Make up air for 900 CFM range exhaust hood
We’re designing a 3500 sq. ft. new home in Southern California, including a kitchen with a professional range that includes a BBQ grill unit. We’ve had one before, we used it 4-5 nights a week so it is essential for our kitchen, but they do produce a fair amount of heat and smoke when the grill is on high. So a professional exhaust hood is necessary, and the one we’re installing will draft 900 CFM on its highest speed. With new construction standards (Title 24 in California), that amount of exhaust necessitates make up air.
I’ve read Martin’s enormously helpful article on the subject, and it looks like wiring the exhaust hood blowers to two Broan 6″ or 8″ automatic dampers that will connect to ductwork that will allow passive makeup air flow out of the kitchen cabinet toekick area may do the trick.
Is there an optimal distance from the hood (which will be located at 6′ above the floor), assuming the passive MUA is coming from floor level? Any suggestions (other than “don’t put in a range that big” or “open a window”) or recommendations to SoCal mechanical engineers who have successfully designed systems that deal with this issue?
Thanks in advance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Sean,
1. If you haven't read it yet, I suggest you read Makeup Air for Range Hoods.
2. What you describe as "an outside air exchanger" is more properly described as "passive outdoor air inlets" or perhaps a "central-fan-integrated supply ventilation system."
3. It's impossible to answer your question without (a) knowing the size of your passive outdoor air inlets, and (b) knowing the details of Title 24 make-up air requirements, which I don't. I don't know whether the California code permits passive make-up air sources, or whether it requires an active make-up air fan.
Sean,
As you guessed, the ideal solution would be not having this thing inside the house. If it must be there and the windows are going to be shut, the best solution might be a dedicated makeup air fan with a supply register in the floor ahead of the range. Wire this to power up with the exhaust hood, and create a short circuit for airflow to focus on the range. That will minimize impact on the rest of the house.
If it were me, I might consider adjusting the architectural program to put this virtual commercial kitchen in a separate building outside the dwelling. A breezeway connection to the house might give you the result you want.
In general, "huge amounts of heat and smoke" aren't very compatible with indoor air quality, especially in an energy-efficient house.
That's why in centuries past, especially in rural homes of the well-to-do, cooking was often done in a separate outbuilding behind the house.
Martin,
Thanks. I raised the question with the architect after reading your article on make up air for range hoods. I can't figure out if an active make up air fan can be part of a central integrated supply fan, or if the central fan drawing air from interior returns and a passive outside air duct with an automatic damper accomplishes the same thing. For now, I'm just trying to put enough information in the spec sheet that the builders who bid the contract can get their HVAC/mechanical subs to give them accurate, useful bids that include addressing the problem.
Thanks.
Sean: Is there any aesthetically pleasing way to be able to enclose the kitchen temporarily, like w/ two sets of large, double doors, an accordion wall, or something similar? That would give you the "outbuilding", sort of. Just a thought. Call me when the burgers are on.
The 2009 Code requires this make-up air, MUA, and for a 1200 cfm hood, should never be tied to the HVAC system, ever. You can possibly double the size of your HVAC system with that amount of MUA. 1200 cfm requires about a 12”-14” pipe coming from the outside; that is a huge amount of uncontrolled and unfiltered air coming into your house.
As far as I know, Broan is the only manufacturer that makes an automatic MUA damper… It synchronizes ventilation with exhaustion when you turn the hood on. It includes a damper, a control module and a transformer. You may need 2-8” dampers, but I would make sure the system is designed by a professional. You can install filters in the line and its best if you installed the air supplies about 10’-12’ from the hood.
Unfortunately, retailers are not informing their clients about this new code, and it can be a huge issue in home performance and health.
Sean,
While our new house doesn't include that grill option on the range, we have a 720 CFM Zephyr 36" range hood that we have to provide make-up air for due to extremely tight construction of our house (.5 ACH at 50 Pa in our initial blower door test last week)..
My discussions with appliance retailers yielded a lot of blank stares and questions about what make-up air was. We did research one self-contained MUA unit that was mentioned in an earlier GBA blog but found it to be cost-prohibitive for our needs (something over $1000 if I recall correctly). Our lower cost solution is to install a passive 6" duct connected to a Broan MD6T motorized damper assembly controlled by a Skuttle A50 current sensing relay (designed for humidifiers) that is sensing the common lead of the range hood. The relay is a 24v system and therefore powered by the transformer that powers the damper. The MD6T is the 6" version while the MD8T is an 8" version Armando mentions. I bought mine through the special order process at Lowes after a very helpful employee called Broan to make sure they could get it (and as a Broan dealer it is an easy process).
