Q: My attic gets above 150 degrees in the sun and I plan to install a temperature-controlled attic vent fan to lower my air-conditioning costs. What type of vent fan is best and how large a vent fan should I get?
A: Installing an automatic attic vent fan can lower the air-conditioning load. By cooling the attic temperature as much as 50 degrees, you should also feel much more comfortable inside your house, especially in second-floor bedrooms. High attic temperatures can also deteriorate the lumber.
There are several attic vent fan choices available. A solar-powered electric attic vent fan is the most efficient, easiest to install and runs for free.
Since it is powered by the sun, the brighter and hotter the sun is, the faster the fan runs. This makes it a perfect fit for attic cooling.
Some designs have the small solar cell panel (converts sunlight directly into electricity) built into the top of the vent fan.
Others use a small (less than one-foot square) solar cell panel on the roof next to the fan. You can use metal angles to face the small panel more directly toward the sun for more electricity output and better cooling.
You remove a few shingles and saw a 12-inch hole in the roof. Nail the 12-volt fan in place and seal around it. Replace the shingles.
Run the wire from the solar cell panel to the fan and it starts. You can also get solar-powered vent fans designed to mount vertically in the gables.
An automatic temperature-controlled 120-volt electric attic vent fan is also effective for cooling your attic. These can provide up to 1,600 cfm (cubic-feet-per-minute) of air flow to cool even large attics.
An adjustable thermostat is used to control the temperature at which the attic vent fan automatically switches on. The thermostat keeps it from running too much and wasting electricity.
In the winter, you can install a humidistat to control the fan. This eliminates potential moisture problems during extremely cold spells. With adequate attic floor insulation, it will not increase your heating bills.
A rule of thumb for sizing an attic vent fan is that you should have a minimum of 10 complete air changes per hour. With a typical pitched roof, a 900-square-foot attic would require a vent fan capacity of 630 cfm. A 1,500-square-foot attic would require 1,050 cfm vent fan.
The best location for the inlet air vents is under the soffits (roof overhang).
You can write to me for "Utility Bills Update No. 023" listing addresses and telephone numbers of manufacturers of solar-powered and electric-powered attic vent fans, air flow capacities in cfm, product information and a chart showing recommended sizes for vent fans for various attic sizes. Please include $1.50 and a self-addressed business-size envelope.
Questions should be addressed to James Dulley, c/o Baltimore Sun, 6906 Royalgreen Drive, Cincinnati, Ohio 45244.