Even with air conditioning you need a whole-house fan

When I installed central air conditioning a few years ago, I could hardly wait to get rid of my 20-inch window fan. At that time, air conditioning cost less than a dollar a day, and the fan was nothing more than a nuisance.

But by last summer, my air-conditioning costs had nearly quadrupled. I began thinking about that slightly ugly and somewhat noisy fan, remembering how I had managed to cool my house with it on moderately hot days at a negligible operating cost. Right now, similar window fans cost up to $180. But for a little more money I found a better way to cut the cost of keeping cool: a whole-house fan.

I installed a fan in the ceiling of my central hallway, and began trial-and-error experiments with the airflow in my house (see drawing). After I developed a good ventilation scheme, I was able to get along without central air conditioning for all but a few days last summer. Depending on your house and climate, you too may be able to take care of most or all of your cooling needs this way. Whole-house fans are designed for easy installation and operation. Here’s how they work:

Ceiling exhaust fans draw air up through the house and push it out through attic gables or eave openings. For the fan to operate properly, you must open some windows, and perhaps exterior doors. Obviously, all hall doors must be ajar. Deciding which windows to open and fine-tuning your plan may take some time, but it will pay off in efficiency and comfort. One word of caution: You may be tempted to open doors and windows in a cool basement, but it’s more effective to bring air in through first-floor windows. And if you live in an area where radon is a concern, it’s especially important to circulate fresh outside air.

Early in the morning, when the house is cool, close your windows and doors. Keep blinds and shades drawn, to reflect sunlight. It takes time for heat to penetrate the walls and ceilings of your house. Outdoors, it’s usually warmest about 3 p.m., but the indoor peak generally occurs between 5 and 6 p.m. Wait until the outside temperature in the shade falls below the indoor temperature, then open your windows and turn on your fan.

As the fan pulls cool air into your house, it dramatically lowers the air temperature in the attic, which can reach 150 degrees F or so without air movement. The fan provides almost instantaneous relief because moving air feels about seven degrees cooler than static air. Makers of whole-house fans say–and I found it to be true–that the fans have limited usefulness when the outdoor temperature rises above 85 degrees F. That’s because most people are comfortable with air temperatures of 78 degrees or less.

Last summer I resorted to my central air only on the few days when temperatures reached the 90s and low 100s and humidity was high. Even then, I first ran the fan at full speed to purge hot air from the attic before starting the air conditioner. As soon as the outside temperature dropped in the evening, I switched back to the fan.

It’s difficult to compare last summer’s cooling costs with those of earlier years, because the weather was unusual in my area–with wide hot-cool swings and a shorter-than-normal hot spell. But I do know that when I run the ceiling fan, I am paying for only 1/3 horsepower. The air conditioner, combined with its fan and the furnace blower, is rated at three horsepower. So the fan’s operating cost is about 1/10 that of air conditioning. I expect to recover its cost in power savings within a couple of years.

My 30-inch variable-speed fan from Sears cost about $280. Smaller whole-house fans are available for as little as $100. The fan blades are direct- or belt-driven, with diameters of 20 to 42 inches. The motors are about 1/4 to 3/4 hp. Some are single-speed; others have two speeds or variable speeds. The fans can move anywhere from 3,000 to 17,000 cubic feet of air per minute.

The most popular fans are 24- and30-inch units, used in 1,200- and 1,700-square-foot houses, respectively. Airflow for these fans is 3,600 to 5,700 cubic feet per minute. Check fan specifications to find out which size you’ll need. When in doubt, choose the larger, higher-volume fan.

You can use a whole-house fan in a two-story house, too, but you should choose a fan based on the total square footage of both floors. Ideally, the fan should be centrally located in the upstairs hallway. One maker has this two-story tip: Keep the upstairs windows closed until evening, pulling all the air through the lower windows. When you retire, close the downstairs windows and open the others.

You may want to add accessories to your fan. I spent about $60 for a thermostat, an insulated winter cover, and a “firestat’–a safety device that shuts off the fan at 204 degrees F. In a fire, a running fan could act like a blast furnace. The fire stat also has a kill switch to shut down the fan when you are working around it. Never work near a powered fan.

My unit also has an automatic temperature control that shuts down the fan when the house cools to a selected temperature, but the fan must be manually started. The reason? To make you remember to open enough doors and windows to provide the minimum air intake specified for your fan. If you don’t furnish an adequate air supply, your fan may draw gases from the flues of fireplaces, furnaces, or water heaters inside your house.

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