If you’re considering a whole house fan to complement your air conditioner, or to take the place of your AC, you first must evaluate your needs. The following guidelines will help:

* Physically go into your attic to see what you have to work with. A sign of poor ventilation is an unbearably hot attic in summer. Check for moisture, evident from any mold, mildew, rusted nail heads, damp or compressed insulation, or wood rot.

* Also look for vents that are already in place. These are gable vents, roof louvers, intake vents, and soffit and eave vents. Note the size and locations of each.

* Finally, estimate the attic’s square footage.

* Call your utility company. Many offer rebates for whole house fan installation. You may also be able to reclaim a portion of the cost as an energy tax credit on your income tax.

* According to most building codes, you need 1 square foot of vent area for each 150 square
feet of attic floor space. If a ventilation system is balanced, half of it will be provided by exhaust
vents in the upper portion of your attic, with the other half provided by intake vents.

* Do not underestimate your ventilation needs. Building codes specify minimums; you may want to increase that to ensure proper ventilation.

* Make sure to have a balance of intake and exhaust avenues to properly ventilate your home. In the attic, remember that the flow of air is limited by the amount of intake.

* Many companies offer 10-year warranties. Make sure you inquire before making a purchase.

Traditional whole house fans tended to be noisy, consume more energy, require maintenance (belt-driven ones needed the belts changed periodically), and were not well insulated. The newer whole house fans are well engineered to take care of these problems in multiple ways. Look for:

* The number of blades—the more blades, the quieter the fan will be because each blade has to do less work.

* Have the whole house fan isolated from your home’s framing with foam strips or rubber mountings. That way the motor hum will not resound through the framing and drywall. Also, make sure your unit has a welded frame so it will not loosen up and begin to squeak.

* Quality shutters—the better the quality, the less thumping noises will occur when the unit shuts off. The latest fans have self-sealing insulated shutters that close when the fan is not operating, which prevents heat from escaping when the unit is not in use.

* Timers are recommended over thermostats for whole house fans because timers won’t overcool the house at night. Thermostats could turn the fan on when no one is home (i.e., no windows or doors are open), or a fire in the fireplace could trigger the fan to go on.

* Ideally, look for a larger fan that will run well at a lower speed. A heavier unit is also recommended because it rests on foam weatherstripping, held down only by its own weight, and the lower speed uses less energy.