Broan advertises that the damper is only for their specially equipped hoods but it is simply a 24v damper that has rubber sealing rings on the damper assembly. The 'tricky' part is that the motor needs to be activated only when the fan is operating hence the current sensing relay. In our case, the lights did not draw enough current to trigger the relay but the blower motor, even on low speed, triggers the relay and the damper opens. One note is that this particular damper is either open or closed but since it's a passive unit the theory is only as much air as is required will be drawn through it. We do have an ERV planned for other air quality and time will tell if there's any impact on one by the other.
I've attached my crude diagram of the connections. All told the components cost less than $200 not including the duct work and external vent. One thing we have done is route the fresh air supply under the refrigerator to pre-condition the air, increase the efficiency of the refrigerator, and as a side benefit hopefully keep pet hair and dirt from building up quickly on the refrigerator coils. That's the theory at least - we have not put this system into operation yet but the ducting is connected and we're sheet rocking the house early next week.
We didn't design in a filter like Armando suggested but that option is still available since the ducting is easily accessible in the unfinished area of the basement. Our supply point is about 7' off the ground while the hood exhausts out the roof so we won't draw the dirty air back into the house. We're also over 10' from the ERV exhaust so no short-circuit there either.
If you're interested in reading more, the blog for our build is htto://pittsboropassivhaus.blogspot.com.
John Clarke,
Thanks for a very helpful and detail-filled answer. My only concern would be that the 6-inch passive duct may be undersized.
This reminds me of the conversation regarding make up air for a clothes dryer, I would think John's setup would work for that also.
John,
Thank you for a very helpful and detailed response. I think two Broan 8-inch passive ducts with automated dampers will probably work for us if we design it right. I'm delighted to hear that it's not super complicated to figure out how to make a non-Best/Broan hood fan trigger the automatic dampers.
Martin, We sized the passive duct based on the size of the range hood duct - 6" in both cases. The other thing I have to consider is that 715 CFM is at full speed. This hood actually has 5 speeds and the only reason we sized the hood to 715 CFM is based on the range manufacturer's recommendations which are based on having all the burners running, the oven on, and the griddle (it's electric) operating at full capacity. While I do occasionally have several burners on at one time, they're typically not all functioning at max capacity.
While it's going to be hard to tell if the passive 6" duct is sufficient without instrumentation I suspect we'll be able to tell the impacts when we try to open any of our doors when the hood is on - any negative pressure will likely cause the outswing doors to be more difficult and the inswing doors to open easier. We may also have to do the smoke pencil test once the range hood is installed just to see if we detect any de de-pressuriation effects I'll try to remember to come back and post an update once that happens which may be within a month or so.
Oh but for a good low-cost solution to this problem. At present I suspect the less stringent certification programs may work against the supply/demand economics for that to happen.
Sean, you might want to have the HVAC engineers look at this solution just to be sure they're comfortable with it. My other caution would be that the amount of air you're supplying will definitely need to be included in your HVAC load calculations. Not knowing the actual volume of your house it's hard to tell the ACH calculations but our house (with vaulted 11' ceilings) has a total volume of around 52000 cubic feet. If I use my 715 CFM fan at max speed that's over .8 air changes per hour just from the make-up air. If I use 1200 CFM for the same size house that's about 1.4 air changes per hour. That can indeed cause HVAC system overload if not appropriately handled.
john, interesting approach. did you get PHIUS sign off for pre-certification? how are you dealing w/ additional moisture issues and heat loss/gain of the MUA?
Sean, you have touched on a subject that is eschewed, ignored, or overlooked by a lot of builders, architects, and code inspectors. As you are in Southern California, an outside kitchen is an option. But, if you are looking to build a real chef's kitchen for cooking (as opposed to those created for the covers of magazines), then I have found that you have to start looking to commercial solutions. The first comment that you get from a residential HVAC designer is that 350 cfm is more than adequate for a four burner stove. They are WRONG! Its fine for cooking ramen noodles, but not for cooking a real dinner that doesn't come from a box or can. For real cooking, you need real heat, great exhaust, and room to move your pots and pans on the piano.
