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  1. #1

    wiring a humidity sensor switch into an existing on / off switch bathroom fan

    i would like to install a humidity sensor switch into our existing bathroom fan set up which just has an on/off switch. i still want the on / off switch to work for the fan, but I also want the humidity sensor to turn on the fan if the humidity gets too high. currently there is only the 3 wires from the switch running to the fan etc.

    do i need to run a wire from before the on / off switch (essentially bi-passing the switch) to the new humidity switch and then the wire from the humidity switch back into the wire that runs to the fan.

    that way there is still only one wire taking power to the fan, but either of the switchs can turn on to allow the power to flow.

    does that make sense?


  2. #2
    Join Date
    Mar 2001
    Ilwaco, Washington
    be careful matching line voltage to a dehumidistat. Most all I've seen are low voltage.
    One way to outthink people is to make them think you think. They'll think you're not really thinking what you're trying to get them to think you think...........

  3. #3
    The installation instructions say for 24v or 120v installations

  4. #4
    Join Date
    Jul 2000
    Northern Wisconsin
    How old is the fan? There are units on the market that have built into them just what you're looking to do. They turn on with a spike in humidity from say a shower and then once the shower is over they run for another 20 minutes. To install a simple humidity control the fan would run anytime the set humidity level was exceeded...... this could be on a humid day in the winter with no one being in the bathroom. Also, when the existing fan did finally need replacing, if it were replaced with a "smart" fan, what you did would be no longer needed. Spend the right money now instead of later?

    As for how to do what you're asking, we can not supply that kind of information because it is considered DIY and that is against site rules to do.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  5. #5
    Join Date
    Jun 2003
    Madison, WI/Cape Coral, FL
    The concept of operating a bath fan on high humidity does not work when the outdoor dew points are +60^F. You will end up with a fan operating 24/7 and a damp house. Better option is an extend run timer as part of the bath on/off control.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  6. #6
    Join Date
    Nov 2006
    Athens, Ohio
    I attended a seminar which included information on the new Panasonic vent fans. Some of them have humidity control built in. You can wire them without a switch, just 120 volts to the fan. I've been considering one for our house.

  7. #7
    Join Date
    Dec 2007
    Just replace the existing switch with a switch that has a timer feature.
    You are only concerned with the humidity created with bathroom use, so run the unit for an hour after the last use and it shuts off automatically. Time is dependent on who you talk to.
    For while house dehumidification you want other things.
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

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