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make up air for everything in a tight house?

BrunoF | Posted in General Questions on

At what point does a tight house get so tight that you need make up air for everything? I am targeting an ACH between 1-3 and will have make up air for my range hood but would I also need it for the clothes dryer & bathroom vents?


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  1. buildzilla | | #1

    can you share your strategy for range-hood make up air?

    also, what kind of range-hood were you planning?

    i was considering attempting to fly beneath the radar with a < 400 CFM range hood,
    but not sure yet how limiting that would be in terms of selection and effectiveness, etc.

  2. Expert Member
    ARMANDO COBO | | #2

    All our homes come around with ±1ACH50, and our typical ventilation strategy is properly designed, installed and commissioned ERVs and dehumidification/humidification systems. MAU units are installed with homes with exhaust fans over 400cfm.

  3. Expert Member
    DCcontrarian | | #3

    Makeup air is only necessary if you are burning fuel inside the house.

    First, a house that is considered tight isn't really that tight. It's not like you need to open your mouth to keep from popping your eardrums when you close a door, or like the bathroom vent is going to suck the water out of the toilet. Think of a house that is 1.0 ACH50, 2000 SF, 8' ceilings. That's 16,000 cubic feet, so at 1 ACH that's 16k CF per hour or 267 CFM. That's at a pressure of 50 Pascal. That's a really low pressure -- 0.00725 or a quarter inch of water. That's the air pressure change associated with a 14 foot change in altitude or about a third of a degree in temperature. It's a pressure difference that isn't detectable without instruments.

    Combustion appliances typically rely on convection to exhaust the combustion products. Slight differences in air pressure can make the difference whether an appliance vents properly. The consequences of improper venting can be severe as well so makeup air needs to be provided.

  4. walta100 | | #4

    Consider tightening up your goal 3 ACH50 is code minimum in a lot of places and 1 ACH50 was a stretch goal 20 years ago but not so much today.

    I agree with DC in that I see no need for makeup air if you have no fuel burning appliances or if the appliances are the sealed combustion variety.


  5. BrunoF | | #5

    Code in my area require makeup air if you are pulling more than 600cfm on the range hood. The hood we selected is 800 or so…well above the code requirement so my hvac guy is planning to install an actuated make up air system. We will be using an induction rage but the main reason for a big hood is to keep odors and cooking particulates out of the house. Although we love the smell of bacon, I don’t like a house that smells like bacon.

    I have no idea how much CFM a dryer moves but it seem like if I’m anything above ACh 1 it will have plenty of incoming air.

    BTW, I don’t think we even have ACH code here in central NC; I can get a pretty decent rebate from the power company just for getting ACH 4 or below.

  6. Expert Member
    ARMANDO COBO | | #6

    In a house with a good and healthy indoor environment, it's not only about the removing of gas contaminants but also removing as many airborne contaminants as possible from electric cooking. A good exhaust system will clean your indoor air and improve your indoor air quality.
    Most electric stoves require ≤400cfm exhust fan, and you can exhaust + ventilate that with an above stove hood plus an ERV, but for all large electric/induction ranges + electric modules (wok, griddle, boiler, fryer, etc), I recommend a range hood of at least 600cfm.
    We've installed a couple of 36" induction stoves/ranges with induction woks and fryer modules that required an exhaust fan over 400cfm, so we also installed MUA units.
    It's all about the level of commitment to good IAQ/IEQ!

  7. maxwell_mcgee | | #7

    Question to add to OPs...

    If you're building a tight house, presumably you'll have an ERV or HRV. Given that, would it be possible to temporarily run the ERV/HRV in imbalanced mode (i.e., intaking more than it is exhausting) while the kitchen exhaust is on, so that it in effect becomes your make-up air system?

    Do any ERV/HRV manufacturers allow this capability in their devices?

    1. jamesboris | | #8

      I think you're unlikely to get quite enough makeup air if you've got a 500+ CFM exhaust. If you're the kind of person that need everything automated, then just open a window when you run the range hood, as that's easier than messing with your ERV every time.

    2. DennisWood | | #9

      None that I'm aware of allow you to run an imbalance. That said, I've used some basic automation bits to tie our induction range's power use into the exhaust fan controlled with 0-10 volt zwave dimmers. The HRV also runs in asymmetric (unbalanced mode) to deliver about 100 CFM of makeup air as our home has gotten fairly tight as the retrofits have been completed. Turning on the induction cooktop triggers the exhaust fan which in turn modulates from about 50 to 130 (measured) CFM at the hood depending on the induction cook top's power use. While the kitchen exhaust is running, the HRV runs an asymmetric profile. You can read a bit about it here:

      More on the 0-10 volt automation for the EC HRV and kitchen exhaust fans:

      The system works quite well. We also have a high efficiency fireplace (outside air supply) that can back puff a bit during loading. The HRV asymmetric mode stops that completely.

      1. nexp | | #10

        Make-up air is essential for proper functioning of a range hood. A hood should generally be sized to maintain 90cfm/ft2 of hood opening, which is 540 cfm for a 36x24 hood. Your make-up air blower would likely need to be more powerful than this to accommodate pressure losses. If a hood has to fight negative pressure, it doesn't move nearly as much air. I don't presently have make-up air, and my hood performs very poorly if I don't open a window. You would not want an ERV to provide this make-up air because 1) They aren't big enough 2) much of the supply is probably to rooms with closed doors and 3) it wouldn't be doing any energy exchange while it is unbalanced. The supply ducts probably would be way-undersized as well.

        1. Expert Member
          DCcontrarian | | #11

          The effect can be estimated if you have a blower door score for your house. Take the rated CFM of the range hood and compare that with the CFM from the blower door. From that you can estimate how much pressure drop that much CFM unbalanced will cause. If you have a fan curve for the range hood you can see how much flow reduction would be associated with that pressure drop. (Since the flow is reduced, the actual pressure drop is reduced too, if you want you can reiterate with the reduced flow as many times as you like to get successively closer approximations. But one iteration is probably sufficient.)

          I think what you'll find is the effect is minimal and that your duct layout has a much bigger impact on fan performance.

        2. DennisWood | | #12

          Those were my observations, even at relatively low hood flows in a tighter home. I was able to pull about .4 negative with just the kitchen exhaust (about 130 CFM, measured, max) and the two bath fans running.

          That said, you can easily get 100 CFM make up (and yes, at lower exchange efficiencies) from a system with 4" intake/exhaust hoods on an ERV. I just ramp the intake fan to 130 CFM and back off the exhaust EC fan to about 30 CFM. The hood is not running all that long so the core efficiency issue is not that great.

          Between the HRV in asymetric mode and the hood working very efficiently (large capture area directly over the cooktop with non turbulent intake) cooking odours are pretty much zero outside the kitchen.

  8. StephenSheehy | | #13

    With a whole house ventilation system and a heat pump, condensing dryer, you'd only need to provide make up air for your stove hood.

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