Association of Home Appliance Manufacturers
Calculations for Power Fan Installations
If you plan on installing a power fan, you can calculate intake and exhaust requirements using the following formulas;

1. Determine the fan capacity needed to provide about 10 to 15 air exchanges per hour.

The formula is:
Attic square feet x 0.7 = CFM capacity

For example, using the same dimensions as the previous example:
Calculation:
1,000 sq. ft. x 0.7 = 700 CFM

Note: For roofs with an 8/12 roof pitch or higher, you may want to add 20 percent more CFM to handle the larger volume of attic space.

2. Determine the amount of intake venting required.

The formula is:
CFM rating of fan + 300 = square feet of intake ventilation needed.

Calculation:
700 + 300 = 23 square feet

To turn that figure into square inches (the common method of specifying net-free area), multiply by 144 (the square inches in a square foot).

Calculation:
2.3 x 144 = 331 square inches of net-free intake area

To find the number of intake vents required, use the Net-Free Area Table as explained in "5 Steps for a fixed vent system"


Source: Air Vent Inc. A Certainteed Company
Reprinted with permission

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