Range Hood and Make up air

9 years ago

Hi. I will be getting a 36" gas range ( no griddle or grill ) all 6 burners. I added up all 6 burners and BTU's are 70,000. I checked to see what some 30" range BTU's and they are around 60,000. I have a 48" wood hood and my cabinet salesman suggested the Zephyr insert with 1000 CFM. I never had a ducted vent that will be added during renovations.
But now I'm freaking out after I read about make up air, I never heard of this before. I live in NY not sure of any codes here? But I'm not just worried about codes I'm also worried about this in general. I can't afford to put in system for make up air, what are my options? lower CFM? 400? 600?
It doesn't seem that my range has that much more BTU's then the average 30" range. I know I would never be using all 6 burners at the same time on max. My house was built in the 80's, the kitchen has vaulted ceilings, window is close to range ( not sure if any of this matters )! What's the lowest CFM I can use? I don't fry a lot other then your basic chicken chicken cutlets. I mean before I had a recirculating hood that never really worked so when I did fry I just cracked my window open. Should I just have stayed with recirculating? Never had a problem before with grease on cabinets or anything!
Sorry for the long post I'm just freaking because my reno starts on Monday and just read about this.
Thanks

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Comments (6)

snoonyb
9 years ago
Generally, 1000CFM, shouldn't trigger that requirement.

There are some areas, which by a function of code, require make-up air for kitchen venting appliances.

However, in general, that determination is based upon the type of construction (how tight) as well as the openness of the interior spaces and the type and location of your comfort heating appliance.

There are also both passive and active make-up air systems.

Your venting appliance may also have speed (volume)
control.

