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.
