# Calculating Ventilation fan / vent "free area"

• posted on November 2, 2006, 2:58 pm

Consider a room that measures 8' x 10' x 8', or 640 cubic feet. I want to install a fan and vents that will exchange the air in this room about once every 2-3 minutes, so a fan should be rated in the range of 210 - 320 cfm.
I need some guidance in how to calculate the appropriate free area of the vent(s) that will allow the fan to pull air from adjoining rooms through this room.
Any suggestions? I expected to be able to find an online "calculator' for this scenario, but no luck so far.
Thanks
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• posted on November 2, 2006, 9:18 pm
snipped-for-privacy@gmail.com says...

That's a heck on an exchange rate, higher than a clean room! Anyway, for a quickie estimate, select your fan first and see what size duct it uses. I'm guessing an 8" round duct. Of course, you don't need your intake air velocity to be anywhere near as fast as the exhaust, so start with maybe double the fan exhaust area. For an 8" fan, of 50 square inches, 100 square inches would give you half the exhaust velocty, 150 would give you 1/3, etc. And if you want efficient 'washing', the intakes need to be distributed in the correct areas.
-- Dennis