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Process of exhaust vent size selection and quantity

beedigs | Posted in General Questions on

HVAC guy says with our master bathroom that is approximately 11.5ft x 17ft that we would only need one bathroom exhaust vent since our toilet is not enclosed in a closet.  Is he right?  Also I am concerned that with one exhaust van it wouldn’t be able to handle the humidity in that bigger bathroom which is not what we want as we don’t want to deal with mold and such.  Is there a standard calculation used to determine the correct CFM of the exhaust fan for a room?  Attaching pic of master bath


  1. stevedavis | | #1

    HVI has suggested sizing for bathrooms. Other green building programs reference this standard. It can be found here:

    Following the standard, you'd probably want a 150 CFM fan which is at the upper range of most bath fans. Make sure the speed is adjustable which most fans are now so you can dial it back if you want.

    Occupant behavior is the big variable. I'd feel fine with this but if you're taking extra long showers you might think differently.

  2. walta100 | | #2

    A few questions
    Are you the type of people that turn on a bath fan when in the shower?

    Are you the type of people that turn on a bath fan when the room may have an odor?

    From looking inside of old fans most appear to have seen little use.

    If you feel you are likely to use the fans put them in if not stick to the code minimum.


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