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Thread: How Many CFM?

  1. #1
    Join Date
    Aug 2009

    Question How Many CFM?

    I am remodeling bathroom. I have removed the tub and added a walkin shower that will be tiled floor to celing. The shower is 42" X 96" with a 36" opening. There will be no door. I will install a fan with a light above the shower should I size the fan to the entire room, or just to the shower area?

  2. #2
    Join Date
    Mar 2010

    Default Re: How Many CFM?

    Wayne -- now that's a large shower.

    A lot depends on the size of the entire bathroom , what other fixtures in the bathroom , as well the number of shower heads , if there is a steam system involved , so on.

    There are guidelines that have been established by the Home Ventilating Institute ( HVI ) determining the ventilation of bathrooms. Basically, this recommends how many cubic feet per minute ( CFM ) of air should be vented for the size and number of fixtures for the bathroom.

    The HVI recommends 1 CFM per 1 square feet for the bathroom venting.
    They also recommend considering the number of fixtures toilets , tubs , showers for large bathrooms over 100 square feet based on the following :

    toilet = 50 CFM
    Shower = 50 CFM( based on common 32 x 60 )
    Bath tub = 50 CFM
    Jetted tub = 100CFM.

    These values are determined with a recommended 8 air exchanges per hour.

    An alternative to installing separate fans is to install a large in-line fan with ducting for to the intake grills . This would simplify the installation and be considerably more quite than individual point of use fans.

    Venting the air out using a bathroom exhaust fan also requires the same amount of air to be replaced , known as air exchange. If this doesn't occur the exhaust fan(s) wouldn't work very well.

  3. #3
    Join Date
    Feb 2010
    Eastern MA

    Default Re: How Many CFM?

    When fans are rated they are tested with a 6 foot piece of straight duct. When longer duct runs are used and fittings, the capacity is reduced substantially.

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