Q. My neighbor says he keeps his home cool by turning on his whole house fan just twice a day, in the morning and late evening, while our house stays too hot to sleep comfortably, even if we keep the windows open all night. How does it work?

A: A whole house fan quickly pulls large amounts of fresh cool air from the outside through the house and pushes all the hot air out the attic in minutes. If the temperature outside drops from 85-degrees to 75-degrees in two hours, for example, the air in the house will take about four hours to drop that much.

A whole house fan can do the job in less than half that time. Since it pushes air out the attic, it also cools the attic and reduces heat gain in the house. So by turning on your fan in the cool morning hours, you can bring in comfortable air, then close up the house and avoid the searing heat of summer. Turning it on again in the evening ensures all day comfort on all but the hottest days.

Q: How much electricity does a whole house fan use?

A: The operating cost of a whole house fan is about one-tenth that of air conditioning, according to Consumer Research. Using a whole house fan instead of air conditioning continuously can cut your electricity usage by 80 percent. The fans typically draw 400 to 600 watts of electricity, operating for eight cents or less per hour, according to Consumer Reports magazine. Therefore, under the right circumstances a whole house fan can ventilate an entire house on the electricity an air conditioner would use to cool one room.

Q: My house is hottest in the late afternoon and early evening, but a whole house fan can't be turned on until much later in the day after the outside temperature has dropped. How can a whole house fan save money if we need to use the air conditioner too?

A: A whole house fan will allow you to run your air conditioner far less often during the day. It is the same principle as getting into a hot automobile: It is much more efficient to open the windows during the first few minutes of driving to push the hot air out than it is to turn on the air conditioner to cool all that hot air.

With a whole house fan, you can cool the house with fresh outside air in the early morning, turn on the air conditioning later in the morning to simply maintain that temperature, then shut if off in the evening once the outside temperature drops enough to turn the whole house fan on again. You will find this much more economical than running the air conditioner all day long.

Q: Aren't whole house fans noisy to operate?

A: That depends on the type you use. Direct-drive fans, which mount the fan blades directly under the motor and are attached to the motor shaft, can be quite noisy. Belt-driven fans are much quieter. They also create higher air flows than direct-drive models, making them much better suited for medium to larger houses.

The quietest, most efficient whole house fan for bigger homes has six blades and spins at 300 to 500 rpm to make a purring sound no louder than the typical air conditioner fan.

Q: How do I know what size fan I need?

A: Fans range from 24 to 36 inches in diameter. You must tally the number of cubic feet of space, excluding closets, store rooms and the attic, to determine which size you need.

Q: In the installation complicated? Could I do it myself?

A: A whole house fan can be wired into an existing electrical circuit and most fans are designed to slide into place easily, requiring no cutting of trusses. Direct-drive whole house fans are available at home centers for do-it-yourself installation, but these are noisier and less efficient than belt-driven models, making them inappropriate for medium and larger homes.

by Kathy Maynard, reprinted courtesy of HomeAdvisor.com