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Multi-port bath fans

Daniel Morrison | Posted in Mechanicals on

I have two bathrooms without exhaust fans and a multi-port fan in a box in the basement. The bathrooms are stacked in a cape. The upstairs ceiling is sloped with a small attic area above the flat segment.

Assuming I can come up with a way to run the ducts from each bathroom to the fan, how should it be wired? Should the fan turn on whenever either light switch is turned on? Should there be some sort of timer involved?

There’s no ventilation in the house — it’s heated hydronically.

Thanks in advance,

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  1. GBA Editor
    MIKE GUERTIN | | #1


    I can't figure out why you'd have a multiport fan box in your basement w/out it being connected to baths already. Is the fan box exhausting anything now?

    In order for a bath exhaust fan to do it's job it needs to run longer than washing activities have ceased in order to get the moisture out; so a timer is the way to go. There are a number of different timers (electronic, mechanical) that work for single point exhausts and ways to wire them in but since you have a multi port - your timer options are limited; most electronic timers can't operate multi-port fans from multi locations. And don't try 3 way switching - if the fan is quiet, someone will inevitably turn OFF a fan that's been turned ON from another location.

    I prefer 60 min. crank timers for simplicity. They are one of the only timers that can be used in multi switch locations plus they don't require neutral wires in the box. I run the supply to the fan itself, then run switch legs to each timer location. Tape off the neutral (white) wire with black tape at both ends and label as switch legs for future reference. Wire nut all the black legs to the black line wire and wire nut all the taped return wires to the fan load black wire. Users can crank a little time or a lot of time depending on exhaust need.

    Dehumidistats can be wired in also to work automatically - but I advise against them. I've used a couple in my house they operation is erratic / unpredictable. Sometimes the fan runs for days, and other times when the bath is steamy - it doesn't turn on at all.

    Just make sure to train occupants how to use the fan controls.

  2. Daniel Morrison | | #2

    No, Mike, the fan is sitting in a box, in the basement with children's toys and camping gear on top of it.

    So I can put a timer in each bathroom connected to the single fan (which will be removed from the box, and installed in the attic)?

    Thanks for the tips,

  3. homedesign | | #3

    Dan, It concens me that you have "no ventilation in the house".
    This may be an older home with lots of infiltration that is providing a healthy amount of fresh air....or perhaps you have made improvements to make your home more airtight?
    Assuming your house is reasonably airtight..then you need more than a simple timer on your bathroom fans. You need a dedicated air intake location(s)and a timer that is designed to meet code standard ventilation rates.
    I am guessing that your "fan in a box" was a "donation" or promo.
    Perhaps you can get a good deal on an HRV and introduce it into your ventilation strategy. Healthy air changes are essential..but there is an energy penalty... so Energy recovery will become more important as we "improve" our housing stock.

  4. 2tePuaao2B | | #4


  5. Riversong | | #5


    I agree with John that, if this multi-port exhaust will be the only ventilation in the house, then it needs to serve as whole house (ASHRAE 62.2) ventilation as well as spot ventilation.

    This requires the spring-wound timers that Mike suggested in each bathroom as well as a centrally-located mechanical 24-hour programmable timer (Grasslin KM2ST) all wired in parallel to the fan.

    If your house is tighter than about 5 ACH50, then you may need passive make-up air inlets (Aldes Airlet 100) located in bedrooms and living spaces.

  6. John Hess | | #6

    If you want a quick, cheap way to reduce the humidity in your bathroom, figure out a way to enclose the bathtub-shower. In my house there is a one-foot gap between the top of the sliding door enclosing the bathtub and the ceiling. I installed a spring-loaded shower curtain rod near the ceiling above the door and draped a folded shower curtain over the rod, down to the door. It greatly reduced the humidity in my bathroom. No more fogged up mirrors or windows. After showering I also wipe down the wet walls with a squeegee, which further reduces the moisture in the bathroom.

    Not sure which ASHRAE codes these techniques are covered by...

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