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Community and Q&A

Passive air inlet for dryer makeup air?

Nic Smith | Posted in Green Products and Materials on

Hi all,

I’d like to solve a problem I have with makeup air for my clothes dryer.

The dryer is located in an unconditioned (but nicely finished) utility room/pantry/storage room that needs to be kept clean. The door to this room is well-sealed as an exterior door. In order to provide makeup air when the dryer is on, I have been opening the only window in the room. This draws dust, pollen, and small bugs through the window screen and onto the folding table and into the air filter of my heat pump water heater.

I recently discovered the Panasonic FV-GKF32S1 passive inlet vent which seems like it would solve my problem. It states that it is rated up to 18 cfm – which is not enough for a dryer (probably about 100 – 150 cfm?) – but I believe this 18cfm rating is “passive”, meaning the amount of air exchange that occurs due to the stack effect.

Link to Panasonic unit:

Is the ~3″ duct in the Panasonic unit large enough to provide approximately 100cfm of makeup air for a dryer? In my case, the duct would be run directly the an exterior wall, so only about 4″ straight-through.

Assuming that the 3″ duct would provide sufficient makeup air, can anyone think of a problem with this setup?

Thanks for reading 🙂

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  1. Trevor Lambert | | #1

    150cfm seems like a lot to pull through 3", but is it even 3"? The specs say 3" wall opening and that it's insulated, which implies a much smaller inside diameter. This would make sense for the 18cfm spec. I doubt they spec'd it based on presumed stack effect, especially since it's purpose is to offset the depressurization of mechanical appliances. It's also not clear this is going to solve the problem you have. It's still going to bring in pollen, dust and bugs unless you install a large filter box in front of it.

  2. Nic Smith | | #2

    The dryer itself has a 4" exhaust duct, so how choked would a 3" intake duct be?

    Also, an Amazon customer states:
    "You need a 3.25 hole saw to install as the description of a 3 inch vent pipe is incorrect."
    And the instructions state to use a 3-11/32" hole saw. So it does seem that the duct is in fact 3" inside diameter.

    In terms of filtration, the Panasonic unit does have a washable bug and pollen filter. They don't give a MERV rating though.

    I've thought about a DIY intake with a large filter but I always prefer to using an engineered solution when available.

  3. Expert Member
    Akos | | #3

    That passive air went is WAY too small for a dryer. The PDF for it shows a flow curve, there is no way it will ever get to near the flow rate of a dryer. The filter in it is also extremely small.

    A well sealed space would still allow a fair bit of flow at the pressure drop a dryer is capable of. Before adding a makeup air, see if you actually have a problem.

    If it is gas dryer, then you need a properly sized makeup air, something much bigger than the Panasonic unit.

    For an electric dryer, you can see if you are actually depressurize the space or if you are loosing flow rate. If the dryer is not giving you a check lint trap error with the window closed, most likely it is getting sufficient air flow.

  4. Jon R | | #4

    IMO, you need either a large passive intake (like a window) or a smaller fan powered intake. Both with filters.

  5. Nic Smith | | #5

    @Akos: the dryer is electric as are all my other appliances (no gas to the home). The dryer does indeed run with the window shut, but perhaps at a reduced capacity because of inadequate makeup air - I don't know. The more pressing problem with this strategy is that the air that is drawn in smells musty and is surely not healthy. It does seem like the best solution is providing makeup air somehow.

    1. Expert Member
      Akos | | #6

      If you have musty smell in your house, that is a whole other problem. You need to fix that first. Dryer performance is the least of your concerns.

      1. Nic Smith | | #7

        Akos - not a musty smell in the house, only in the utility room, and only when the dryer is drawing air in through the roof and walls.

        This room was originally (1960s build) an attached storage/utility room and does not share the main home's attic. With the window closed, the dryer draws air in from the small, unvented, uninsulated 2x4 roof assembly and also the uninsulated wall cavities. I do not believe the smell is moisture, mold, or rot related, but rather exactly what you'd expect an old unvented attic space to smell like - tar, hot asphalt shingles, old wood, dust, etc. I don't think there is a problem to fix here, unless "fix" means tearing down a portion of the home. Does attic and wall cavity air usually smell pleasant?

        1. Expert Member
          Akos | | #9

          All building materials have some smell, you can definitely smell older structures. The smell is not musty though. Smelling of old wood, dust and shingles is normal, nothing needed to be fixed.

          If you want a filtered inlet, probably your best bet is a 3x10 wall cap with adjustable grill over it. You can then cut a piece of washable furnace filter material as a filter. I use this for the inlet filter on my ERV and works quite well (see attached). This also lets you close the grill when you don't need to run the dryer.

          1. Nic Smith | | #10

            I guess I shouldn't have used the word "musty"! I agree that older structures can smell old.

            I really like your ERV inlet filter solution. Very simple. I could easily do something similar. Thanks a lot for posting the photos.

  6. Ethan Foley | | #8

    Using the same 3 pa pressure difference method used for sizing jump ducts and transfer grilles, you would need a 10" passive duct. Might as well open a window at that point. I think your best bet if you are having problems is to install a supply fan with a relay to turn it on when the dryer turns on. Or open a window if you have it. If you are just trying to get rid of the musty smell, adding a 3" duct to the outside "might" allow enough fresh air in to dilute the smell. Can you crack a window or something to find out without cutting a hole?

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