Q: Dear Jim: On mild days and evenings, I like to open windows to get some fresh, cool air into my house. Would using a whole-house fan draw the air in quicker? Do these fans use much less electricity than my central air conditioner? - Gary W.

A: Dear Gary: With today's airtight, energy efficient homes, it is nice to bring in cool outdoor air whenever possible and accomplish this without driving up your utility bills. Using a whole-house fan is probably the most effective and efficient means to quickly cool your home when the outdoor temperature drops overnight or on moderately warm days.

Pulling fresh outdoor air into your house not only lowers the temperature indoors, but it creates a gentle breeze similar to a ceiling paddle fan. With the air moving throughout your home and across your skin, you can feel five to ten degrees cooler than in still air at the same temperature. Actually, just the sound of the moving air can have the psychological effect of making you feel cooler.

A whole-house fan is a large exhaust fan that is usually mounted in the attic floor. It is often located in a hallway ceiling to draw air from the entire house without creating drafts in the rooms. Since it is most often used at night, this location also minimizes the noise level in the bedrooms. The fan draws outdoor air in through opened windows and exhausts the air into the attic area and out the attic vents.

A secondary benefit of using a whole-house fan is the airflow from the house into the attic helps to quickly cool the attic and the roof. A hot attic and roof can radiate heat down through the attic insulation to the living areas below. This will occur all day long and, without a fan, well into the evening until the roof and attic structure slowly cool down.

Running a whole-house fan uses about 80 to 90 percent less electricity than operating a central air conditioner. Using one can easily save more than $100 per year on your electric bills. Since your air conditioner is used

Fan provides efficient, comfortable cooling

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less, less maintenance is needed and its life will be longer.

Whenever the outdoor air temperature drops to about five degrees lower than your thermostat setting, running the whole-house fan should comfortably cool your house and create air movement indoors. This means you may run your central air conditioner during the daytime and run the whole-house fan at night.

There are many whole-house fan designs with various features from which to choose. For most average-sized homes, a direct-drive design is a good and easy-to-install choice. It has the motor located in the center of the housing with the fan blades attached directly to the motor shaft. The most quiet models use special vibration-blocking rubber hubs and sound-absorbing air flow shrouds to reduce the noise level to a whisper.

For larger houses that require a higher airflow capacity to cool them, a belt-drive model is often used. With this design, the motor is mounted on the corner of the housing frame. A belt runs from a pulley on the motor to a pulley on the fan blade hub. With the motor out of the airflow path and a larger unobstructed blade diameter, the airflow can be greater. These often use a steeper blade pitch and run at a lower speed to further reduce the noise level.

When sizing a whole-house fan, a good rule of thumb is the air-flow rating in cubic feet per minute should be three times the house size in square feet. If you choose a model with two or variable speeds, you can install a larger, higher capacity fan. This will allow you to run it on high speed initially to quickly cool your house and then set it to the correct continuous speed for your house size.

For the greatest convenience, choose one with a built-in timer, thermostat or humidistat for automatic operation. Some automatically switch from high to low speed after 30 minutes. Most models offer air-sealing shutters to seal off the fan when it is not running. Motorized insulated shutters, to block winter heat loss more effectively, are also available on some models.

Write for (instantly download - www.dulley.com) Utility Bills Update No. 641 - buyer's guide of eight whole-house fan manufacturers (28 models) listing diameters, airflow capacities, speeds, drive types, wattages, features, prices, and Do-It-Yourself instructions. Please include $3.00 and a business-size SASE. James Dulley, Publication Name, 6906 Royalgreen Dr., Cincinnati, OH 45244.

James Dulley is a mechanical engineer who writes on a wide variety of energy and utility topics. His column appears in a large number of daily newspapers.