Heat pump trims energy bill of renovated gristmill

Ask Neil and Karen De Riemer how they slashed almost two-thirds from their heating bill last winter, and they're likely to point to the centuries-old well just a few feet from the side door, and to the new heat pump recently installed in the basement.

Working in tandem, the well and the heat pump produced economical heat all winter long. The same combination is expected to appreciably lower air-conditioning costs this summer as well.

''So far,'' says Mrs. De Riemer, ''it's keeping us wonderfully cool.''

When the De Riemers bought the one-time Hershey gristmill here and made it home, they acquired charm, character, and better than 4,000 square feet of living space that wasn't exactly cheap to heat.

That first winter, it cost an average of $360 a month to stay warm. Last winter, without adding any further insulation (''We still don't have storm windows,'' Mrs. De Riemer points out), the electric heating bills were down to $ 115 a month. Admittedly, last winter was somewhat milder than usual, but not enough to lower heating bills so dramatically.

How, then, did the lower costs come about? The answer lies in the ability of the heat pump to tap the tremendous surplus of heat energy in well water that stays a constant 55 degrees F. year-round in the region. In summer that same 55 -degree water will soak up unwanted heat, if the heat pump is used in reverse mode.

This will be the De Riemers' first summer with the new system, so they cannot yet compare it with conventional air conditioning. Yet they expect the savings to be significant for this reason: With relatively cool water to absorb unwanted heat, the compressor (a heavy user of energy in conventional air conditioning) will not need to operate at all.

In simple terms, a heat pump takes heat from one area and transfers it - pumps it, you might say - to another area. A refrigerator, which is a form of heat pump, simply takes unwanted heat from inside the cabinet and blows it out into your kitchen. Because it is much cheaper to move heat than to make it, the heat pump is an energy-efficient instrument.

Heat pumps weren't widely talked about before the Arab oil boycott a decade ago, because there was little need for efficiency when energy was dirt cheap.

Moreover, such heat pumps as were available in that period were not particularly reliable. Lord Kelvin, an Irishman who first expounded on the principles of the heat pump at the Royal Society in London in 1850, said his discovery would not be widely accepted until fossil fuels became scarce and expensive.

The more common heat pumps extract heat from outside air in winter and bring it indoors. They can do that reasonably well even in winter because at zero degrees F. air still contains 89 percent of the heat it has at 100 degrees F. This means that heat pumps can work effectively at temperatures cold enough to keep ice cream brick-hard.

Heat pumps work in these wintry conditions because compressing the air causes the temperature to increase dramatically (Lord Kelvin even called his discovery the ''heat multiplier''). As a result, by using one unit of energy, a heat pump can frequently deliver 10 units of energy to the home or office building.

Obviously, though, the warmer the outside air, the easier it is for the heat pump to extract heat from it in winter. For instance, a study a few years back showed heat pumps provided a 29 percent saving over conventional electric heat in cold Minneapolis; in much more temperate San Jose, Calif., the savings were 56 percent.

Recognizing this, experts in the field developed a heat-pump system that would extract heat from, or discard heat into, a fluid with temperature changes that are generally far less extreme: water. Deep ponds or ground water (wells) are ideal for this purpose.

Insulated by the earth, ground water remains at an almost constant temperature year-round. Temperatures vary depending on a region's latitude, or distance from the equator. In the Philadelphia region, for example, ground-water temperatures hover around 55 degrees F.; in northern Maine they are some 10 degrees cooler.

In extremely cold weather - say on a midwinter day in Maine, where the temperature may plunge to minus 30 degrees or lower - it is difficult for an air-to-air heat pump to do its work efficiently, but the heat pump would have no difficulty in extracting heat from 45-degree water.

The De Riemers certainly appreciate the even temperature of their ground water. Knowing what they know now, they would probably be prepared to dig a well to serve the heat pump, had one not conveniently come with the property.

Meanwhile, according to the Air Conditioning and Refrigeration Institute in Arlington, Va., heat-pump factory shipments - both of air-to-air and air-to-water units - have risen over 300 percent in the past decade as more homeowners have learned of the 130-year-old technology and the savings it can offer.

A good book on the subject, written in layman's terms, is: ''Heat Pumps,'' written by Dermot McGuigan ($6.95, Garden Way Publishing, Charlotte, Vt. 05445).