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Passive air inlets still useless?

Roger Lin | Posted in Green Products and Materials on

I remember passive air inlets made it on Martin’s list of useless products a few years ago. Is that still true for a home that has been retrofitted to be pretty tight? Haven’t done the blower door yet but we are projecting about 1.5 to 2.5 ach 50. The home uses exhaust-only ventilation with a panasonic continuous bath fan in the hallway set to 60 cfm.

Thanks

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Roger,
    I think passive air inlets start to make sense if you achieve Passivhaus levels of airtightness (assuming, of course, that you have an exhaust-only ventilation system -- which is not the case with the typical Passivhaus). I wouldn't advise installing an exhaust-only ventilation system in a house that was tightened down to 0.6 ach50 unless you included a few passive air inlets. If you house is between 1.5 and 2.5 ach50, you are probably OK without the passive air inlets -- but I'm not sure.

    Needless to say, any tight house with an exhaust-only ventilation system shouldn't have any atmospherically vented combustion appliances. Any combustion appliances should be sealed-combustion units.

    Here's a good way to verify that your exhaust-only system is working: commission the system. In other words, if you need 60 cfm of air flow, and if you have installed a 60 cfm fan, measure the airflow after the unit is installed. If the unit is exhausting 60 cfm, it works. Obviously, if it works, the house has enough cracks to provide 60 cfm of makeup air -- otherwise, it would be physically impossible for the fan to be exhausting 60 cfm.

  2. Michael Blasnik | | #2

    Even a house at 0.6 ACH50 shouldn't have too much trouble providing the 60 CFM exhaust (whether exhaust only is a good idea is another question). You can get a pretty good idea of the depressurization caused by an exhaust fan by calculating:

    50 * (CFMfan/CFM50)^(1/0.65)

    For a 16000 cubic foot home at 0.6 ACH50, the CFM50 would be 160 and the equation indicates about 11 Pa of depressurization. That's noticeable and probably not desirable but shouldn't slow down the fan that much. If the house was at 0.3 ACH50, then the depressurization would climb to 32 Pa, which may reduce fan flow noticeably (depends on the fan curve). On the other hand, at 1.5 ACH50, the house would be depressurized by less than 3 Pa. The ability to tolerate some exhaust flows (e.g. a kitchen exhaust fan or conventional clothes dryer) is one of the reasons I wouldn't want to live in a home that's just 0.6 ACH50.

  3. Roger Lin | | #3

    Martin,
    Thanks for the tip on testing if the exhaust-only system works. I am waiting until we do the blower door test to decide whether to put in the passive air inlets.

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