Summer heat can mean soaring energy bills for operating your air conditioner, but there’s a low-cost alternative that often gets overlooked: the whole-house fan.

Whole-house fans offer an installed cost that’s a fraction of central air conditioning, and most installations can be handled by a do-it-yourselfer. Best of all, operating costs can be as little as 10 percent of the cost of operating an air conditioner. A whole-house fan may be all the cooling you need in milder climates, while in hotter areas it can supplement your air conditioning and reduce your energy bills.

How they work

The fan is mounted in the attic so that it opens down through the ceiling in a central location, typically a central hallway in a one-story house or above the stairs in two-story homes. The fan is activated once the air temperature outside is cooler than inside. It draws a large volume of air into the house through open windows, pushing the warmer inside air into the attic and then out through the roof and gable end vents.

This results in three different cooling actions. Hot, stale inside air is replaced with cool, fresh air; hot air is flushed out of the attic, lowering the attic temperature and reducing heat that radiates back into the house; and a gentle breeze is created that cools you naturally due to its evaporative effect across your skin.

Also, by removing all that hot air from the attic, you’re helping extend the life of your roofing. And, flushing the stale air out of living spaces and replacing it with fresh air helps rid your home of unhealthy concentrations of indoor pollutants. Whole-house fans are also useful on mild winter days for flushing out stale odors.

Shopping for a whole-house fan

You’ll find a wide variety of whole-house fans on the market, but they fall into two broad categories: ducted and nonducted. The nonducted fan is the traditional design, and is the most common. It consists of a large, horizontally mounted, propeller-style fan blade in a square box that’s open on the top and bottom. Depending on the size and design, the box may sit on top of the ceiling joists, or it may sit between them, in contact with the top of the ceiling. Some types require cutting and boxing the joists, while others use a skirt instead.

Smaller nonducted fans are direct-drive, with the fan blade attached directly to the motor. These are the least expensive models, but they have less capacity and are also noisier and tend to have more vibration. More common is the belt-drive design, with the motor to the side of the fan blade and connected to it with a belt and pulley system. This allows for larger-diameter, more steeply pitched fan blades that move more air, in addition to quieter operation and less vibration.

On the ceiling is a large louvered cover. Springs hold the louvers closed, and when the fan is activated, the air flow pulls the louvers open. The louvers make a fairly decent seal against air leakage when closed, but winter covers are available for greater energy efficiency, and are highly recommended. Some styles have retractable energy-efficient covers in the attic that open and close with the fan.

With the ducted style, you have a fan box that sits on top of the joists in the attic. From the box, three or four individual ducts extend to different rooms or zones in the house, and are connected to a louvered vent that’s cut into the ceiling. The exhaust port from the fan box exhausts directly into the attic.

Ducted whole-house fans are typically quieter, because the fan motor is enclosed and more isolated. Since the box sits on top of the joists, there’s no cutting involved in the installation. Also, the ducting allows greater regulation of cooling and ventilation, even with the doors closed. On the downside, these types of units are more expensive to purchase.

Residential whole-house fans are 115 volts, and they draw between 3.5 and 9 amps. Depending on the size and design of the unit, they can be controlled by a simple on/off switch, a two-speed or variable-speed switch, or a 12-hour timer. Consult with the manufacturer’s instructions or a licensed electrician for complete wiring and installation details.

Sizing the fan

Whole-house fans are rated in cubic feet per minute (CFM) of air flow, and need to be sized to the square footage of your home. It’s typically recommended you have a minimum of 2 CFM of air flow per square foot of living space. For faster, more efficient cooling and to feel more of the effects of the evaporative cooling on your skin, you should increase that to 3 CFM per square foot. So, a fan rated at 4,500 CFM would be good for a home of 1,500 to around 2,250 square feet. Larger homes can be served by two fans to provide better air flow and more control over cooling.

The stale air in the attic needs to exit to the outside, so attic ventilation is a very important part of your calculations. Here, the rule of thumb is to divide the capacity of the fan in CFM by 750 to arrive at the total square footage of attic ventilation area needed. For a 4,500 CFM fan, you would need approximately 6 square feet of attic ventilation area.

Whole-house fans are available at many home centers, lumberyards, and plumbing and electrical retailers, in stock or by special order, as well as on the Internet.

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