A whole house fan benefits building occupants by cooling a structure and exhausting stale air. It can be used in conjunction with A/C to reduce cooling costs, or alone in less humid regions of the country to create a more natural way to beat the heat. At just a fraction of the cost of A/C ($150 to $350 vs. $2,000 to $4,000), whole house fans can run at .01 to .05 cents per hour. Compare that to the .20 to .25 cents per hour it takes to cool with A/C.
Whole house fans work best when used at night, when outdoor air is cooler than warm indoor air. In high heat and humidity, run the A/C during the day. Once the temperature starts to drop for the evening, turn off the A/C, open the windows, and run the fan. Closing windows in unused rooms will increase cooling to more populated rooms of the house. The fan can be run during the day if temperatures are below 85 degrees F. If temperatures are predictable, a timer can be used to turn the fan on and off—just be sure to open windows before turning on the fan and close windows before running A/C.
Sometimes you can find a two-speed model that will allow ventilation on high speed for immediate cooling and low speed for gentle, quieter circulation. For sizing, it’s important to strike a balance between exhaust and air intake. Look for a design that will provide one square foot of attic ventilation for every 300 square feet of total attic space. You can also select a fan based on total home square footage x ceiling height (usually eight feet). The fan should deliver 1/2 to 1 times the amount of CFM (cubic feet per minute) at .1 inch static pressure (pressure exerted by air).
Since whole house fans can pull large quantities of air from a home, take care to avoid backdrafts in combustion appliances. Be sure to keep windows open to avoid accidents. Educate occupants on the use of the fan—let them know that the fan should only be turned on when at least two windows are open and A/C units are turned off. It is not recommended to use a thermostat to control a whole house fan.