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Community and Q&A

How Much Make Up Air ?

tundracycle | Posted in Mechanicals on

We are building a house in Minnesota that we expect to have an ACH50 of just under 1.

Exhausting appliances include:

Range Hood – 200-1400 CFM variable
Wood Burning Fireplace (yeah, I know) – 400-800 CFM
Clothes Dryers – 200-400? each (there are 3 but likely only 2 ever running at a time)
Vacuum – 240
Bath exhausts: 1 200, 5 100 (all are fantech inline so good fan curves).

Windows are all Loewen triple glaze.

Worst case: It will not be unusual to have the range hood, fireplace, vacuum, a clothes dryer and a couple of bath fans all going at once on a -10°f Christmas eve just prior to 30 people arriving for dinner.

HVAC contractor says two MUA systems; one 1200 CFM for range hood, one 600 CFM for fireplace. He thinks everything else will be able to pull whatever is necessary through leakage. True even with ACH50 of 1?

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  1. Expert Member
    Michael Maines | | #1

    W, do you mean an open fireplace, or a fireplace insert? If an insert, I think your CFM numbers are high by a factor of 10 or 20. If it is a traditional fireplace, I doubt you'll get down to 1.0 ACH50, and you will need to be extra careful about depressurizing the house. Backpuffing from a sealed woodstove or insert is an annoyance; backdrafting down a fireplace is a serious fire hazard.

    Clothes dryers are usually around 125 CFM but commercial machines are probably higher.

    Have you used a 1400 CFM range hood before? I think you'll find that high speed is for when something is burning; otherwise you'll run it on low or medium speed.

    How big will your house be?

  2. tundracycle | | #2

    This is a traditional wood burning open fireplace but an Isokern rather than masonry. According to Isokern, their flue dampers, when closed, will support an ACH50 of below 1.

    We currently have a 1200 CFM hood. Yes, probably 90% of its use is 300 or 600. 1200 isn't just for burning but anytime you are pan frying, sautéing or stir frying (wok) and prefer to eliminate as much odor and grease coating every surface in the kitchen as possible. For us that is 2 or 3 times per week. There are also large dinners when we'll have 4 burners plus 24" griddle going at once so over 100k BTU's of heat being generated along with all the steam, grease and odors.

    We are planning a larger hood with more open volume that I hope will do a better job of capturing stuff so that the blower doesn't have to run full bore as often.

    Total finished; LL=2101, 1st=2944, 2nd=2149, Attic=1220. Total interior area is 9200.

    And yes, you raise a great point about backdrafting the fireplace. This especially since it will be necessary to leave the flues at least partially open for several hours after a fire and there will be many times that we'll forget to close it so it may remain open for several days.


  3. Expert Member
    Michael Maines | | #3

    W, this is not really my area of expertise, but for some back-of-the-envelope math:

    Assuming most of your spaces have 8' ceilings and the areas you list are all within the air barrier, you have a little over 70,000 ft³ of interior air.

    70,000 ft³ at 1.0 ACH50 converts to around 50-100 ft³ natural air changes per hour. That's roughly what you can expect for makeup air through gaps and cracks without depressurizing the house. In your situation, it's low enough to be ignored. At 50 pascals pressure you have 70,000 ft³/hr of air flow, or 1166 cfm.

    Your worst-case typical exhaust situation is about 3,000 cfm. If your maximum makeup air is 1800 cfm, you're still losing 1200 cfm during your worst-case situations.

    You're basically creating ACH50-level negative pressure, even with both makeup air fans running. With the relatively large volume of your house it may not be a problem if this occurs for just a few minutes, but it wouldn't take long for it to be a problem. You're losing 1.7% of your total air volume every minute. I would want a more robust makeup air system for a situation like yours, especially with an open fireplace--that's where the extra air will come from, at least when the damper is open.

  4. Jon_R | | #4

    Maybe a couple of these (apparently it automatically controls pressure). Add a furnace to each to handle the additional heat load.

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