Attic fans work by pulling heated air out of the attic, allowing it to be replaced by cooler outdoor air through the soffit and gable vents. Their effectiveness depends on exterior air temperatures, humidity and whether the attic fan is designed to vent only the attic or the whole house.
Attic fans designed to vent only the attic usually fail to lower cooling bills. The reason is that even in a home with good insulation between the living space and the attic, the attic fan tends to pull up air conditioned air into the attic, forcing the air conditioner to work harder to keep the living areas of the house cool. Attic fans can also cause backdrafting from combustion gas appliances, possibly introducing carbon monoxide vapor to the interior atmosphere.
Whole house fans are mounted in the attic, but are designed to pull exterior air through the entire house. Their primary use is as a substitute for air conditioning in areas with low relative humidity, cool nighttime temperatures and mild winters. To work properly, at least a few downstairs windows must be open while the fan is in operation, making whole house fans impractical in areas where security concerns forbid leaving a window open. Whole house fans need at least 1 square foot of net-free attic venting or 1 1/2 square feet of venting with insect screens for every 750 cubic feet per minute of fan capacity, which can make the house harder to heat in winter.