Q - I was considering installing an electric attic fan to exhaust the hot air from my attic. Are they really effective and how can I determine the proper-sized attic fan to install? B. G.

A - Much of the heat entering your home is from your roof and attic. An electric attic vent fan is a very effective method to lower attic temperatures. That significantly reduces the heat flow into your house and your air-conditioning costs.There are two basic types of attic vent fans. One type mounts in the gable and the other type in the roof near the peak. Both types are very easy to install yourself. Always have a helper and use safety lines when you are working on a roof.

If you have soffit vents under the roof overhang, then a roof-mounted attic vent fan is very effective in both summer and winter. In the summer, it brings in the cool outdoor air over the hot insulation and exhausts it near the peak of the roof. That cools the insulation and the entire attic, so less heat is transferred into your house.

In the winter, the air flow over the insulation keeps moisture-laden air from becoming stagnant in your attic. During cold spells, water droplets can form in stagnant moisture-laden air. That can soak your attic insulation and cause serious problems.

Attic vent fan capacities are measured by the volume of air flow that they produce, in cubic feet per minute (CFM). A rule of thumb is that you need 0.7 CFM of fan capacity for each square foot of attic floor area. If you have a dark roof, add an additional 15-percent capacity. The capacities of various vent fans range from about 900 CFM to 1,600 CFM.

In order to be effective, you must have adequate inlet vent area to let the cooler air into your attic. Typically, one square foot of inlet vent area, preferably in the soffits, is needed for each 300 CFM of attic vent fan capacity. That is actual net open area, not just the dimensions of the vent. The vent packaging usually lists the net free vent area.

Many attic vent fans come with an adjustable thermostatic control that mounts in the attic.

You can write to me for UTILITY BILLS UPDATE No. 097 showing a buyer's guide list of attic vent fan manufactures, model numbers, and CFM capacities, product information and specifications, and a chart to determine the proper size of fan for your attic. Write to James Dulley, The Deseret News, 6906 Royal Green Dr., Cincinnati, Ohio 45244. Please include 75 cents (no checks, please) and a self-addressed envelope.

Q - Our vented crawl space always seems cool. Is it possible to draw the cool air up into our house with an exhaust fan in a window? R. C.

A - A crawl space can work as a mini cool tube. The ground and the concrete foundation walls act as a great heat sink to cool the air drawn through it during the day. If it gets cool enough at night, the air flowing through it keeps it cool for the next day.

Cut an inlet hole in your floor and place a strong register/grill over it.

You should cover the dirt crawl space floor with polyethylene vapor barrier film. This reduces the amount of moisture picked up by the air and minimizes a warm muggy feeling.