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Community and Q&A

Running Make-Up Air Through Existing Supply Line

Carter189 | Posted in General Questions on

I am adding in a Make up air damper because of a large exhaust fan in my kitchen. I happen to have an old 8″ exhaust pipe that goes through the roof and I would like to put the damper in there and run it into my main AC trunk supply line. I live in Florida, and I cant think of a reason to run it all the way to the other end of the house where the return line is. In my mind this is the same as just opening a window which I have been doing and is getting annoying! Will this be an acceptable way of doing this?

Thanks for the help

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  1. Nola_Sweats | | #1

    I'm not an expert, just a hot-humid homeowner (New Orleans). But it seems like adding Florida outdoor air directly to your supply line would introduce a ton of moisture into a cold metal tube, with potential condensation there and in the downstream ducts. If the outdoor air is added to the return line, it at least gets run through the coil and dehumidified in a place where the condensation can drain.

  2. Carter189 | | #2

    Thats a really good point! Didnt think of that, are there any better options than run it into the return?

  3. Expert Member
    BILL WICHERS | | #3

    If you need makeup air, you are stuck bringing in outside air, regardless of the conditions. You are blowing air to the outdoors with your exhaust fan which will depressurize your house. You have no choice but to replace that "exhausted" air with new air from outdoors. There is no way around this.

    In the commerical world, there are special air conditioning units made for this purpose, that have loads of latent heat capacity to allow for lots of dehumidyfing. The downside is they're expensive units. I don't think an HRV (or similar) setup is going to be able to help you much with humidity in this application, either. If your air conditioner happens to be running while you're bringing in outside air, it will at least help to dehumidify that air.

    I think the best advice here is probably to make sure that damper you install seals well and is properly interlocked with the exhaust fan so that at leasy you are sure to ONLY bring in outside air when you NEED to.


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