House & Home: Fan-Tastic Gizmos

March 31, 2007

James Dulley

Plan now for a cool, energy-efficient summer with attic and whole-house fans.
by James Dulley
Q: Our house feels warm even with the air conditioner running. The temperature seems to be low enough for comfort. Could the heat be coming from the attic? Will solar-powered attic fans help much? – Caroline J.

A: Your situation is not uncommon. My guess is, if you put your hand on the ceiling, it will feel warm. Even though the room air is cool, you feel warm from radiant heat transfer. It is the reverse of when you feel chilly during winter sitting near an outside wall or window.

In addition, your central air conditioner is running overtime to remove this excess radiant heat coming down through the ceiling from a hot attic. If you compensate by setting the thermostat lower to stay comfortable, your electric bills go even higher.

A solar-powered attic exhaust fan is a good choice to help keep your attic and roof, and thus the ceiling below, cooler. A solar-powered fan is an ideal match for your needs because as the sun is more intense on the roof, it also creates more free solar electricity so the fan runs faster.

Another monetary benefit of keeping your attic and roof cooler is the life of the shingles will be extended. High heat hastens the degradation of just about any material used in a house. By locating one or two solar fans up near the peak of the roof, the hottest air will be drawn out of the attic.

You don't have to be an electrician to install a solar attic fan. The solar panel is built into the top of the fan and it produces free electricity. To install one, saw a 12-inch-diameter hole in the roof and loosen a few shingles. Nail the fan in place and replace the shingles. Curb-mounted models are also available for flat and tile roofs.

If the area of your roof where you want to install the fan does not face the sun directly, you can install a remote fan. It uses a similar fan, but it has a separate solar panel. The one-foot-square solar panel, on an adjustable stand, can be mounted up to 20 feet away in a sunny location.

The remote panel may be the most efficient option because the small solar panel stays cooler than when it is built into the top of the fan. The hotter solar cells get, the less free electricity they produce. The small wire that carries the safe 12 volts can barely be seen on the roof. You may also consider attaching two solar panels to one fan for extra cooling.

Yet another installation option to consider is a built-in design with a hinged solar panel. It can be tilted up from the fan top to face the sun more directly. Also, with the solar panel tilted upward, even at a slight angle, air flows around it to keep it cooler and raise its efficiency.

Download bulletin No. 987-buyer's guide of 11 solar-powered attic vent fan and turbine fan manufacturers listing air flow ratings, watts, sizes, features, prices, sizing chart for number of fans required per square foot of attic, installation methods with illustrations, and descriptions/illustrations of various models, $3: www.dulley.com

Q: On mild evenings, I open windows to get some fresh, cool air into my house. Would using a whole-house fan draw fresh air in quicker? Do these fans cost less to operate than my central air conditioner?

 – Gary W.

A: With today's airtight, energy efficient homes, it is nice to get fresh air indoors whenever possible without driving up your utility bills. Using a whole-house fan is the most effective and efficient means of bringing fresh air indoors quickly when the outdoor temperature drops.

A whole-house fan is a large fan that mounts in the attic floor. It is often located over a hallway to draw air from the entire house without creating drafts or noise in the rooms. The fan draws outdoor air in through opened windows and exhausts the air into the attic area and out the vents.

A secondary benefit of using a whole-house fan is the air flow through the attic helps to cool the attic and the roof. A hot attic and roof can radiate heat down through the attic insulation to the living areas below.

A whole-house fan uses about 80-90% less electricity than a central air conditioner. Using one can save more than $100 a year on your electric bills and reduce the maintenance needed on your air conditioner.

During hot summer weather, run the central air conditioner during the day and then use the whole-house at night. Whenever the outdoor air temperature drops to about five degrees lower than your thermostat setting, running the whole-house fan will comfortably cool your house and create a gentle breeze indoors.

There are many whole-house fan designs with various features from which to choose. For most average-sized homes, a direct-drive model with the motor in the center of the fan is a good choice. The quietest models have vibration-blocking rubber hubs and sound-absorbing air flow shrouds.

For higher air flow capacity, a belt-drive model is often used. With this design, the motor is mounted on the corner of the frame. A belt runs from a pulley on the motor to a pulley on the fan hub. With the motor out of the air flow path and a large open diameter, the air flow can be greater.

For sizing a whole-house fan, a rule of thumb is the air flow in cubic feet per minute should be three times the house size in square feet. Some fans have two or variable speeds, so you can size it bigger for quick cooling.

For the greatest convenience, choose one with a built-in timer, thermostat or humidistat for automatic operation. Some automatically switch from high to low speed after 30 minutes. Most models offer air sealing shutters to seal off the fan when it is not running. Motorized insulated shutters are also available.

Download bulletin No. 641 – buyer's guide of eight whole-house fan manufacturers (28 models) listing blade diameters, maximum air flows, rpms, drive types (direct or belt drive), features, prices, suggested operation of a whole-house ventilation fan, and do-it-yourself installation instructions with a diagram, illustrations, diagram and explanation of how whole-house fan draws in cool air and expels hot air, $3:

www.dulley.com Send questions to James Dulley c/o greta@catalystmagazine.net.