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Warning on House Fans, Attic Ventilators

House Doctor | Gary Dymski
September 7, 2000

Q. I read your Aug. 10 column on attic ventilators and whole-house fans with increased feelings of foreboding. It has been my experience that improper use of ventilators and whole-house fans can be dangerous and life-threatening.

I have been an oil-heat service technician for more than 25 years. In that time I have found that the use of whole-house fans should be avoided in homes with oil or gas heat.

Whole-house fans exhaust the interior living space air and draw in outside air to replace it. The problems occur when the fan is attempting to exhaust the air faster than the replacement air can enter the home. When this happens, a negative air pressure occurs within the home and the potential for catastrophe is great.

A negative air pressure may cause the air flow within the home's chimney to be reversed. If the chimney's air flow is reversed while a gas or oil heating unit is operating, the combustion gases from those units will be drawn into the living spaces of the home. When combustion gases are drawn into the living spaces, there are chances of carbon-monoxide poisoning or a fire.

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My advice is that if a homeowner insists on using a whole-house fan, take steps to assure that all gas or oil units won't operate simultaneously with the fan.

-Andrew A. Amador, Holtsville

A. Negative pressure can be caused when whole-house fans and ventilators are improperly installed or used. But negative air pressure also can be caused by improperly venting a central air conditioning system. It's not the appliance -- the fan or the air conditioner -- that's the problem; it's the installation and usage.

I still think these fans and ventilators can do an excellent job of cooling during the summer months. They are less expensive to operate than air conditioners. For some people, air conditioning works too well, and they can't help but feel cold all the time.

The solution, of course, is to make sure your whole-house fan or attic ventilator is installed correctly and doesn't create a negative air flow.

First, never use a whole-house fan with the windows closed or while a furnace or air conditioner is operating. In fact, I'd advise opening every window in the house when the fan is in use. Also, install carbon-monoxide detectors in all bedrooms. If you have CO detectors, then make sure they are working properly.

Whole-house fans and attic ventilators need fresh air to operate properly. Be sure your roof has the proper venting for the fan or ventilator you are using. Most manufacturers provide excellent installation instructions, including the number of exhaust and intake vents that should be used for each model of fan. The rule is that a fan should have one square foot of exhaust vent space for every 750 CFM (cubic feet of air per minute) the fan moves.

Finally, if you aren't sure about installing a fan correctly, have a professional do it. Consult with a qualified roofing contractor or heating and cooling contractor. If you are concerned about negative air pressure, ask a heating and cooling contractor to inspect your fuel-burning appliances. Each appliance should have adequate sources of intake air and exhaust.

Copyright © 2008, Newsday Inc.

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