Dear Jim: I want to landscape my house to cool it with less electricity, but I do not want to block the winter sun. Will trees help much, and if so, where should I locate them and what types are best?

- Steve K.

Dear Steve: By planting the proper types of trees in the proper locations in your yard, you can cut your air-conditioning costs by 25 percent or more. By shading your walls, your house stays cooler and you feel more comfortable. You can feel the difference when sitting indoors near a shaded wall.

Trees are nature's air conditioners. A single large tree can produce as much actual cooling in a single day as running a large room air conditioner. When you include the benefit from the shade that it provides, the combined cooling effect from just one tree is significant.

Trees cool the air by a natural process called transpiration. The tree continually draws water from its roots, which eventually evaporates from its leaves. This evaporation process cools the tree and the air around it just like when you perspire. The air near a tree can be 10 degrees cooler.

On a grand scale, planting trees reduces global warming. Trees consume carbon dioxide (greenhouse gas) and produce oxygen. Also, by reducing electricity usage, less carbon dioxide is emitted from power plants.

To landscape efficiently with trees, the basic goal is to block the cold winter winds and the intense summer sun while letting the beneficial winter sun and summer breezes through.

This typically includes a row of deciduous trees (ones that lose their leaves in the winter) from the southeast to the southwest sides of your house. Plant evergreens to the north. In most climates, leave a small gap on the southwest side to allow the prevailing evening breezes through.

When selecting trees, consider the height and shape of the tree as it matures. This allows you to determine how many to plant and how far to locate them from your house for shading. Growth rates and winter hardiness are also important selection criteria.

The branch pattern of some trees is too dense to be used on the south side. They are great for blocking the summer sun, but they also block too much of the winter sun's free heat. Check out varieties with a local landscaper.

Write for (or instant download - Update Bulletin No. 872 - a selector guide of 100 types of trees, mature heights and shapes, growth rates, hardiness zones and recommended landscape layouts for the various climates. Please include $3 and a business-size SAE.

James Dulley, Deseret News, 6906 Royalgreen Drive, Cincinnati, OH 45244

Dear Jim: I installed a whole-house fan in our hall ceiling last fall. I know it saves electricity as compared to air-conditioning, but how do I know when to turn it on?

- Sue C.

Dear Sue: Running a whole-house fan, especially the new small twin-fan models, is much less expensive than running a central air conditioner.

If you feel comfortable with the fan on, it probably is the proper time to use it. As a rule of thumb, in dry climates, turn it on when the outdoor temperature drops to the same as indoors. In humid climates, use it only when the outdoor temperature is at least 10 degrees cooler than indoors.