Bathroom Exhaust Fan/Light w/ Damper??

Discussion in 'Electrical Forum discussion & Blog' started by KimiS, Oct 28, 2008.

  1. KimiS

    KimiS New Member

    Oct 28, 2008
    Hi all! Long time reader here, first time poster.
    I am looking for a bathroom exhaust fan/light that has an electric/motorized backdraft damper (if there is such a thing!!)
    We have a standard "builders model" fan in our bathroom that has a horrible backdraft! I am told that it has a backdraft damper in it, but it must be a piece of you-know-what because when the wind blows it's like your standing outside in the bathroom.
    The duct vents to the soffit with a square cover that has slots on all four sides. This is on the north side of our house. Bbbrrrrrrr in the winter time!
    Is there any such thing? :confused:
    Thanks in advance!
  2. geniescience

    geniescience Homeowner

    Nov 27, 2005
    humid summers hot, humid winters cold
    take it out and see if the backdraft thingie is stuck. You can weight it down a bit so it doesn't just fly up when someone opens or closes a door. It's light as a feather.

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  4. KimiS

    KimiS New Member

    Oct 28, 2008
    I will check the damper to see if it's stuck open. But what I really want is something that I can make sure is closed - when you shut the fan off it shuts the damper and only opens when the fan is turned on.
  5. Thatguy

    Thatguy Homeowner

    Aug 27, 2008
    A bounty hunter like in "Raising Arizona"
    Gravity is more dependable, but. . .
    you need a fan with an electrically operated (probably using a solenoid) damper door, if someone makes such a thing.
    Try these guys
    to see if someone makes it.
    Call 'em on the phone, pretend to be fetching parts on behalf of a contractor.

    If not, this forum may tell you how to build your own.

    At minimum you want a solenoid with a 120vac coil and a mechanical linkage to close the damper door. Try
    for the solenoid.

    Get one with the minimum power consumption; there is no lighter load than one of these sheet metal damper doors. You put the solenoid coil in parallel with the fan motor AC input.
    Motor on=solenoid pulls in and holds the damper open.
    Motor off=damper slams shut because of the solenoid return spring and the weak spring (see below).
    You'll need wirenuts for the elec. part. The mechanical part will be the cut-and-try part.

    I'm not a mechanical engineer but you might want to couple the solenoid armature to the damper door with a weak spring so you don't whack the daylights out of the damper door.
    The fan motor is already in a fireproof box but for extra safety and to show "due diligence" and "duty of care" put a fast-blow fuse in series with solenoid coil. If the coil pulls X milliamps then fuse it at ~1.5x X milliamps.

    If you go this DIY route you will probably be paying yourself 25 tax free dollars per hour when you compare the time you spend with the price of a commercially available unit.

    BTW, your fan should exhaust to the outside, and this mechanism also has a damper in it. The likelihood of both dampers failing is low, so I'd do some investigation first.

    Good luck!
    Post back with your decision!
    Last edited: Oct 28, 2008
  6. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 2004
    Cave Creek, Arizona

    You might have better luck finding something with a damper for the soffit, although if it is facing downward you would need something with counterbalanced vanes to keep them closed between usages.
  7. Thatguy

    Thatguy Homeowner

    Aug 27, 2008
    A bounty hunter like in "Raising Arizona"
    I suppose the wind could create a slight vacuum which temporarily opens at least one damper because the air pressure in the house would be momentarily higher than outside.
    Same principle as when a passing hurricane sucks windows out (I've seen this one myself). The National Weather Service can relate pressure drop to wind gust, if they'll help you with this. I'd think your neighbors who are not protected from the wind would have the same problem.

    In this case there would actually be a draft in the bathroom during the gust. You can confirm this with a candle flame. It's worse if you're wet, because the draft helps the water to evaporate, further dropping your skin temperature.
    If the bathroom door seals tightly (and what interior door does this?), no draft, no damper opening. Also, no bathroom ventilation.

    Making a baffle that redirects the wind around the damper opening probably won't work and on the outside of the house it would be unsightly.

    Still, if the outside pressure is lower than the inside pressure, all doors of this type will open.
    If you have a kitchen fan damper, it may do the same thing, but you may not notice it. Rarely in the DC area do I hear the kitchen fan damper opening and then slamming shut from the mild vacuum caused by wind gusts, but it happens.
    If the light is just right, you will also notice the glass surfaces of large windows bowing slightly outward during the gust. The effect will be stronger if your house is not drafty (i.e., tightly sealed).

    Sound like a motor-actuated damper for you, unless you want to operate it manually, like a fireplace damper.

    Atmospheric pressure is 760 mm of mercury. If the damper is 3" round, the area is 7 sq. in. If one ounce, the weight of six sheets of paper, is enough to open this door then a sudden pressure drop of 1/2 mm of mercury should do it. If you have a quick responding barometer you should see at least this much pressure drop when the damper opens.
    Knowing the pressure drop and the damper door size we can figure out how much force it will take to keep the door closed against the gust.

    With any entry door or window open in the house and all interior doors open, the pressure inside and outside will be equal even with a wind gust.
    I think. . .
    Last edited: Oct 30, 2008
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