When Bob Bunting's old electric hot-water heater gave out in 1979, Potomac Edison Co. made him a free offer of a new heater--a heat-pump hot-water heater--in return for two years of monitoring privileges on the relatively novel device.

Last summer, a year and some months later, PE was so impressed that it decided to sell 35 more of the units at cost to its employes, so it could do further testing.

The results? Bunting and the 35 other hot-water heat-pump users are saving an average of half their previous hot water heating bills. Although no conclusions will be drawn until after the winter, the utility's research manager, Joseph Staley, calls the preliminary findings "very promising."

For a customer using 1,800 gallons of hot water a month and paying PE's current rate of 5 1/2 cents per kilowatt hour, the new hot-water heater can mean a saving of 200 kilowatt hours a month or $132 a year over the cost of heating water with a standard electric-resistance unit. For those using more hot water or paying higher electric rates, savings can be even greater.

Because they're relatively new to the market, heat-pump hot-water heaters don't come cheap: They are $800 to $1,000 installed, utility officials said. At that price, the devices pay for themselves in energy savings in six to eight years. But in fact, payback times are expected to be shorter, with electric rates continuing to go up and retail prices expected to come down.

"We expect the price to drop as the product is used and as competition comes into the field," says PE's Staley, who notes that the unit used in the tests, made by Energy Utilization Systems, Inc., was "practically the only" one available when the project was started two years ago. Today a number of other manufacturers have gotten into the market.

Compared with an electric resistance hot water heater at about $250, the new device is expensive partly because of the heat-pump unit, mounted on top of the storage tank like a cap. Instead of using a 4,500-watt coil to heat water like the resistance unit does, the heat pump operates on a mere 800 watts, drawing heat from the air, putting it into the hot water tank, and releasing cool air back into the atmosphere--something like a window air conditioner.

Because the heat pump works more slowly than the 4,500-watt resistance coil in a standard heater, the water storage section is larger--usually 80 gallons versus 52 gallons common in most homes today. Each heat pump unit also contains an electric resistance coil for backup.

In the south, heat-pump hot-water heaters are generally located in utility rooms, where they provide air conditioning up to 10 months of the year. Here in the north, they are most efficient in insulated basements of 500 square feet or more, where they dehumidify the area in summer and use waste heat in winter. They work especially well near gas or oil furnaces or appliances (such as freezers) that give off excess heat when they operate.

In smaller unheated basements (under 500 square feet), the units have to revert to the backup coils for about five months of the winter, according to Joe Staley, minimizing savings.

From Potomac Edison's point of view, one of the most exciting findings so far is that the heat-pump water heaters appear to put only about a third of the demand on the utility's generating capacity as standard electric units.

"Demand is what makes us build new generating facilities," Staley said. "And a lot of our rate is represented by building costs." Since hot-water heaters in most homes are major energy consumers after heat and air-conditioning, the drop in demand could eventually become significant.

On a smaller scale, Potomac Edison and Washington's Potomac Electric Power Co. are also looking at add-on units, in which the heat pump is hooked to an existing hot water heater. Unlike the all-in-one devices, the add-ons are "as simple to install as attaching a garden hose to your faucet," according to Pepco energy management specialist Gerry Lopez. Many come with a kit to allow the homeowner to do the hook-up himself or herself.

In the three add-on units Pepco has been monitoring since this summer, maximum savings have been 40 to 50 percent over standard electric heaters, somewhat less over gas heaters, which are cheaper to run at current rates.

But one problem with adding a heat-pump unit onto what is probably a 52-gallon existing heater is that the heat-pump recovery rate is slower. Large families using a lot of hot water sometimes find that they run out during peak demand periods unless they leave one of the electric resistance elements hooked up, or add a larger water storage tank--both of which minimize savings.

"You almost need an 80-gallon tank to make it effective," said Don Whipp, whose family of five (including three teenagers) tested the first add-on unit for Potomac Edison and found it wanting until they added extra storage capacity. But a five-member family in the Pepco test had no such problems, and officials feel that families wishing to schedule showers and wash loads at different times of the day can surmount the difficulties and still save money.

Since the add-on units cost about the same as the all-in-one unit, PE's Staley says it may be more practical for large users to wait until the current hot-water heater needs replacement, then buy the all-in-one device with its 80-gallon storage capacity.

Still to be determined by PE tests on 10 add-on units that will be installed shortly is whether sediment builds up in the add-ons which, unlike the all-in-one device, run the water through the heat pump section. In PE's largely limestone service area, sediment build-up in pipes is a common problem.