Can Attic Fans Reduce Air Conditioning Bills?

| 8/12/2009 12:00:00 AM

I’ve seen conflicting information about attic fans. Some sources say they’ll lower air conditioning bills by reducing the temperature of air in the attic. Others say they’ll draw conditioned air into the attic and increase energy use. Are attic fans effective in reducing air conditioning bills?

In general, attic fans are not effective at reducing air conditioning costs. The benefit from lowering temperatures in the attic (and thus reducing heat gain from the attic into the house) is usually off-set by the electricity use by the fan and the increase in air leakage from the house into the attic that can result from the pressure drop created by the attic fan. It’s usually better to take the money you would spend on an attic fan and instead spend it on increasing the insulation levels in the attic and the airtightness of the attic floor plane.

Now, a whole-house fan is a different beast, and these can make sense in certain climates when used properly. This is a high-volume fan (sometimes installed in a ceiling opening that extends into the attic) that is used in place of mechanical air conditioning at night. This cooling strategy only works in climates where there is a significant day-night temperature swing, where the nighttime temperature drops below about 65 degrees, and where outdoor humidity levels aren’t too high. Here’s how it works: During the day, the house is kept pretty closed up with windows closed and blinds drawn in unused rooms to minimize unwanted heat gain. Then at night, windows are opened up and the whole-house fan is operated. The fan exhausts air from the house, and cool outside air is drawn in, replacing it. The energy needed to operate the fan is a lot less than the energy needed to operate compression-cycle air conditioning.

— Alex Wilson, Founder of Environmental Building News, Publisher of the GreenSpec Directory  of green building products, Publisher of  and  

M Fowler
11/3/2010 8:52:20 AM

I am a fan of the whole-house fan. My grandparents had one in Louisiana. You had to get a blanket before the end of the night. Now, in middle FL, the fan does a great job on most days. There are times here that the humidity and night temperature is too high. We adjust.

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