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Heating Make-up Air

Bill L | Posted in Mechanicals on

Hello,
First the details that I hope will answer questions you might have before considering this:
Zone 5a, R20/40/60 dense-pack cellulose, triple-pane and super tight. 3600sf interior floor space includes an in-law with its own laundry room. ERV pulling on bathrooms, laundry rooms and back corner closets. Air-source minisplits, sealed combustion water heater, direct vent gas fireplace and serious central vac blowing to outside. Very open floor plan with a ‘command central’ kitchen that will get a lot of use, and is adjacent to a wide opening to second floor (yes, I know, but can’t change this). Electric oven and a gas cooktop around 52,000 BTU.

I would have gone with a carbon filter for the kitchen hood (and added an ERV return near the kitchen), but the range has to be gas, so I’m looking at the largest reasonable hood area with the smallest fan to suitably exhaust the cooktop (somewhere below 400 cfm). No appetite to push into the bleeding-edge of ductless dryers, and although line drying will be done, it won’t always be done.

Even though none of the exhausting appliances by themselves push me into a code requirement for make-up air, it is conceivable that a total draw of around 900 cfm could occur if the kitchen hood, two (high-capacity) dryers and central vac are all running at the same time. I’m convinced that during such a perfect storm, somewhere around 600 cfm make-up air could be needed as the house is expected to be very tight.

I was originally planning to passively deliver all make-up air to the back of the refrigerator through a power-open damper, as I don’t want that much air being delivered straight to the cooktop, but I’m doubtful that either of those solutions would satisfy code requirements for heating make-up air.

Because the laundry rooms can have their doors closed to create a self-contained system, I have been considering a dedicated, relay-driven, power-open make-up duct delivering air in close proximity to the dryer intake for each of them. If I take care of the dryers that could be running at the same time as the kitchen hood, perhaps I can just leave leakage to handle the kitchen hood’s makeup air. Again, however, I’m doubtful such an approach would satisfy code requirements for heating make-up air.

If I bring _any_ makeup air into the building in Zone 5A, am I running a risk that the building inspector may require me to heat it?

I imagine there may be some alternative ways of looking at this, and will, of course, appreciate all of them 🙂

thanks!
Bill

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Bill,
    Have you seen this article: Makeup Air for Range Hoods?

    As far as I know, the building code only requires makeup air for range hoods that exceed 400 cfm. And as far as I know, it's up to you to decide whether or not you want to condition the makeup air. Unconditioned makeup air may be uncomfortable, but I think it still meets code requirements.

  2. Bill L | | #2

    Thank you Martin,
    Yes, that article has been an important resource for me, thanks for writing it!

    I've read a lot of 'gossip' about needing to heat make-up air, but recently found these exceptions in excerpts that claim to be from the IMC (I don't have an official copy of that code to verify the authenticity of this):
    IMC 403.1 & 508.1.1 / COMM 64.0309(1),
    The temperature differential between makeup air & the air in the conditioned space shall not exceed 10 deg. F
    Exceptions:
    Makeup air that is part of the air conditioning system
    Makeup air that does not decrease the comfort conditions of the occupied space

    I will check with my HVAC guy, and realize that our local (stretch) codes will govern, but I'm thinking it could turn out be a somewhat subjective call by the building inspector.
    thanks,
    Bill

  3. Pat Rowland | | #3

    Hi Bill

    same for us... (or very similar boat, sans the 2 floor vaulted and no central vac)... tight, good insulation, etc.

    We are pretty sure we wont do an external hood-fan. With 36cfm sucking (Zehnder ERV) the "dirty" air in the kitchen, a carbon filter hood recirculating hood-fan should get the grease out of the air and all the bad stuff off the stove before sending it up to the Zehnder.. which has merv 13 out of the box (and 15 and charcoal available we need even "cleaner" air).

    We have a 36" 6 burner crazy btu Viking stove (elec wall oven by LG), rarely are 4 burners lit and I don't ever remember all 6 lit - even at Christmas or Thanksgiving (we had the same stove set up in our old non pretty good design house and really only needed the giant hoodfan for frying chicken, meatballs, or other splatter type cooking; it is not necessary for boiling macaroni and making spaghetti sauce). so, read the requirements for cfm carefully and with a grain of salt. If you insist on external vent, a 400cfm should suffice (and I understand cooking a lot - we are both CIA trained chefs... and still think a filter recirc system with the ERV air will provide clean and comfortable air in the kitchen and rest of the house.

    buuuuut, we have a wood stove too and that causes me more consternation. we think that we'll wind up with a make up air solution for that... right now, we are thinking of a separate port with a small motor ducted into the back of a myson toe kick heater mounted flush in the hallway near the mudroom. This will allow us to heat the incoming air a little bit if needed (three settings, no heat, one unit on and both units on). The unit is probably not enough watts to heat 600cfm make up much, but it is better than no heat. on high, it uses 1600 watts (think hair dryer) at 13 amps. The unit is designed to take air from the room and heat it and spit it back out... if we go this route, we are configuring it so that the supply air comes from outside.

    also, just having a port to the outside wont provide enough make up air - it has to be mechanically activated.. heated or not.

    lastly, if infiltration is sufficient to handle the hoodfan make up the 2-3 hours a week that you are cooking copious amounts of food, then INfiltration will be providing cold air, the other 160 hours a week you are not cooking; another roll of air tape or SA flashing and even a can of foam might be a good investment, no?

  4. Bill L | | #4

    Hi Pat,
    It sounds like you have plans for a gas stove. Am I wrong in thinking that a recirculating hood can only be used for an electric stove?

    Nice idea to temper with a low watt toe kick heater. Would you have to operate it manually?

    I've tried to stop infiltration everywhere I can, and expect it to be extremely tight. I'm sure there will be some leaks regardless, and am hoping that they won't be in spots where water could get drawn in if I don't provide adequate makeup air.

    thanks!
    Bill

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