and at what height should it exit the wall? Level with the stove? I was think 2 1/2" PVC might work for thay CFM. Also I was considering one for my drier. I susoect that this one need be larger. Any thoughts?
Response to Joe Suhrada
Joe,
From a technical standpoint, the answer to your question is complicated. Many variables will affect the answer.
However, the usual rule of thumb is that the diameter of the makeup air duct should be at least as large as the diameter of the exhaust duct. If your range hood has an 8-inch-diameter exhaust duct, then you need a makeup air duct that is at least 8 inches in diameter.
Response to Joe Suhrada by Martin Holladay, GBA Advisor Read m
Ultra aire says that their ventilating dehumidifiers put a house under slight positive pressure. Would that make the large kitchen fan operate effectively without additional backup air?
Response to Matthew Sagui
Matthew,
Q. "Ultra-Aire says that their ventilating dehumidifiers put a house under slight positive pressure. Would that make the large kitchen fan operate effectively without additional backup air?"
A. Probably not. The answer depends on the airflow rate (cfm) of outdoor air that the Ultra-Aire unit introduces into your house. It's unlikely that the dehumidifier introduces the high rates of airflow (generally between 400 cfm and 1,200 cfm) required as makeup air for a powerful range hood.
Response to Ian Garradd
Ian,
Most of what you wrote sounds sensible. The only thing I didn't understand is "a motorized damper with no motor." I'm not sure what that is.
Woops just fixed that -
Woops just fixed that - motorized damper with no fan - so just a fan on the outlet and not the inlet
Response to Ian Garradd
Ian,
Yes, a passive air inlet equipped with a motorized damper is one of the options I listed in the bullet points at the end of the article. That approach will work.
Makeup air
After reading most of the above, I am thinking that a rangehood ducted to the outside under the eaves, and makeup air supply via a duct from outside air located near floor level could work.
Currently I am thinking that the makeup air could be distributed through an outlet just under the front of the benchtop surface, in front of the stovetop. It might be about 900 mm (36") wide and about 25 mm (1") high
(Luckily where we are, the temp of our makeup air would be at worst a min of 4 deg C (40F) or 35 deg C (95F) max.
Currently considering an EC fan with speed control on the exhaust, and a motorized damper with no fan on the intake - somehow interconnected, and perhaps a couple of pressure sensors to minimise external pressure differences. (Not sure who can arrange this yet but might try some industrial fan suppliers such as Fantech for direction)
General goals - During our renovation, we are aiming to seal the envelope to ensure greater energy efficiency, and sorting the key issues related to minimising the wastage of conditioned air, and the need to minimise / eliminate condensation problems.
In general, it would appear that it is best to eliminate dangers to health by getting rid of internal combustion appliances, and so we would be getting rid of our gas stove top and replacing with an induction unit (anyway that will save energy, give better heat control, and be easier to clean too)
Then the only issues would be to get rid of the moisture from the kettle, saucepans and fatty fumes from frying etc.
While I am a great supporter of the Passivehouse principles, I reckon it is really necessary to get rid of cooking fumes and damp air outside, and anyway for all the time that cooking is not happening- which is most of the time, the building would function very efficiently.
Any comments and ideas would be appreciated thanks
Here we are 10 years later, with even more mounting evidence of the dangers of cooking fumes, and few commercially available solutions. And manufacturers still don't publish hood capture efficiency.
My Zehnder rep says my system will automatically kick into more airflow when the dryer and central vac turn on. But that even with a 400 cfm hood I should have makeup air or open a window. But it's cold where I live.
Lstiburek says "With a 200 cfm kitchen range hood we interlock the range hood exhaust with a supply fan that provides make-up air at roughly a 10 percent greater flow rate than the hood exhaust rate. This make-up air is typically provided in a separate zone like a basement that provides ample tempering of the make-up air by mixing. It then is introduced via floor registers adjacent the cook surface."
Since my crawl is fully conditioned, I think he's saying if I all the make-up air to come up from the crawl it will naturally be warmer than the outside air and may not have to be conditioned. So am I right to assume I should have a motorized damper to allow the extra air in, and that it should be at the other end of the house? And why does it need a fan; why can't it be passive?
Qofmiwok,
Q. "Why does it need a fan; why can't it be passive?"
A. The main advantage of a supply fan is that the airflow provided by the fan can be measured and adjusted during commissioning. That way you know how many cfm of outdoor air you are introducing. If you provide a passive hole in your envelope controlled by a motorized damper, the air flow will be greatly affected by outdoor wind speed -- meaning that the airflow will be variable.
Thanks. That makes sense.
But, I would imagine that often times I would use a hood at lower speeds, and only occasionally depending on what I'm cooking would I crank it up. I will have a Zhender 600 which they tell me can detect when the system goes out of balance and easily supply about 200 cfm makeup air to the hood. So I would only have an issue above that.
So I don't see how the "locking the makeup air into the hood" approach works for this, nor how you would commission it for various different airflows.
Maybe:
1) Use a pressure switch to determine when to open the makeup air. Then it would only open when it's above what the Zehnder can supply, such as on high speed.
or
2) Open the vent (or turn on the makeup air fan) manually with a switch when running the hood on high.
Either way, commission it with the hood on high speed?
Note:
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