More Q&A Spotlight
A range hood that whisks away cooking odors, moisture, and grease is almost always recommended as a way of keeping indoor air healthy. But what happens to the equation when the range hood is a behemoth, capable of sucking up 900 cubic feet of air per minute?
That’s the dilemma that Sean McLoughlin is facing as he designs a 3,500-sq. ft. house in southern California. The kitchen will be outfitted with a professional-size range and barbecue grill.
“We’ve had one before — we used it 4 or 5 nights a week — and they produce huge amounts of heat and smoke,” McLoughlin writes in a Q&A post at GreenBuildingAdvisor.
California requires that houses with high-power exhaust fans include equipment to supply makeup air, to ensure that the exhaust fan doesn’t depressurize the house, and McLoughlin is trying to sort out the details of how to accomplish that.
“The HVAC system we’re planning will include an outside air exchanger (not an energy-recovery ventilator or heat-recovery ventilator, just a controlled damper) ducted into the return before the filter stage on each of the two forced air units/systems in the house,” McLouglin says.
“Is it possible that those two sources of outside air can be tied in to the exhaust hood to provide sufficient makeup air for the kitchen exhaust hood? Or should we be planning to add non-centrally tied in make up air ducts direct from outside using automatic dampers?”
McLoughlin’s search for a practical solution to this problem is the starting point for this week’s Q&A Spotlight.
How about a change in plans?
Anticipating an obvious reaction, McLoughlin says he’d rather not hear suggestions to install a smaller kitchen range or simply…
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16 Comments
Range Hood on Steroids
The subject of your article comes up now and then at an HVAC discussion site I'm also active in. What is consistent there and what I read above is when it is suggested to the homeowner to consider more reasonably scaled equipment for a residence, the response is flaty "no!". Rather than scale back expectations, up the energy draw to get what one wants, is what I see in translation. Sure, we can come up with a way to make it work so the hood doesn't pull a 500 micron vacuum on the house, but we're also talking about a range hood that itself is an energy pig, and must draw in make-up air from outdoors to avoid the house going negative. If the hood does not introduce make-up air right at its perimeter, like many commercial hoods do, the make-up air WILL come from the rest of the house. And I'm not confident just having a dampered opening to the great outdoors feeding that perimeter make-up air feed would take care of everything.
Typical commercial installations have forced make-up air fed in at slightly less CFM than what the hood draws out. Who is positioned to properly engineer and commission such a thing for a residential setting? All of it could be avoided or minimized by merely scaling back one's appetite for shiny, energy grabbing bling...but that's asking a lot from some folks, I suppose.
Will there ever come a time when "passive house" and other practical "green" strategies (not gimmicks) become the desired "shiny bling" vs. grossly oversized rangetops and range hoods? Priorities are misplaced here, IMO. And will continue in that vein until it hurts to spend that much energy for not much real return.
Makeup Air Information
I am constantly amazed by the lack of information about makeup air on building and green building forums. It is a very difficult subject and high power residential range manufacturers would rather gloss over this issue so they can sell more ranges without scaring their customers.
The fact is makeup air has been dealt with in commercial kitchens for years. Homeowners with Near-Commercial stoves in residences could benefit from the knowledge of makeup air in commercial kitchens. But this knowledge ain't pretty or cheap!
See http://www.energy.ca.gov/reports/2003-06-13_500-03-034F.PDF
Do what you gotta do, but...
I would never try to tell anyone how to build their own kitchen, but I would definitely go for the semi-attached summer kitchen on my house. Wouldn't it be nice to be able to open it up and get fresh air in the warmer months (and even in most of the cooler months too)?
Professional cooking AND traditional fireplaces . . .
Thanks for addressing this very real problem. I was recently working on a newly built home with an impressive professional style kitchen - 6-burner gas range, etc. - and FIVE very large masonry fireplaces.
The Architect (who is otherwise a creative genius) has a personal prejudice against direct vent fireplaces ("fire in an aquarium"), ergo the selection of mansonry fireplaces, but my company is now tasked with converting the fireplaces to gas logs for the convenience of the homeowner.
Long story short, with a little testing it was determined that it would take about 2300 c.f.m. to positively pressurize the house adequately for the fireplaces to draw properly, and that was WITHOUT the range hood turned on. How can we, in good conscience, install gas appliances (logs) that we are pretty sure will not vent properly, however legal it may be to do so?
How do we get the word out that this is serious business, especially when we consider the potential for carbon monoxide spillage?
P.S. In a tangential issue, the health dept.in one of our jurisdictions requires that the make-up air fan in a commercial kitchen provides MORE c.f.m. of air than the exhaust fan so as not to draw insects into the buildings. How often do you think the HVAC guy compensates for that?
make up air
I have installed several professional hoods that have great make up air systems incorporated within the hood. If Mr. McLoughlin is specifying a hood for the project, it should be of this type only. Much more expensive then the home style look alike, but a hood that will, in my opinion only, work with the least amount of interior air loss. Go to a local restaurant supplier and he will help you spec the correct hood/ air recovery system. Positive pressure sounds good too, not much though.
My house is so tight...
1.
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. I didn't install the kitchen vent to deal with a grotesque gas range and its unburned hydrocarbons, I installed the vent to deal only with the added humidity that cooking might introduce from the magnetic vibrations of my induction cooktop. Right now the kitchen exhaust doesn't really get used because I'm still designing the custom hood (try finding a hood that doesn't come with a motor). After reading this article and the subsequent comments, I turned the fan on and opened the casement window a smidgeon and a blast of air came in so hard the window wanted 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? Is my self-balancing ERV dealing with it? Fortunately the 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? Is it just me that finds it ironic that we build air-tight houses with ERV's only to discover that we still need to open windows after all?
