Measuring vent area for whole house fan
Considering installing a 6000 CFM whole house fan in our 3200 square foot house. The fan we are considering is the QuietCool QC-CL-6400, and the manufacturer recommends 8 square feet of vent area in the attic.
Two questions:
1. What vents can I count towards the required vent area? I would think, with the attic pressurized, any “official” opening (gable vents, ridge vent, soffit vents, etc.) could be counted. Is this true?
2. Is there are good rule of thumb for converting measured vent square footage to actual vent square footage? For example, if my gable vents are 1′ x 1.5′, that’s 1.5 measured square feet. However, the screen and louvers add resistance that should be accounted for. Is there a simple estimated way to convert?
Thanks.
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Replies
Matt,
First of all, I advise you to read this GBA article: Fans in the Attic. The article has a lot of information to help you.
Q. "What vents can I count towards the required vent area? I would think, with the attic pressurized, any 'official' opening (gable vents, ridge vent, soffit vents, etc.) could be counted. Is this true?"
A. Yes.
Q. "Is there are good rule of thumb for converting measured vent square footage to actual vent square footage? For example, if my gable vents are 1' x 1.5', that's 1.5 measured square feet. However, the screen and louvers add resistance that should be accounted for. Is there a simple estimated way to convert?"
A. As noted in the article, "If the vent has insect screening, remember to make the opening 50% larger than the rule of thumb dictates. It’s better to have too much vent area than not enough. Manufacturers of ridge vents and soffit vents provide information on the net free area of ventilation per linear foot of their products; for example, [a web page] from the Air Vent website lists different ridge vent products that provide between 9 and 18 square inches of net free area per linear foot of product."