I'm in the middle of a home addition and am thinking about adding a whole house fan. The finished size of the house will be about 2000 square feet – approx 1500 on the main level, and 500 on a lower level. I'm currently looking at the Quiet Cool Line of fans.
My question: How do I size the fan? Is the relevant number to use the total square (really cubic footage, but the marketing material usually uses square) footage of the house, or just the upstairs square footage? Seemingly it would just be the upstairs square footage, since the downstairs warm air would be rising anyway. Or is there some other criteria I should be using?
Best Answer
The guidelines I've seen (Dayton?) figure the cubic footage of the living area of the house (sum of all room & hallway height * width * ceiling height), then multiply that by 30 (assuming an air change every 30 minutes), divide by 60. That will give you a suggested CFM capacity for the fan. It'll be most effective if you can have air intake at the lowest level and at the extreme ends of the house on the larger upper level.
Also check your attic ventilation to make sure the air can get out. Divide the fan CFM rating by 750 to get sq ft of net free vent area (NFVA) needed. If your attic is ventilated to the 1:150 standard, that's probably enough. If it's ventilated to the 1:300 standard, or less, you'll need to look at adding more.
The Quiet Cool fans are very quiet, but very expensive. A much cheaper way to get the same effect is to suspend the fan from the roof joists on nylon webbing (at least 18-24", preferably 1.5x blade diameter or more below the roof deck), then use rubber backed carpet to make a tunnel from the fan to the ceiling louver. (Make sure to seal it airtight at both ends to draw air from the house, not from the attic.) This isolates the fan so it doesn't vibrate the interior walls and ceiling. Doing this cut the noise directly under my fan from 92db to 65db, quiet enough I can have a quiet conversation directly under it and can't hear it at all from 2 rooms away.
As others have suggested, if you're going for a high efficiency, airtight house an ERV/HRV would be better, but if your house is at least 10-15 years old it probably leaks enough that a house fan would be more cost effective.