Q&A / 

Whole House Fan Sizing

DEAR TIM: I have a whole house fan located 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 in other houses very effectively. What could be the problem? Laurie B., Beavercreek, OH

DEAR LAURIE: 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 setup features. Everything must be just right, or the fan will not operate properly.

Whole house fans are work horses. They are designed to move great quantities of air. That is why they are able to provide comfort during periods of hot weather. The principal by which they operate is extremely simple. During the daytime, the air inside of your house gets hot and stale.

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Generally speaking, after sunset the air temperature outside of your house is lower than the air temperature inside. A whole house fan pushes the hot air out of your house and replaces it with the cooler, outside air.

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.

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Because houses come in a variety of sizes, whole house fans must be made to accommodate the different volumes of air. The strength or power of a whole house fan is measured by the amount of cubic feet of air per minute (CFM) that it can push. These ratings are commonly in the range of thousands of feet per minute.

There is a simple way to determine the size fan you need for your house. Assuming you have standard eight foot ceilings, simply multiply the total square footage of the finished living space of your house by three.

This number represents the minimum cubic feet per minute that the fan must push. Simply purchase a fan that has 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, you need a minimum of one square foot of free open exhaust area. Beware, 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 multi-speed 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 the fan on? Remember, you have to provide the fan with a readily available and ample supply of replacement air.

Author's Notes

February, 1999

There are new variations on the traditional monster attic fans. You can now purchase quiet dual blade fans that operate at lower CFM rates - up to 1,000 CFM - and still do a very good job of moving air through your home. These fans have insulated doors as well.

CLICK HERE to get FREE & FAST BIDS from local contractors who can install the correct size house fan for your home.

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