Q-I have a whole-house fan in the second-floor hall ceiling of a house I just purchased. The fan and louvers look new and seem to operate smoothly, but the fan doesn’t seem to exhaust very much air. I have seen these fans operate very effectively in other houses. What could be the problem?
A-I hate to be the bearer of bad news, but you may have several problems. Fortunately, they are quite simple to diagnose. The effectiveness of a whole-house fan depends upon several key factors and everything must be just right or the fan won’t operate properly.
Whole-house fans are workhorses, designed to move great quantities of air. That is why they are able to provide comfort during periods of hot weather.
Their effectiveness is based upon an extremely simple principle. During the daytime, the air inside a house gets hot and stale. After sunset, the air temperature outside of your house usually is lower than the air temperature inside. A whole-house fan pushes the hot air out of your house and draws in cooler, outside air.
The air movement creates a cooling effect by increasing the rate of evaporation from your skin. The faster that perspiration evaporates, the cooler you will feel. If you can completely replace the air inside of your house every two minutes, you should experience this cooling effect.
Because houses come in a variety of sizes, whole-house fans come with various power ratings to accommodate the different volumes of air. The strength or power of a whole-house fan is measured by the cubic feet of air per minute (CFM) that it can push.
There is a simple way to determine the size fan you need for your house. Assuming you have standard 8-foot ceilings, simply multiply the total square footage of the finished living space of your house by three. This number will represent the minimum cubic feet per minute that the fan must push. Your whole-house fan should have a CFM rating equal to or greater than the number you calculated.
There is one other major consideration. These powerful fans must be able to easily push the hot air out of your house. They require openings in the roof or gable ends of an attic space to do this. If these openings are not large enough, the fan will spin ineffectively.
For every 750 CFM that the fan is rated for, you need a minimum of one square foot of free open exhaust area. Please note that insect screening placed over these openings can significantly reduce their free exhaust area. Many louvers and roof vents are stamped with a rating of their free open exhaust area.
Now let’s solve your problem. First, check to see if you have the right size fan for your house. Then, check to see if the exhaust areas in your attic are large enough to handle the CFM capacity of the whole house fan.
If your fan is belt-driven, make sure that the belt is adjusted to the proper tension. Based upon the model number of your fan, check to see if it is a multispeed fan or variable speed. Possibly your fan was operating at its lowest speed when you checked it.
By the way, did you have several windows or doors open when you turned on the fan? Remember, the fan must have a readily available and ample supply of replacement air.
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Have questions about the remodeling process? Write to Tim Carter, c/o the Chicago Tribune, P.O. Box 36352, Cincinnati, Ohio 45236-0352. Questions will be answered only in the column.
For a listing of manufacturers of whole-house fans and more tips on whole-house fan ventilation, send for Builder Bulletin No. 156. Please send $1.50 and a business-size, stamped, self-addressed envelope to Tim Carter, c/o the Chicago Tribune, P.O. Box 36352, Cincinnati, Ohio 45236-0352. You can obtain a free order form for a wide variety of individual job bid sheets by sending a business-size SASE to the same address.