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.
Response to Martin
Thanks for the response. Though in this case I shouldn't be replacing the brand-new fan if I'm not getting the cfm, I should be sawzal-ling a hole in my wall for make-up air, right? I think there is little doubt that I'll need to open the window when I use the 450cfm kitchen exhaust. I'll do the test though and let you know!
Many thanks
Dealing with a High-Capacity Range Hood
Try this twist on the Sean McLoughlin’s High-Capacity Range Hood.
It seem this client knows what he wants, has deep pockets and is not too concerned with his eco-footprint.
Provide an air intake and fan to equal the 900 exhaust fan with a similar fan to drive it.
Lead the air to a pipe that follows the perimeter of the cooking surfaces – that will probably also be the outer edge of the range hood. There should be holes on the top of the pipe every couple inches. The size of the holes will require some tweeking, but they should not create much back-pressure in the pipe. That back-pressure will determine the amount of room air that will be expelled.
One switch turns on both fans.
This will create an “Air-Curtain” that will have a similar effect as a heavy plastic sheet.
I have built two tunnel kilns for pottery using these “air doors” that had a very small air flow. The temperature differential was about 400 degrees F on four-inch travel through, what looked like open space.
Gordon Barnes MFA Alfred U – Ceramic Design
Gordon,
That is very
Gordon,
That is very interesting. I have read several instances where people build air curtain exhaust hoods that worked well. Thank you for posting this!
This study claims the air curtain makeup air doesn't work well:
http://www.energy.ca.gov/reports/2003-06-13_500-03-034F.PDF
But I like the design and would be inclined to try it, especially because it worked so well for you. What type of overhang do you have around the hood perimeter? For example, if this type of hood is built for a cooktop, how much wider and deeper would the hood including makeup air delivery need to be than the cooktop?
HRV/ERV + Dampers = MUA Heaven?
As someone planning a commercial-size kitchen in a residential home, I do take exception to the assumption that this MUST be excess. Granted, for most people it is, but two things to consider:
1) those with trophy kitchens who don't cook aren't creating an environmental impact
2) those who do frequently use them to full capacity by cooking for large groups (like us) are simply "range pooling": we use a lot of energy in our kitchen, but 20-40 people AREN'T doing so in their kitchens or in restaurants that meal.
Now the real question: I've heard people on this forum state that you never tie range hoods into HRV/ERVs. Why is that? Seems like the perfect solution: a range hood needs an exhaust fan and ideally, an intake (MUA) fan. Rather than purchasing 1) a blower for exhaust, 2) a blower for MUA, 3) a conditioning solution (and dampers and such), and 4) a separate HRV/ERV for your airtight home, why not tie the range hood to the HRV/ERV?
The HRV/ERV has both exhaust and intake fans as well as conditioned air: seems like a match made in heaven. Add dampers that switch between range hood duty and whole house duty, and this seems to me to be the ideal solution. This assumes, of course, complete grease removal from the range hood exhaust to reduce fire risk.
Am I missing something here? Please educate me! Thanks in advance for your expertise.
David
Response to David Ahn
David,
Q. "I've heard people on this forum state that you never tie range hoods into HRV/ERVs. Why is that?"
A. In a word: grease.
Every HRV and ERV manufacturer includes a warning that a range hood exhaust fan should never be hooked up to an HRV or ERV. The cooking grease will quickly gum up the HRV core and ruin the appliance.
Range hood manufacturers have experimented with heat-recovery equipment, but they all gum up fast. You really want to get the hot grease out of your house before it cools and gets gummy. A heat exchanger cools the grease, so it coagulates and coats the ducts. What a mess.
This phenomenon is similar to what happens when creosote builds up on a chimney flue hooked up to a wood-burning appliance. The cooler the smoke, the greater the rate of creosote accumulation.
Bummer...
Thank you, Martin! I knew grease was an issue, but I was hoping there was a reliable method of removing ALL grease prior to passing through the HRV.
What about recirculating hoods? How effective do you think they are compared to exhausting? It would obviate the need for MUA. I do think it would be a nightmare in the summer with heat rapidly building up in the kitchen area.
David
Response to David Ahn
David,
You might want to read my article on the topic: Makeup Air for Range Hoods.
The article discusses the approach taken by some Passivhaus builders: installing a recirculating range hood and an exhaust grille in the kitchen ceiling, as far as possible from the range. This exhaust grille is connected to the home's HRV. One major problem with this approach: it violates the building code. However, local code officials may approve such a system if you can provide a good rationale for why it makes sense.
However, I don't think the Passivhaus approach will work for what you want to build -- "a commercial-size kitchen."
I spent years as a commercial mechanical contractor. Every time we did a commercial kitchen we had to install dedicated makeup air systems for the kitchen. These were large expensive units providing 100% outside air, and with heat for winter operation. I can not figure how commercial size cooking appliances and exhaust hoods are installed in homes without makeup air systems. Turning on an 900 to 1500 CFM exhaust without an interlocked makeup air system is a code violation in commercial applications, and it should be in residential too.
Of course the easy solution is to buy residential cooking appliances and range hoods designed for homes, and stop pretending you must have appliances designed to cook dinner for 200 people. An electric range and a 300 CFM hood is the most that people really need in a home, and that would take care of most issues right off the bat.
Dennis,
This article is from 2011. Residential building codes do now require make-up air for large kitchen fans.
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