Most homeowners think of a greenhouse as a luxury. Sure, a sunny sanctuary for house plants and a year-round source of fresh vegetables would be nice, but who can afford to heat a greenhouse these days?

Actually, a well-designed greenhouse should be able to heat itself, and might even gather a little extra solar energy to help you heat your home.

But not just any greenhouse will do. Several critical design requirements must be met, or the greenhouse can quickly become an energy drain.

The best type is the attached greenhouse -- a sort of a lean-to or a shed built against the south wall of your home. The attached design has several things going for it:

Since it only has three walls, it's cheaper to build. It also has less surface area to lose heat. Any excess heat it collects during the day can be fed directly into your house during the daytime. If necessary, the greenhouse can then borrow some of that heat back at night when you are asleep and don't need the heat in your home anyway.

Attaching the greenhouse also gives you the access to it directly from your house. No need to go outdoors, letting cold air into your home and the greenhouse in the process.

The attached greenhouse must go against the south wall of your home, plus or minus about 15 degrees, in order to receive the full heat of the sun. The south wall of the greenhouse, and its roof, should be double-glazed for maximum heat retention, and the east and west walls should be opaque and insulated to at least R-20 (the equivalent of six inches of mineral wool insulation).

The glazed surfaces should also be fitted with some sort of movable insulation that can be opened during the day, then closed to retain heat at night. All construction should be tight, and well sealed with caulk and weatherstripping to avoid infiltration of cold air.

Finally, the greenhouse will need some means of storing the excess heat it can collect during the daytime. Storing this heat will help keep the greenhouse from overheating when the sun is shining, yet keep the heat in the greenhouse where it can be used when needed after the sun sets.

Storing heat is a relatively simple matter. All you have to do is to provide a lot of mass to soak the heat up. There are several common ways of doing this: Steel drums painted black and filled with about 50 gallons of water can store a lot of heat. So can a concrete slab floor, masonry such as block or stone along the north wall of the greenhouse, or loose rocks held in place behind heavy wire mesh.

No matter what form the storage medium takes, it should be a dark color to help it absorb heat. How much storage will you need? As a rule of thumb, figure on about five gallons of water per square foot of glazed greenhouse sufrace. If you choose rock or masonry, figure about 1 2/3 cubic foot of rock or gravel for each square foot of glass.

Follow all these rules to the letter and you should end up with a little bit of summer all year round -- and maybe even a little free heat for your home to boot. Q: A few years ago I bought some bentwood chairs at a garage sale. I stored them away in my attic. Now I want to refinish them, but two have developed large cracks in the bent wooden backs. Are the cracks repairable ? A: You should be able to fix those chairs without much trouble. To start with, wrap the cracked areas with rags. Then pour some boiling water over the rags. Add some fresh boiling water every few minutes to keep the rags hot. You can also go over them with a hot iron. After about 20 minutes, remove the rags. Quickly clamp the cracked sections back together using automotive hose clamps. Before using the clamps, apply masking tape to the areas the clamps will go on, to keep them from marring the wood. You will probably need two clamps for each chair.

Apply one clamp near the base of the crack. Then add the second near the tip of the piece that is peeling away.

You can use heavy nylon cord instead of the clamps if you like. Just wrap it as tightly as you can around the break, starting at the base and working completely over the cracked area, secure the end of the cord with several half hitches and some tape.

If you do use cord, make sure it is nylon. It will stretch slightly as you wrap it in place. Then it will try to shrink back to its original length. That will increase the pressure on the crack and help assure a good fit.

Now let the repair dry out for a few days. Remove the cord or clamps and you are ready to glue. Use a clear, slow-setting epoxy. Don't use a fast-setting "five-minute" type -- it may set up before you can get the parts properly clamped.

Apply the glue to the crack between the two pieces of wood, making sure to cover the entire area of the crack. Then reclamp or rewrap. Let the glue set for a day or more. Then remove the clamps or cord. Bits of tape or cord will probably stick to the excess glue that has oozed out of the repair. Remove this and the excess glue by careful sanding along the grain of the wood.

You are now ready to refinish. If you have used clear glue, the repair should be nearly invisible. Q: Could you suggest a treatment for restoring color to outdoor man-made stone that has faded to an ugly pink color? Could it be re-stained or painted in some way ? A: Paint is your best bet. Any latex house paint should do the job. Stains might work, but I'm not sure how well they will penetrate man-made stone. You might write to Rohloff & Co., 918 North Western Avenue, Los Angeles, California 90029 and ask if their Kemiko concrete stains would work on your stone. You could try a sample on a small area as a test, and if it doesn't work as you like, you can always paint over it. Q: How can I remove exterior latex paint from concrete? It dripped onto the concrete several months ago . A: Apply a thick coat of paint and varnish remover. Let it soak in well. Then scrape off as much paint as possible. Apply some more remover and let it soak in again. Then attack the residue with a wire brush. Finally, finish up with detergent, hot water and a scrub brush.