The heat pump: It's both an air conditioner and a heater - and quite possibly the furnace of the future. In this climate, one can actually deliver more heat than you put into it. Sounds impossible, but it's not.

A heat pump is essentially a central air conditioner that can run backward. Air conditioner cools your home by picking up heat inside and dumping it outside. In it's cooling mode, a heat pump does the same thing. But when it runs in reverse for heating, it picks up heat from outside your home and delivers it insside. And believe it or not, there's heat outdoors even when the temperature is below freezing. When the outside temperature is 50 degrees, a good heat pump can deliver three BTUs for every BTU of electricity you put into it. As the outside temperature drops, so does the heat pump's efficiency: It may give you two-for-one performance at 32 degrees. By the time the mercury dips to 15, the pump will be barely breaking even, and below that it will strain so hard that automatic switches will cut it off and turn the heating over to conventional electrical resistance units.

A recent computer study suggests that a heat pump will heat and cool for less than an electric furnace/central air conditioner combination, break even with oil heat-/central air conditioning tandem, but cost more to buy and run over the long haul than a gas system.

Another factor to consider: Don't convert to a heat pump if your home is not already equipped with the necessary ducts. Retrofitting will probably be too expensive.

Pepco has a phamphlet detailing plusses and minuses of heat pumps in this area, available from the receptionist at 1900 Pennsylvania Ave. NW; it also has an expert for you to talk to by calling 872-2465. Vepco's heat pump expert's number is 836-5900 ext. 237. Or send a self-addressed business-size envelope to Dept. HP, ARI, 1815 N. Fort Myer Dr., Arlington 22209.