Combine the frostiest winter in living memory with a 3-ton reversed-cycle refrigeration unit and you come up with, not a gigantic ice cube, but what some mechanical engineers see as the best available heating system for a well-insulated new house of average size. What we're talking about is the device called a "heat pump."

Other kinds of refrigeration systems, like air conditioners and refrigerators, pump heat from a place where it is not wanted to some other place where it will not be objectionable. The heat pump can operate that way, too, and when installed in a building it will take the place of conventional air conditioning.

But what the heat pump does most efficiently, in a reversal of its cooling operation, is to draw up heat where there apparently is none (outdoors, in the dead of winter) and deposit it where it is needed (indoors, to warm living space). Heat can be wrung from the air even at temperatures substantially below freezing, but the pump's efficiency drops rapidly below 20 F.

When a heat pump is used for heating, it draws on electric power to blow outside air over an even colder outdoor evaporator coil. Heat from the air evaporates the liquid refrigerant in the coil, which is raised above room temperature by compression and then condensed in an indoor coil. Air warmed by that coil is then circulated through ducts, as in any forced hot-air system.

For air conditioning purposes, a valve reverses the flow of refrigerant in the system, and the indoor and outdoor coils exchange functions. In its cooling cycle, the heat pump is so much like a conventional air conditioner that in the 1950s, when heat pumps were first being marketed, some manufacturers did nothing more than put a reversing valve on an air conditioner and call it a heat pump. Such units could not deal effectively with problems peculiar to the heating cycle (such as ice buildup on the outside coil), and failures began to show up in epidemic proportions.

Industry and the Edison Electric Institute responded by organizing an intensified research effort that led to the development of a much more precisely engineered device. These "second-generation" heat pumps "have proven very reliable when a proper maintenance program is followed," according to the National Bureau of Standards, which has been evaluating heat pump performance since 1959.

By the early 1970s manufacturers of electric resistance heating equipment had won away a sizeable share of the heating market from producers of combustion furnaces; all-electric homes had become common. But the revamped heat pump was garnering little interest. Sales might still be in the doldrums today without the impetus of two rapidly occurring changes in the availability of cheap fossil fuels.

First, natural gas supplies were curtailed. Across a wide swath of the country's midsection, from the Mississippi to the Atlantic, builders were told that natural gas would no longer be available for new buildings. There have been no new hookups in the Washington area since March 1972. Next, a series of hefty OPEC price increases doubled the cost of heating oil and drove up the cost of the residual oil burned by many electric utilities. Consumers began to view their oil burners with suspicion, and the all-electric house drew some outright hostility.

At this point, the heat pump began to exercise a powerful economic attraction on builders and developers. Although operated by electricity, it can deliver two or more times as much heat as that available if the electricity were used directly in resistance heating, that is, in a hot-air furnace or in baseboard units. This is because the energy available for heating comes not only from the work energy put into the heat pump's compressor and indoor fan, but also from the heat extracted from the outdoor environment. The latter is free solar heat - warmth transferred to out atmosphere by the sun.

When that warmth is insufficient, usually when the outside temperature has dropped to about 30 F, supplementary resistance heaters, installed as part of the heat pump system, take over the extra heating load.

The heat pump is only beginning to come into its own, but prospects are promising. Nationwide, 312,000 units were sold in 1976. In the Washington area, over 12 per cent of new houses built last year were equipped with heat pumps. PEPCO reports that they are now regarded as standard equipment by the builders of more than a dozen subdivisions in its service area.

In the local market, however, fewer sales are made for new housing than to replace straight resistance heating in all-electric houses. Typically, a 2-ton unit installed in a medium-sized townhouse will cost $1,300. A larger house will need at least a 3-ton unit at perhaps double the price.

A good quality heat pump that replaces resistance heaters is virtually certain to reduce heating costs by 40 per cent, according to Dr. David Didion, who heads a heat-pump research team at the National Bureau of Standards. Even larger reductions are possible.

Take the case of Dale Berman, an energy consultant who last August spent $2,800 on a heat pump "retrofitting" for the large (2,100 square feet), all-electric Chevy Chase townhouse that he and his wife share with their three teen-agers. Recently he compared the six electric bills he has paid since then with those covering the same period a year earlier. Winter charges of $250 to $300 monthly had been cut as much as 50 per cent. The total bill for January 1977, a month of record-breaking cold temperatures, came to just $150.

So far, the Bermans' new heat pump has operated during only one summer month, August 1976. Total charges for that month were $130, 40 per cent less than the average previous summer billing. However, summer savings are typical. (When cooling, a heat pump operates like - and cannot outperform - conventional central air conditioning.) Berman ascribes his summer savings (and a portion of winter savings as well) to two factors. First, the discarded heating and cooling equipment, installed when the house was built in 1967, was "probably low quality and certainly low efficiency." Second, while the heat pump was on order, he added $1,000 worth of storm doors and windows to his otherwise well-insulated house.

While it makes good sense, both economically and from an energy savings standpoint, to get rid of electric furnaces and baseboard units, there is no real justification for replacing a properly sized, properly operating gas-or oil-fired furnace. There is considerable controversy over the seasonal efficiency of combustion furnaces, but NBS engineer Didion believes that in the Washington area there is neither any demonstrable economic advantage, nor any clear-cut energy savings, in choosing a heat pump over a gas-or oil-fired system. Under price controls gas is still the cheapest heating fuel.

Nevertheless, if the decision has been made to add air conditioning to a house heated by an oil-fired forced-air furnace, it could be a wise move to purchase a heat pump to perform the cooling and also to assume the primary heating load. With the addition of a simple control device, the oil furnace can provide the heating backup for the heat pump at much higher energy efficiency than resistance heating. Since no resistance heaters would be needed, the household would not likely be affected by future utility rate increases related to peak loads in winter.

George Lindamood, an NBS computer specialist, has found that the money saved by heating with a heat pump can pay the cost of summer cooling. Three years ago, during the last season that an antiquated oil burner heated his 16-room brick house in rural Woodsboro, Md., it consumed 1,700 gallons of heating oil. That quantity of oil would have cost Lindamood $680 during the '75-'76 heating season, but, as it turns out, he wasn't buying oil last year. In the meantime, he had fully insulated his 60-year-old house and fitted it with a dual heat pump system.Instead of paying $680 for oil alone, he paid $650 for the entire year's heating and cooling.

The system Lindamood installed was quite complex, "with a whopping lot of ductwork," electrostatic air filters (because he and his two young sons suffer from asthma), and two 4-ton split-system heat pumps (with one air-handling unit in the attic, ducted to the third floor and to second-floor ceiling outlets, and the other in the basement ducted to first-floor registers).

It was also quite expensive ($10,000). But Lindamood is well pleased. For one thing, the system is amortizing at a nice rate, and it cost no more than an oil-fired forced-air furnace combined with central air conditioning. For another, he thought long and hard before making his capital investment in the cheapest available way of converting electricity to heat. "You see," he explains, "I wanted to stop being caught up in the energy crisis, and I felt quite safe going to electricity. My local power company, Potomac Edison, sits on top of a coal mine."