In nine out of 10 homes in this country the second-largest consumer of energy is the hot-water heater (space heating or central air conditioning is first). Anything that promises to cut down on the amount of energy consumed for making hot water is of prime interest to every homeowner, as well as to utility companies and federal, state and private energy-conservation authorities.

In addition to advising homeowners to insulate their hot-water tanks and, by observing all possible conservation measures, cut down on the amount of hot water wasted each day (wash clothes in warm or cold water when possible; don't let the shower run any longer than necessary; shut off the hot water when lathering; repair leaks promptly), a great deal of interest has been raised by one of the most promising new developments in recent years -- the heat-pump water heater.

This appliance operates on the same principle as a refrigerator or air conditioner (both of which are actually heat pumps). It draws heat from the surrounding air in the room in which it is located, then transfers or "pumps" this heat into the water that is stored in the hot-water tank.

Like an air conditioner or refrigerator, the heat-pump water heater has a compressor and an evaporator, and in some models there is also a small circulating pump. All have motors that use electricity, buy when compared with a typical resistance-type electric hot-water heater, they will cut down on the amount of electricity consumed by anwhere from 50 to 60 percent -- and even more in those communities where electric rates are higher than average.

Heat-pump water heaters are costly -- prices currently range from $700 to $1,000 installed -- but savings are impressive enough to result in a comparatively short pay-back period for most homeowners. Depending on local utility rates, and on ambient temperatures of the space where the heat-pump water heater is installed, the average homeowner can expect to save enough on oil bills or electric bills to pay for the cost of a new heat-pump water heater in from 21 to 42 months. (At present, those making hot water with gas are still better off staying with their gas water heaters, at least in the colder climates.)

Studies made by the Department of Energy and HUD indicate that the average American family of four spends about $300 per year for electricity to make hot water, so switching from an electric hot-water heater to a heat-pump hot water heater will save at least $150 to $200 per year.

Savings are even greater for homes with oil hot water heaters, or where electric rates are higher than 6 cents to 7 cents per kilowatt hour (as they are in much of the Northeast). In one area where many homes use oil to make hot water, local utility companies and oil companies have estimated that the annual cost of making hot water in a typical house costs over $400 per year; the annual saving that would result if those homeowners switched to a heat-pump water heater could run as high as $250 to $300 per year.

Because these heat pumps operate on the same principle as an air conditioner, they have a sizable side effect on the air surrounding them: They will cool and dehumidify the air in that space. This is an advantage in southern climates where air conditioning is needed much of the year, as well as in the summer in the rest of the country where the additional cooling and dehumidifying will help lower summer air-conditioning costs. In a basement it may even eliminate the need for running a dehumidifier (thus saving even more on the monthly electric bills).

In the winter, when the house is being heated, the extra cooling is not as welcome since it could add to the amount of heat that must be generated by the furnace (depending on where in the house the water heater is located). However, in homes that are air conditioned in the summer, the savings that result from the extra cooling will usually more than offset the losses incurred by having to generate a small amount of extra heat in the winter.

In addition, in most homes where the water heater is in a basement that is not heated, except by waste heat cast off by the furnace, this is almost a negligible factor. The same is true where heat is supplied by a wood stove or other heating unit that uses low-cost fuel or where the new water heater will be located in an attached garage or underground crawl space.

To work effectively, these units must be located in space where temperatures do not drop down below 45 degrees or 50 degrees (depending on model), but this limitation is seldom a problem in crawl spaces and basements since the ground maintains this temperature even when air temperatures outside drop well below freezing. Even an attached garage that is otherwise unheated will seldom, if ever, drop lower than this.

For those installations where exceptionally low temperatures may be a problem on occasion, one of the companies mentioned below has an automatic relay that will switch the original electric heating element back on whenever temperatures in the area drop below a safe operating level.

Heat-pump water heaters currently are being produced by at least three companies who market them through local dealers and installers under their own brand names: Temcor, made by Energy Utilization Systems, Inc. (365 Plum Industrial Court, Pittsburgh, Pa. 15239); Efficiency II, made by E-Tech, Inc. (3570 American Drive, Atlanta, Ga. 30341); and Fedders, made by Fedders, Inc. (edison, N.J. 08817).

The Temcor unit is a self-contained water heater that includes its own storage tank for hot water; this makes it slightly higher in price than the others ($800 to $1,000 installed). The compressor and evaporator are mounted in an enclosed chamber on the top of the tank, with a built-in heat-exchanger coil immersed inside the tank.

The other two brands are designed for retrofitting -- that is, for connecting to existing hot-water heaters. The Fedders and E-Tech units sell for about $700 to $900 installed.

In both cases the units come in compact, self-contained cases that can be set up next to the existing water heater, then connected to it with flexible tubing (supplied by the manufacturer). Labor costs for installation are about $75 to $150, but for those reasonably familiar with doing plumbing around the house, do-it-yourself installation is entirely practical.