SOME PEOPLE are looking at the prospect of a 50 percent increase in their home heating oil bill and wondering if they can even afford another winter. They may have to make some difficult choices. One of these could be whether to buy a heat pump. The newest use of a heat pump is in conjunction with an existing oil or gas furnace.

Both York and Lennox make heat pump systems that can be added to your hot-air furnace. Attaching the pump to the furnace eliminates the need for the indoor blower and the backup electric resistance heating coil that normally comes with the heat pump.

The pump and the furnace are wired together and equipped with sensors. They are programmed to operate under a system that takes best advantage of each device's "efficiency factor" according to the different costs of fuels. The heat pump heats first until it reaches a "balance point" temperature, the temperature at which it makes more sense to heat with the furnace. Once the furnace has raised the temperature sufficiently, the heat pump switches on again.

While heat pumps are no panacea and do not warrant throwing out your gas or oil furnace, they can be added to your present system to save fuel.

"I personally might be tempted to do that because then I might be able to get by on only one tank of oil for the whole winter," said David Didion, head of mechanical the National Bureau of Standards. "The manufacturers originally developed the hybrid [heat pump/oil or gas furnace combination] for the New England area."

"This industry right now doesn't have any better answer to the fuel shortage crisis than the add-on heat pump," said Don Rittgers, general product manager of York Division of Borg-Warner.

From roughly 20 years of relative obscurity, the heat pump is emerging as a means of relieving the energy pinch. Although it still does not compete economically with natural gas prices, operating one is half as expensive as running an electric resistance heating system; and if oil prices keep going up, it may soon be cheaper than using oil as well.

"The truth is somewhere in the middle, depending on the product you pick," said Didion. "Washington, D.C., tends to be on the borderline of this issue. Further south, heat pumps tend to be more effective. Further north, they are not."

The reason is that heat pumps heat with warmth extracted from the outside air. This heat is always present, even in the winter, as long as there is a sun to shine.

The pump works like a backwards refrigerator (refrigerators exhaust warm air). A closed loop coil contains a refrigerant that expands in the outdoor cycle and gives off cold. The refrigerant is then pumped back into the house and condensed to give off heat. (In summer, the cycles are reversed and the heat pump becomes an air conditioner.)

Heat pumps have been found to provide warmth even at temperatures of minus 30 degrees, though their efficiency begins to drop off at around 20-30 degrees and diminishes at an increasing rate at temperatures below that. Rittgers estimates that a sound heat pump, properly sized, can handle the complete heating needs of a home at temperatures between 23-43 degrees. Most heat pumps have an electric resistance heating coil as a backup system.

There are dozens of heat pump manufacturers, and the cost of one 3-ton unit, enough to heat the average-size home, can vary. Both the add-on heat pumps and the standard ones with electric resistance heating seem to run consistenly around $2,000. Westinghouse, for instance, makes two models. The lower-priced one retails for $1,600-$2,000; the higher-priced for $1,800-$2,200.

Didion suggests whatever manufacturer you buy from, you purchase his top-of-the-line product. The difference in costs, he said, will pay for itself in added efficiency. When you shop for one, ask for the "energy efficiency rating" (EER). The higher the better.

To operate efficiently, heat pumps need the proper duct work. If you have an old house with dated duct work, you may need to make alterations first. Before you buy the pump, be sure the duct work is inspected.

If you want to heat and cool only a small area of the house, you may only need a room heat pump. Carrier is one manufacturer of window units, and should have its latest model out this month. Carrier makes two types: one with an automatic defrost control and the other without.

Heat pumps can develop ice problems in the winter. The Carrier without defroster is recommended only for areas where the outdoor temperature stays above 45 degrees. The defrosting unit switches automatically at 35 degrees to the electric resistance coil.

Prices on Carrier's four window models, which range in BTU output from 9,600 to almost 18,000, are $640- $725.

Be sure to check the warranty and service clauses carefully.If you want to know more about heat pumps, you can call the Air Conditioning and Refrigeration Institute at 703/524--8800.