MY PARENTS finally sold the house we built together. It wasn't easy.

One of the reasons they had so much trouble was that my father, being a thinking man far ahead of his time, put radiant heat in the house. He put it in, I guess, because he saved money doing it himself. We simply soldered copper tubing together, tacked it to the ceiling joists and ran hot water through it.

Chicago, however, is like a sauna in summer months, and you can't easily run air conditioning through copper tubes.

You need ducts, of which there were none.

Prospective buyers found the duct shortage especially conspicuous in August.

But in the winter months, the radiant heat was quiet, comfortable and clean. It was pleasing, for a change, to walk into a ceramic tiled bathroom where we had installed tubes in the floor and not have to hop-scotch into the shower.

We thought this worth the lack of ducts in the summer.

Radiant heat is a mysterious thing. It floats through the air invisibly, without a sound, and hits you in the head.

A good many stores around the country are selling an electric space heater that works according to these principles. It is called the Quartz Energy Saver and is made by a company called Boekamp in La Mesa, Calif. (It is available locally at Hecht stores for $120.)

Energy Efficient Systems Inc. in Middletown, Conn., is marketing the device on the East Coast. The president of Energy Efficient Systems, Jack White, said the company sold 150,000 of them in the last year.

The Energy Saver, which uses conventional electric resistance wires in two quartz elements to produce radiant heat, is a smaller version of other quartz heaters that have been around a long time. Perhaps as long as 20 years. But until now they have been used mostly for industrial and commercial purposes: to heat a small area in a large warehouse, for instance, or to warm hotel clients waiting for a taxi outside the door.

Instead of a bare wire that heats to approximately 1,500 degrees fahrenheit, as on most convection (hot air) space heaters, the metal alloy wires are encased in quartz. This allows them to burn about 400 degrees hotter by protecting them from surrounding elements and machinery.

Quartz does not magnify heat. Infrared radiation passes through it, however. And while all bodies warmer than absolute zero (including convection space heaters, light bulbs and a well-done T-bone steak) give off some infra-red radiation, the hotter-burning quartz elements emit more.

(The elements in quartz lamps used to bake enamel on cars, for instance, burn at temepratures up to 4,000 degrees. Unlike the quartz heater, however, the filament is housed in an evacuated tube, like a light bulb. This prevents the wire from burning itself up at high temperatures.

While some literature about the device claims greater efficiency with it, heating experts argue that to compare these to convection heaters can easily mislead. To insist that one is more efficient than the other, they say, is to argue personal preferences, not physics.

Like the headlights on a car, radiant heaters are directional -- you can point the heat where you want it to go. The infra-red radiation passes through the air and is absorbed by solid objects. Convection heaters heat air, which floats and circulates and in turn warms objects. The Energy Saver, and other radiant heaters, will heat more quickly.

All space heaters, authorities say, are nearly 100 percent efficient. The Energy Saver is a 1,500-watt unit that gives off 5,120 BTUs (British thermal units). Sears sells a 1,500-watt convection space heater that gives off 5,100 BTUs.

The amount of electricity consumed by fans on convection heaters is minimal, they say. "I may even want to put a fan on the radiant heater," said one engineer, "to get more heat into the air."

Since radiant heaters, like a fireplace, heat those areas of the body exposed to them, other parts may feel cold. "Suppose your back is facing a cold window?" said Tamami Kusuda, a thermal analyst at the National Bureau of Standards.

Others even argue that radiant heaters can lose heat to many objects in a room that don't reflect it back and don't need to be heated, such as walls.

All agree, however, that zoned heating conserves energy. You can usually save money, they say, by turning down the heat in the house and putting a space heater, either radiant or convection, in the room where you spend most of your time.

A few companies have jazzed up their designs. National Presto industries makes a high-tech orange and black model, tube-shaped like the barrel of a huge hair dryer. Another Presto heater looks like a stereo speaker. Both are sold at Woodward and Lothrop.

Ursell's home furnishings store in Georgetown sells three models from the design award-winning Braun company in Germany. The 1,200-watt units are available in wood grain or various colors. One looks like a radio and is small enough for desktop use. They range in price from $39 to $60.

Others are available in many hardware and department stores, in sizes ranging from 500 to 1,500 BTUs and prices from as little as $13 to $135. Some are box-shaped, others long and slim. The least expensive have no fan and one temperature setting. The more expensive have fans as well as switches to vary both heat and blower. Some stores, such as Hechinger, carry portable baseboard heaters.

Besides the Energy Saver, there are a few other radiant heaters on the market. Instead of bouncing radiation from the heating elements off a reflector, however, they are built as a panel to set on a table top or hang from a wall or ceiling.

The electric Dazey Sun Square Panel sold at Woodies is a tabletop model, only 15-by-23-inches square. It sells for $60. Larger radiant heat panels, to place on a wall, for instance, or over a bed, are approximately 2-by-4-foot square and sell for $55-$70 at electric supply stores.

And for those who can't bear to part with their faithful radiators, Hammacher and Schlemmer in New York sells one that moves from room to room on wheels. The 1,500-watt radiator is filled with oil, imported from England, and sells for about $130.

Another method of heating is called Getting Close to a Friend (provides both radiant and convection warmth). Unfortunately, none of the local stores seem to carry it.