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Wayne Hale
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?

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.

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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