Sizing an Exhaust Fan
Sizing an Exhaust Fan TIPS
- Different rooms require minimum amounts of air
- Kitchens need twice as much air as bathrooms
- WATCH exhaust fan videos below!
- Exhaust air out the roof except in heavy snow areas
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Size Is Everything!
Can you imagine trying to cool yourself on a hot summer day with just the air that's allowed to pass through a tiny cocktail straw? Just a tiny stream of air would pass through and it would be virtually worthless.
Move The Air
To cool something or provide enough air to keep things fresh, you have to move air. The room you're trying to ventilate dictates the amount of air that needs to be moved in a given amount of time.
Air Movement Guidelines
For years the Home Ventilating Institute said that all the air in a kitchen should be changed out for new at least fifteen times an hour. They felt that bathrooms should have a complete air change eight times an hour.
Shhhhhhhhsss! Quiet Fans
Are you upset with how loud some bathroom or kitchen exhaust fans are?
Did you know you can get wonderful fans for bathroom exhaust where the fan is way up in the attic and there's no noise in the bathroom while the air is being sucked out of the room?
It's possible! Look at the following fan I installed in my own home:
A Fast Example
Let's take a small kitchen for an example. Say the kitchen measures 10 feet by 12 feet with an 8-foot ceiling. Remember how to compute cubic feet in a room?
You multiply length times width times height: 10 feet X 12 feet X 8 feet
That computes to 960 cubic feet of air in the room.
The minimum requirements say that this air should be changed 15 times an hour.
You now need to multiply 960 X 15.
This means that a fan must have the ability to move 14,400 cubic feet of air in an hour.
Exhaust Fan Videos
Watch these helpful videos about exhaust fans and how to vent them out of roofs.
The following video shows you how to prevent roof leaks around bathroom exhaust vents.
Fans Are Rated Cubic Feet Per Minute Not Hour
Since there are 60 minutes in an hour, we must divide the 14,400 by 60 to get CFM (cubic feet per minute).
Divid 14,400 by 60 and you get 240 CFM. Nothing to it!
Handy Table For Fan Values
The following table does lots of the math for you. All you need to do is calculate the square feet of a room and the table tells you the CFM for either a kitchen or bathroom.
Square feet is easy. Just multiply the length of the room by the width. The table assumes a common 8-foot ceiling height.
The room area is listed in square feet. All values below the listed room size represent the fan's minimum capacity shown in cubic feet per minute.
|Room Area in Square Feet||Fan Capacity in CFM for a KITCHEN||Fan Capacity in CFM for a BATHROOM||Fan Capacity in CFM for Other Rooms|
Table Values courtesy of Nutone, Inc.