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Community and Q&A

Double backdraft damper needed for bathroom fan?

Jeremy Turner | Posted in Energy Efficiency and Durability on

I am going to install Panasonic exhaust fans in our bathrooms in new construction through an unvented attic.. The Panasonic fan comes with a built-in damper and their installation guides mention using a wall cap with a damper too. This means there are two dampers.

The Panasonic damper is very light plastic does not seal well and opens easily. The wall cap damper is typically metal, making it a little harder to open. Our HVAC contractor suggests not using a damper at the wall cap but this worries me.

Is the best approach to use two dampers in line?

Thanks for your advice.

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  1. Steve Knapp CZ 3A Georgia | | #1

    I have Panasonic fans and used exhaust covers with dampers. I would be concerned that the internal damper would not seal sufficiently and outdoor air might infiltrate into the exhaust line whenever the fan is not on.

    Maybe your contractor is used to the cheap junk exhaust fans that most people install. The Panasonic is an entirely different product.

    On a related note, be sure to use smooth, metal, insulated ducts and ensure that all connection points are sealed.

  2. Jeremy Turner | | #2

    Steve: thanks for your reply, my thoughts too. But since I am often wrong additional advice is most welcome.

  3. GBA Editor
    Martin Holladay | | #3

    For additional advice on duct details for exhaust fans, see this article: Bathroom Exhaust Fans.

  4. Jeremy Turner | | #4

    Thanks, I missed it during my search

  5. Brian P | | #5

    We have Panasonic exhaust fans in our two bathrooms. You're right...the built in fan damper isn't really much of a solid seal, but probably meant more to block large gusts of air.

    We installed these Broan wall caps:
    I'd rate them pretty good as they are well made with a solid screen and ok damper. I say ok damper because the weather stripping associated with the damper started to separate and needed replacement (can just use stuff found at local hardware stores).

    To truly seal out cold winter winds, we also installed these:
    Our duct runs are very short, so I wasn't concerned about reduced air flow. If you did install these, I think it would make sense to do so in an accessible location, in case they ever need to be replaced or cleaned, but maybe that concern is unfounded.

    I've seen people mention Seiho products:
    Looks like the dryer vents have a damper but no screen...and the wall caps have a screen but no damper?

    These look like a decent wall cap:

    To more directly answer your question, I would use a wall cap with damper...even at 30 CFM, the fans easily open the wall cap damper.

  6. David Meiland | | #6

    I have used Panasonic fans on many projects, and always install Seiho wall caps. Make your duct runs as clean as possible and your fan will work fine.

  7. Jeremy Turner | | #7

    Brian, thanks for your suggestions, good to know the dampers open easily, I was concerned about that though we are using 6" duct and have short runs.. I have some of the Famco vents on order but I had not come across the Tamarack damper, they look very interesting.

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