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Two Panasonic fans on the same 6-inch exhaust duct through the roof

Don Harto | Posted in General Questions on

Have 2 panasonic whisper green fans, 80 cfm bath and 50 cfm laundry room. Looking to minimize roof penetrations and only have one vent. Best practices for integrating these 2 into one 6 inch roof exhaust.
Both are using 6 inch galvanized duct.
Marine zone 4c.
Also about air sealing, should I put sheetrock boxes around before blowing in cellulose?
Is insulating exhaust ducts in vented attic necessary in marine 4c?

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Replies

  1. Brian Pontolilo | | #1

    I don't think it would be best practice to put them both into the same exhaust duct, particularly not if they will both be used at the same time. I'm not sure about the sheet rock boxes since it is a fan with a duct to the exterior. Make sure the back-draft damper is working and you can caulk the flange to prevent leakage around the fan. And you can insulate the pipe or at least make sure that it pitches down towards the exterior termination to allow condensation to flow out of the house.

    Here's are some helpful articles:

    https://www.greenbuildingadvisor.com/article/bathroom-exhaust-fans
    https://www.greenbuildingadvisor.com/article/air-sealing-an-attic

  2. Expert Member
    Zephyr7 | | #2

    I discussed this in another recent Q and A here. Basically, I don’t recommend sharing a single vent. If you really want to though, the important thing to worry about is back pressure seen by the fans and dampers. Don’t bring the two ducts into the common duct in a way they will blow into each other, make sure each duct blows down the main vent and not down the other fans duct. In big chilled water systems, I use Y fittings for this with each pump going into the branch of the Y and the main line in the straight part of the Y. The Y then “points” in the direction of flow. Hopefully that makes sense. This minimizes the back pressure seen by each pump (or fan, in your case).

    Size the main vent line to have the same area as the combined area of the two fan ducts. You could probably get away with a single 6-8” vent, but larger would be better.

    Best is just to use seperate vents for each fan.

    Bill

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