How to size an attic fan

How to size attic venting:

When you choose a fan, divide that fans cfm by 750. This number determines the amount of square footage of net free air required to properly vent.

To allow for any restrictions created by screening and louvers multiply that number by 2. This number is the minimum square footage of venting required for the fan to exhaust properly.

One can choose a combination of ridge and gable venting to make up the square footage.

For Example:

A 1,200 square foot home with 8 foot ceilings has 9,600 cubic feet of air.

The Tamarack whole house fan model # HV1000 moves 1,150 CFM per minute.

Divide 1,150 into 9,600 = 8.30.

This is the estimated time in minutes, to perform one air exchange.

Divide 60 (the number of minutes in an hour) by 8.30 to determine the approximate number of air changes per hour the fan will provide. The answer is 7.23 so; the estimated number of air exchanges per hour is 7.

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