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Thread: Can I pipe two bathroom fans to one roof vent?

  1. #1

    Default Can I pipe two bathroom fans to one roof vent?

    I have two adjacent bathrooms that I would like to add fans in.

    The fans would be located within 5-6 feet of each other and could easily exit the roof at the same point.

    If I remember correctly, fans all have a "check valve" or flapper preventing back flow and drafting back into the room.

    With that in mind, if they were both plumbed into one vent, it should be no problem right? Turn one fan on, the flap on the other one closes. Turn both on and they both vent.

    Good idea or bad? Saves me a little bit of money and one hole in the roof.

  2. #2
    Master Plumber Dunbar Plumbing's Avatar
    Join Date
    Apr 2005
    Northern Kentucky/Greater Cincinnati Area


    I did this once and it failed miserably. One could be taking a dump or shower and the user in the other bathroom would nearly pass out from the smells/moisture.

    Those swinging flaps are not air tight to say the least.

    The reason I had to do this was because the customer refused to go out the roof twice. It was highly likely though that both bathrooms would be in use due to the amount of people in the home *5*.
    Read what the end of this sentence means.

  3. #3


    The problem as I understand it is that if both fans are using the same duct and are both running at the same time, the fans will be less efficient and more noisy because it will be like installing a fan with a smaller duct that it was rated for. The fan(s) would have to work harder and would expel less air. I doubt any inspector would pass such an install. But this is just my opinion since I was looking to do this kind of setup until I realized it was not worth it.

    Good luck

  4. #4
    Forum Admin, Expert Plumber Terry's Avatar
    Join Date
    Aug 2004
    Bothell, Washington
    Blog Entries


    They also make fans that mount on the roof, and "pull" air from the bathrooms.
    These are very quiet, and you can run more than one inlet (bathroom), several can be done this way using one fan motor.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    Fantec makes some nice multiple input fan systems. You can turn it on or off from either location. As Terry mentioned, these use a remote fan and are very quiet, so much so that you may want to put a pilot light to indicate that they are on.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    Homeowner geniescience's Avatar
    Join Date
    Nov 2005
    humid summers hot, humid winters cold

    Default splitter-combiner for two inlets.

    i think it may be possible to arrange for one fan to suck air equally from two inlets. So both are ON or both are OFF. Not one or the other.

    a month ago i looked on the Broan and Nutone web sites (they are sister companies), and I saw a remote-installed squirrel cage fan that came with two vent inlets attached to a splitter-combiner, so when it was ON, it pulled from both inlets. Hopefully equally, but that would depend on the length and elbows after the combiner-splitter.

    I have a squirrel cage installed remotely. The noise is nice because it is only the noise of air moving through the vent opening, not a motor noise. I am planning on putting a variable speed controller on it because it is too powerful by a factor or two or three, and I would like to be able to leave it ON, on "low", for hours and hours, when the outdoor temperature is just right.



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