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Thread: Bathroom Vent Fans

  1. #1
    Test, Don't Guess! cacher_chick's Avatar
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    Default Bathroom Vent Fans

    I recently installed 2 Panasonic vent fans due to their extremely good ratings here on the forums. I noticed when I installed them that the door for preventing backflow draft has a notable gap all the way around it, and does very little to actually stop cold air from coming back into the room.

    These were 2 different models, and both use the same type of draft door. Not only can cold air come in, but I would imagine that there is plenty of warm air escaping out these fans, even when they are not on.

    Given all the interest in energy savings and keeping a tight house, how can the losses from these vent fans be considered acceptable, and what if anything is done to improve the situation?

  2. #2
    DIY Senior Member dlarrivee's Avatar
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    Make them really tight so they get stuck.

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    Jack of all trades DonL's Avatar
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    The back-draft damper is probable designed that way because many Low Power Fan motors have a hard time working against a completely closed damper.

    The small gap gives the Fan Motor a chance to come up to speed, then it can open the back-draft damper properly.
    Theory only works perfect in a vacuum.

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    DIY Senior Member Chad Schloss's Avatar
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    you should also have a damper installed in the roof jack where the fan discharges. this one is a better seal (on the one i have installed) than the panasonic damper. if your fan is not mounted properly, the damper may not be closing fully either.

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    In the Trades mtcummins's Avatar
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    I typically have 3 dampers in total... factory one at the fan, one mid-line somewhere near the outside wall, and a louvered side wall discharge (don't usually go through the roof, prefer to not have roof penetrations if possible). The fans kick all 3 open very quickly, don't seem to have any trouble with it. There will always be some backflow, but this seems to stop most of it.

    I also tape all the seams and insulate around the vent line when possible - spray foam around the terminal through the wall/roof, and whatever you like in the bay around the exhaust line. Heat will slowly escape through the thin wall of an exhaust line otherwise.

    Fans are an inherent inefficiency though, there's only so much you can do.

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    DIY Senior Member dlarrivee's Avatar
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    I agree.

    I also have a damper on the galvanized louver on the outside wall.

  7. #7
    Test, Don't Guess! cacher_chick's Avatar
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    Thanks for your input guys. In retrospect, I think when I had my roof done they installed a regular attic vent louver over the outlet of the main bath fan. This will need to be tended to.

    On the basement bath I did install an aluminum louver with a damper on the outside wall. I'm a bit shocked to find how much cold is absorbed through the louver and inside along the vent pipe. The vent pipe is like ice 4 feet in from the outside wall. (I don't have the ceiling in yet.) I guess wrapping this with insulation is all that I can do.

    Moving south for the winter sounds like a pretty good alternative right now.

  8. #8
    In the Trades mtcummins's Avatar
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    for your basement one at least, buy an additional in-line damper and install it just inside the exterior wall. tape all of the seams along the line, then spray foam around the penetration through the wall and along the sides of the new damper (or at least wrap this portion of the line with duct insulation). this should slow down the infiltration substantially, but there will always be cold air infiltration through a vent line like this. insulating around it will take care of most of the rest of the cold.

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