Living in Eastern Washington, I don't have the option of cooking in an outdoor kitchen during many months of the year, unless I want to cook popsicles. So the only alternative I have here is to incorporate make-up air. You can get a decent 5' or 6' hood with makeup air for around $2500 from shops like hoodmart. Unfortunately, the makeup air built into the hood is not well located for efficient smoke removal, nor is it heated by default. Most professional places use a gas heater to pre-heat the make-up air before it comes in a building. That is just too inefficient for a house that is trying to save on energy. So, I came up with the following design for my kitchen. I placed the stove in an inglenook at the end of the kitchen so that I could partially contain any smoke and odors coming from the stove. The inglenook contains a 48" range, 2 ovens, and a charbroiler. Because I'm using a larger hood (6') I can also put a deep fryer to the side and have it adequately exhausted. I chose a variable speed hood, but a single speed make-up air fan. The makeup air comes through its own ducting from outside, and is passed through a hydronic heat exchanger that is connected to my radiant floor system. When the make-up air fan is turned on, an internal thermostat will kick on the circulating pump if the incoming air is below 55 degrees. Living in the Northwest, I run the pump on a separate line filled with glycol so it doesn't freeze during the winter. The makeup air is then sent through ducts contained in two soffits on each side of the kitchen and exhausted at the opposite end so that the kitchen air gets changed without affecting the rest of the house while the system is on.
A helpful guide for commercial kitchen ventilation can be found at http://www.foodservicetechnologycenter.com/ventilation/designguides/ You will just need to modify the ideas to fit your kitchen.
I hope that this helps.
Mike,
Sorry, haven't been to the site in a while since I've been busy at the build site. We did get pre-cert with the PHIUS. The make-up air is included in our geothermal/ERV calculations and they are sized appropriately to handle the extra load. As an interesting side note, when we were drying the air from the drywall mud/tape and painting we used a portable dehumidifier and that kept the interior temps at between 60 and 65 degrees when the outside temps were 30 degrees overnight.
John
Check out the Make Up Air Kit that CCB Innovations offers, Their Make Up Air System will lat longer than those pressure based systems
I realize this is an old post but to John Clarke, how did the make up air under the fridge work out? Im at the point in my build now and am debating directly under range vs fridge. My concern is having unconditioned air come across the kitchen from fridge vs just under the range.
There are a couple of studies that have looked at makeup air for commercial range hoods. Probably the best spot to start for figuring out what works.
https://www.energy.ca.gov/reports/2003-04-10_500-03-007F.PDF
Be careful with anything blowing cold air near ground level. A couple hundred CFM of winter air over feet is very unpleasant. A fridge would do next to nothing to tamper it. You need serious BTU to preheat the air, for example 400CFM of makeup air would need around 90kBTU for a 50F rise, pretty much a dedicated furnace.
The physically ideal place for the range hood is "Just high enough above the grill that it doesn't get spattered with grease".
If you want to do a lot of smoky, smelly, or spicy work, it may be worthwhile cordoning off a walk-in-closet-sized segment of kitchen with sealed walls lined with tile, with a doorway (but no door). Put the makeup air ducts next to the doorway at floor & ceiling level, put the crazy-powerful range hood at the opposite wall of this mini-kitchen, and you've created a mostly sealed system that doesn't impact indoor air quality much. Air gets sucked towards the stove and up & out of the house. Because the makeup air is passive, a tiny fraction of air is sucked through the doorway (and cracks in the building airsealing envelope) instead of the makeup vents, which keeps smells from permeating through the house. You've built yourself a walk-in fume hood.
For better air quality results, seal up the doorway better, first with a curtain, then with a door, then with weatherstripping etc, up until you're happy with the results.
I feel like there should be some type of simple thermal buffer solution that could work for makeup air, even if you didn't try to harvest exhaust heat (which could be an flammability issue with fine filtering of greasy smoke). Something conceptually like an ambient-temperature water heater connected to a heater exchanger inside the passive makeup air vent, and another heat exchanger in indoor air. Storage of ambient-temperature water doesn't cost anything, so it's 'free' until you want to turn on the hood (after which it will slowly rebuild ambient temperature). It can be a simple storage tank without insulation. The heat exchanger doesn't suffer from the same cost:usage ratio as buying a dedicated furnace or plumbing in a sufficiently complex air to air heat pump to function against such huge pressure differentials, for only a few hours a week (or a few hours a month) of usage.