Budgeting for contingency, the recommended venting of the cooking appliance, by that mfg., should be followed, not, well maybe I'll only use 1 burner today.
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sjhockeyfan325
9 years ago
I've read on the appliance forum that the requirement for makeup air can be triggered by a hood as low as 400cfm. You have to check your local code.
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See Martin Holladay's article. If you don't get makeup air from a dedicated intake, it will be drawn through all the air leaks in your home and possibly even backdrafting CO from the furnace and water heater if these are not closed units. Though I know you're looking for a cheap answer, I'd suggest finding a more educated HVAC designer with commercial experience to be safe. However, if you don't run the exhaust all out all the time there may be no need for makeup air. We had a 700 cfm unit that was only run full throttle when my spouse cooked fish and offal. I just cracked open a patio door and all was OK.
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In order to begin to understand it, you would need to buy ANSI Z21.1 for about $650. There is some very technical engineering, even though it doesn't look like it, in the ventilation between walls of the oven and cooktop sides that ensures that the cabinets up against the range will not get hotter than 194°F. If you block the airflow by lowering the range (some people try modifying the feet for thin countertops that make them less than 36" tall), or putting ducts in for MUA, the outside wall of the range will get hotter. If you force that much extra air through the range, (push or pull) you change the way the gas burns, and create CO problems. Even if you can adjust the air shutters to compensate, you would then have to turn on the entire system just to make a pot of tea, in order to get enough air to get a clean burn. With holes in the floor or back wall, the natural draft intended/engineered to cool the outer walls of the range can be disrupted or diverted, leaving the heat to build up around the range - nothing to do with heat in other areas of the home. What can't tolerate more heat than the wood cabinets around the range? Another separate reason for no holes is the possibility of a gas leak. How much gas do you think that duct will hold, just waiting for someone to create a spark? The testing standards do not say no holes "unless someone thinks of some unusual excuse".
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There are myriad posts about it here. Search on "kaseki" and "MUA" to find some in the Appliance forum. (The Kitchen forum is for marble vs. granite vs. soapstone, inter alia.) Here is a link to and image from a 2012 thread. There are earlier ones. http://ths.gardenweb.com/forums/load/appl/msg0711051415510.html For a better understanding of what is going on with kitchen ventilation, I also recommend reading the first dozen of so pages of the Greenheck Guide: http://www.greenheck.com/media/pdf/otherinfo/KVSApplDesign_catalog.pdf We can start at some basic principles. One is that no air goes up the vent hood without getting into the house to begin with. Hence there is always make-up air, but it might be insufficient to meet all requirements. Another principle is that if moving air up the vent hood reduces the pressure in the house relative to outdoors, several undesirable things happen: - You may back\-draft combustion appliances (hot water heaters, fireplaces, etc.) and cause a safety hazard from carbon monoxide - You may pull outside air moisture into the walls - You may unbalance your heating system, if moving air based - You certainly will reduce the flow rate you are getting from the hood blower. I would prefer not to address your MUA options in the context of your architecture and hood until you have become familiarized with the issues and selected a hood and blower that meet your ventilation requirements. As to "why?" there wasn't information put in front of you, it may be that the people who should be advising you are ignorant of basic HVAC issues, quasi-informed but too lazy to delve more deeply to fully learn their trade, or not in the business of selling MUA system components.
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Wow! Some basics are needed here. First, no air goes up the hood, through the duct, and to the outside that didn't get into the house. Seal the house and the hood flows no air. Second, the hood will try to move air and if it can't the house pressure will fall to the zero CFM value at the left edge of the hood blower's fan curve, perhaps a few tenths of an inch of water column. This pressure, and likely the pressures corresponding to a good portion of the fan curve (less the losses from hood filter to outside), can cause back-drafting of combustion appliances. Back-drafting is a carbon monoxide hazard. Some combustion appliances can back-draft at a mere 0.03 inches of water column. Fireplaces are just a bit higher than that number, depending on draft achieved. Third, while low CFM levels may be supplied by the house leakage as make-up air, this is not usually good for the interiors of house walls, and can pull dust into the interior. Fourth, there are many ways to supply make-up air (MUA) and generally some heating of the air is needed (possibly required by code) in northern climes. Fifth, the comments about wall vs. island hoods are valid, but the result is somewhat confusing. Without a back wall, the island hood needs to be larger front-to-back (deeper) to accommodate the rising expanding cooking plumes it is to capture from the rear burners. This larger area still requires the needed flow velocity (90 ft/min suggested), so island hoods will in general require more CFM than the same performance hood mounted to a wall. (Feet per minute equals CFM per square foot of hood entry aperture area.) If drafts are accounted for, including those caused by moving people, the island hood may need to be wider as well as deeper. In some cases, cold MUA can be brought into a room having an oversized heater (think Modine type) and preheated that way. In others, an electric or hydronic heat exchanger may be needed somewhere in the MUA ducting. Low CFM requirements might be addressed by an existing hot air furnace, but usually these are sized for normal house heat loss. In all cases, even in houses with separate MUA for combustion appliances, the MUA configuration should be imagined as having ducting commensurate with that needed for the hood system. Last, all of the above is related to having a hood that removes most of the cooking effluent, leaving the walls clean and the air relatively odor free. As far as I know, there is no Code requirement to keep one's house clean and thus the options of no hood, or of a barely filtered recirculating hood, or a barely useful low CFM exhausting hood are available choices.
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missymoo12
9 years ago
My Pa 2009 codes required make up air for anything over 400 CFM.
I am getting an American Range 36 inch and the manual doesn't give an exact requirement but I have installed a Zephyr monsoon DCBL it is 715 CFM
I bought the Broan Make up air unit from amazon because I needed an 8"diameter vent duct for that Zephyr unit. The HVHC guys have it installed and everything works well. The MUA duct opens when an air finger trigger is activated in the range hood duct the make up air unit is connected via 12 volt wire. The duct pulls outside air into my air return vent prior to it going into the geothermal unit to be heated.
Zephyr now makes their own MUA unit identical to the Broan.
The inspector was of no help in choosing anything.
The HVAC guys said just get a 300 CFM range hood. Knew about makeup air but had never installed.
The Appliance saleperson gave me NO guidance.
I got more info here on GW in the appliance forum.
I basically had to wing it myself and tell the HVAC guys what to do.
Wont know about inspections until Dec or so when we get our final.
This is a major gray area in my locale. I am hoping I do not have to get an air heat exchanger to warm the air prior to it loading into the main air return. This is required with some HVAC units but may not be an issue with my geothermal. And that I am not under the newer 2012 codes.
I love the Zephry. BTW - very quiet.
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juliekcmo
9 years ago
I will say that if your hood vent is straight with no elbows then that is probably more cfm than you may need. If there are longer duct runs, or elbows, that does compromise the velocity of the air in the ducting.

My previous house had a Thermador all gas range with 90,000 total BTUH output possible on the cooktop. I had straight up and very short duct run, and a good quality 600 cfm hood was very adequate.
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kksmama
9 years ago
Good for you, getting a proper vent hood! You may enjoy the linked thread and NY Times article which inspired it.

I chose to evade the expense and logistics of mua because my rangetop is the only gas appliance in my home (so no worry about backdrafting) and I rarely use high speed. When I do use high speed, I open a window.

I calculated that if my hood really did exhaust 1200 cfm, it would take 20 to 30 minutes to replace all the air in my house. Hard to imagine that being necessary, but I do enjoy cranking it up on cool mornings and opening furthest window to air out the house.

Here is a link that might be useful: spirited venting discussion
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rwiegand
9 years ago
MA has no MUA requirement for a 1200 cfm residential hood. Our house is very tight so I need to open a window for the hood to work well on medium to high. The few minutes a week of having an open window seems a simple and sensible solution. (all of our gas appliances other than the range have sealed combustion chambers and their own MUA supplies, so backdrafting is not a concern.) There is no downside to having a big blower, as long as it has a speed control. Low for routine use and typhoon for charring the chili peppers in the wok.
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Range Hood and Make up air

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