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Community and Q&A

Measuring interior air pressure drawdown?

W Ramsay | Posted in General Questions on

Our building inspector had said that we’d be required to include a makeup air system equal to our whole house fan. 6400 CFM of MUA!  That totally defeats the purpose of a whole house fan!

We’ve got him to back down to locking out being able to turn the fan on if windows aren’t open. We’d planned to contact a number of windows and so long as at least two were ‘not closed’ then the fan will run. The inspector may require the opposite and want to know that some bit of windows are ‘open’ which is vastly more complicated and expensive.

So, another option may be to measure the air flow in to the house for a passive vent located in the mechanical room. I think we should be able to do a manometer on it that will either give us a contact closure if FPM reaches some threshold indicating a large volume of air being sucked in or a 0-12v output based on FPM that we can trigger off of.

If there is much of a breeze outside then we’d not be using the WHF so I don’t think there would be windward/leeward type pressure issues.

Is this reasonable? Any idea how to calculate the threshold value that would indicate the house is in a state of depressurization and thus the fan should not run?


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  1. GBA Editor
    Peter Yost | | #1

    Hi W Ramsay -

    When you say "whole house fan" are you talking about this:

    or a whole-house mechanical ventilation system, such as this:

    Big difference in terms of the answer to your question and what applies from the building code.


  2. Jon R | | #2

    You can install a differential pressure switch (plus timer) that will turn the whole house fan off if it excessively depressurizes the house. Use pascals, not FPM.

  3. Expert Member
    Zephyr7 | | #3

    You have to provide makeup air while a whole house fan is running. The purpose of the whole house fan is to draw air through the house, usually coming in through first floor windows and exhausting through the fan and out the attic vents.

    I’d you need to interlock the system, a simple pressure switch is all you’d need. These are similar to the “proving” switches used in gas-fired furnaces to ensure the inducer (combustion) blower is working prior to starting gas flow. They are very expensive. What you’d want is a setup that would lock out the fan if the fan produced excessive negative pressure within the house. Such a system would automagically work to keep the fan from running if too few windows were open too. There are electronic pressure sensors available that will give an analog output (0-10v is standard, not 0-12v). Digikey has a large selection as do other places. If you can, consider using a 4-20mA current loop interface instead of 0-10v since the current loop will have much better noise tolerance and will be more reliable with longer wire runs than the 0-10v Interface will be.


  4. W Ramsay | | #4

    Thanks. The former (I've always, rightly or wrongly, referred to the latter as a ventilator). Likely this: In our case though it will not exhaust in to the attic but directly outside as our attic is conditioned space.

    A differential pressure switch seems like it could be a better option. I'd not thought about that. I know that in earlier discussions about using these to manage indoor air pressure in general, such as to control MUA more accurately across a variety of exhausting devices rather than having it tied only to a range hood*, the consensus was that they could not be nearly accurate enough. This particularly given the significant pressure differences between the windward and leeward sides of a house. In this case we'd be unlikely to use the WHF when there is any wind so perhaps that'd not be an issue? Any idea what the accuracy would be? What pressure differential should be set?

    How would a simple pressure switch work? Don't we need to know the inside and outside pressure to know if the house is being depressurized?

    * This discussion was about houses of less than 1 ACH50 and the possibility of having far more than 400 or even 800 CFM of exhausting appliances without any MUA. E.G., several bath exhaust fans, central vacuum and clothes dryer all going at once. The primary issue we were discussing was adequate MUA to avoid any backdrafting but the secondary issue was that many of these very quickly drop to quite poor performance with high static pressure on the supply side.

    Thanks all.

  5. Expert Member
    Akos | | #5

    I've used an Emerson 770-1 Air Switch for a dryer booster, should work for a house fan as well.

    Depending on the power of your fan, you might need an external relay.

  6. W Ramsay | | #6

    Thanks all. What kind of pressure differential would be appropriate?

    1. Jon R | | #7

      It depends on the devices you have. 3 pascals is usually safe, so set for 2 pascals.

