AVS Special Member
Join Date: Dec 2008
Location: L.A. - Lower Alabama
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From what I've been able to figure out by reading the forum, there are few rules of thumb to help determine the size of your dead vent and fan.
You need to shoot for six air exchanges every hour. So calculate the total volume of your room (length x width x height for a rectangular room). Multiply that number by six (you want six exchanges per hour). This number is the total cubic feet per hour, so divide that by 60 to convert to cubic feet per minute. You can use that number to pick a fan.
The next catch is to keep your air velocity below 250 feet per minute. That will determine the size of the dead vent opening, or you can increase the number of vents and move less air through each. To get a rough estimate of the air velocity you divide the cfm in your pipe by the cross sectional area of the pipe or AC register. This is a rough estimate because air is compressible.