Q - I would like to use some free solar heating to lower my utility bills. Would you explain how a solar Trombe wall works? Is it something that I can attach to my house myself? K.L.

A - A Trombe wall can be a very effective solar heating method for new and existing houses. With the proper design, size and number of air vents, you can control the amount and timing of heat output into your house.You should be able to add a Trombe wall yourself. If your house already has masonry walls, you can add a glass covering over a small section of it. For a frame house, you can remove part of the wall and add a small masonry section. A modified design can also be built below a window.

A Trombe wall is basically a large glass window placed a few inches outside of a masonry wall. It is often called one of the "mass under glass" solar heating techniques that requires no fans or pumps.

Vent holes are cut through the wall at its bottom and top. As the sun heats the brick, block or stone wall section behind the large glass, the air between the wall and the glass gets hot.

This hot air rises and flows out the top vent holes into your room. Cool room air is drawn in the bottom vents. During the daytime, this provides heated air flow as the wall itself slowly gets hot and stores solar heat.

At night, you close off the vents to block any reverse air flow between the wall and the glass. Then heat from the warm masonry wall slowly radiates into the room to provide even heating.

For a kitchen or dining area where you want heat as early as possible, larger vents carry in heated air more quickly. For a bedroom or living room, where you want heat later in the evening, you may use smaller vents or close them to store more heat in the masonry wall throughout the daytime.

There are many design options available for a solar Trombe wall, and much depends on your climate. In very cold climates, a selective solar coating on the wall is often necessary for maximum effectiveness. Nighttime insulated coverings for the glass are also helpful.

Adding outdoor upper vents can help ventilate your house in the summer, too.

To avoid overheating in extremely mild Southern climates, a Trombe wall must be properly designed. In these areas, it is a good idea to contact a solar expert to do some basic design analysis for you initially.

You can write to me for UTILITY BILLS UPDATE No. 079 showing do-it-yourself instructions and diagrams of how to make a solar Trombe wall and a chart of common design features for the various climate zones of the country. Write to James Dulley, The Deseret News, 6906 Royal Green Drive, Cincinnati, OH 45244. Please include $1 and a self-addressed STAMPED BUSINESS-SIZE envelope.

Q - I have a high-efficiency wood-burning fireplace with glass doors and a blower. The instructions say to keep the doors closed, but I feel much more heat when they are opened. Which should I do? S.F.

A - Although you feel warm directly in front of the fire with the doors opened (direct radiant heating), it is actually wasting energy and firewood. You should definitely keep the glass doors closed if you are trying to heat your house with it. With them open, too much heated room air is lost and the rest of your house gets very chilly. Firewood also burns extremely fast from excess combustion air with the doors opened.