      1. W Ramsay | | #8

        Thanks Jon. Do you know if there is a document anywhere that discusses this that we can provide to our building inspector?

        1. Jon R | | #12

          Maybe Table 1 below (or similar for your state).

          I'd measure the inside/outside differential pressure right next to some window that you expect to be opened.

  7. GBA Editor
    Peter Yost | | #9

    I have never heard of a whole-house fan being controlled by a pressure switch; going to the Tamarack Technologies website, I don't see any controllers for their high-tech whole-house fans that use pressure.

    I would check with the manufacturer on appropriate ways to control and code issues. Tamarack Technologies has excellent tech support.

    The problem with controlling by pressure is wind: you can easily get pressures of considerably more than 2 - 3 Pascals acting on the building. The other issue would be where do you locate the pressure sensor?


    1. Expert Member
      Zephyr7 | | #14

      I don’t think the fan is being controlled by the pressure switch, the idea is just to interlock the fan such that too much negative pressure (such as with the fan running but all the windows closed), the fan would be locked out.

      To isolate the outdoor pressure sensor from the wind, it needs to measure pressure perpendicular to the wind direction. I’d probably use an H Shaped PVC guard (three T fittings piped together) with the sensor in the T in the middle of the H. That’s probably the best protection you could get for the sensor without getting really complicated.

      It would probably be worth dealing with the inspector on this one. If it’s really important to you, an engineer can overrule an inspector. The other option is wait until after the inspection to install the fan. Inspectors can be a problem when they see something outside the norm that they don’t understand. Ive run into that plenty of time on special facilities.


  8. W Ramsay | | #10

    Hi Peter, that makes two of us.

    Our building inspector originally said that we'd have to have mechanical MUA equal to the exhaust of the WHF - 6400 CFM. Or no WHF. We got him to back off to allow it so long as it could only be turned on if x number of windows are open. I'm exploring the pressure differential as a better alternative.

  9. Walter Ahlgrim | | #11

    Will you have any fuel burning appliances in the house? Fire place, wood stove, gas water heater or gas furnace.

    Without any fuel burning appliances there is no safety risk from depressurization.


  10. Expert Member
    Peter Engle | | #13

    I'm with Walta. If all appliances are electric, or sealed combustion, there's no need for makeup air. You can depressurize the house all you want without any safety issues. I'd maintain that even a fireplace/woodstove is not a safety issue, because you will only run the WHF once with windows closed and suck all the wood ashes into the house. That's a self-correcting behavior.

    If the building inspector is being that much of a *[email protected]#, I'd be very tempted to install the WHF after the house is complete. Run the wiring, frame the opening, and just leave the panel of drywall in place. Not necessarily kosher, but certainly expedient.

    All this talk of pressure switches, interlocks, "not closed" vs. "open" is making my head spin. Of course you can engineer a solution to a problem created only by an aggressive inspector. But you're talking about real money to solve a problem that doesn't exist. A house with <1ACH shouldn't have any combustion appliances anyhow. When the weather is nice, operating a WHF saves energy on A/C at zero risk to the occupants. Sheesh. It's a freakin' fan. Tell your inspector to go hassle some HVAC guy who is oversizing his equipment by 300%.

  11. W Ramsay | | #15

    Furnace is sealed combustion, hot water is from a sealed combustion boiler that also serves hydronic heat. There is a fireplace but agree that it is a non-issue. As well, when weather is cold enough to have a fire it will be too cold to run the WHF. Inspector doesn't seem to care. He says that MN Code states that any appliance exhausting more than 400 CFM requires an equal amount of MUA - period.

    Numerous people suggested that after his first comment that we skip it until later but my preference is to include it with the initial build and we're sure that at this point the inspector will be looking for anything extra done for a future installation and tag us on it. We've still got a long way to go with this build and can't afford to tick him off. As well, he seems like a genuinely nice guy, just extremely ignorant.

    Peter, I have been in contact with Tamarack and agree, their CS folks and Nelson have been quite good.

    Bill, yes, this is purely just a lockout. Thanks for the tips on the T/H piping. That will help.

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