With the price of fuel oil and natural gas going up and up, an inexhaustible supply of energy at a fixed price may by only a dream.

But is it, really? Certainly not to John Jones, a Dayton, Ohio mechanical engineer and owner of the Jones Heating & Cooling Co.

According to Jones, "When solar heat is combined with geothermal heat, the result is an endless fund of energy at a fixed cost. I don't know why someone hasn't tried linking these two energy sources together in residential buildings."

Jones began building small solar units more than 30 years ago while he was an engineer at Wright-Patterson Air Force Base in Dayton.

After years of experimenting with alternative energy sources, he installed his first combined solar-gheothermal system about four years ago in his own home. It has been working without a problem ever since.

The heart of the Jones system is a water-to-air heat pump. Water is supplied to the unit from the earth at a constant temperature. Then the heat that is extracted from it is used to warm the house. The cooled water returns to the earth where it again is heated, thus continuing the cycle.

"Below ground, the temperature of the earth remains constant the year around," Jones said, "anywhere from 47 to 74 degrees, depending upon the location. Here in the Dayton area it is 55 degrees. We can use that temperature at no cost to preheat or precool the water we use to heat or air-condition our homes."

When air conditioning is called for, the cycle is reversed. The heat pump, instead of extracting heat from the water, takes it from the house and transfers it to the water, which in turn cools the house. The heated water then returns to the earth to be cooled.

The system is being installed at Oak Creek South, a 300-house development in Washington, Ohio, just south of Dayton.

In the past it has been installed successfully in 500 houses. The first combined system, the one in Jones's own home, worked so well it used 93 percent less electricity than an electric baseboard heating system installed in a similar house put up by the same builder.

Jones said the system will save homeowners up to 80 percent on their heating bills, compared with electric resistance (baseboard) heat, and 20 percent compared with gas heat. In the summer, the savings could be 80 percent of the cost of a conventional air-conditioning system.

In its heating mode, the Jones system gathers radiant energy from the sun in fluid flowing through solar-collector panels that are mounted on the top of a house. Heat to water in a 1,000-gallon underground storage tank.

"On cloudy days, when direct solar energy is unavailable, the system reverts to heat energy stored in the tank," Jones said. "It can go about 20 days without collecting radiant energy."

Should the stored solar energy be unavailable, the system reverts to heat energy stored in the tank," Jones said. "It can go about 20 days without collecting radiant energy."

Should the stored solar energy in the tank be depleted, the heat pump then will extract heat from ground water. In effect, the geothermal portion of the system acts as a backup for the solar portion.

In the cooling mode, the system uses the earth's constant temperature as a heat sink rather than a heat source. In other words, with the pump operating in reverse, heat is drawn from the house and dissipated in the groundwater.

When the system is heating or cooling with groundwater, the required 20 to 30 gallons a minute is supplied by a well, lake or some other source. However, when the flows is inadequate, a closed loop of plastic pipe is buried several feet beneath the ground. Functioning as a heat exchanger, this loop can be used independently or in conjunction with low-yield wells.

Jones recommended the use of the loop, since it eliminates the chance of contaminants getting into the system and disrupting the operation of the heat pump. It is used in most of the houses at Oak Creek South. All of the homes are two-and three-bedroom models in the 2,200-to 2,500-square-foot class and sell for $75,000 to $100,000.

Specially designed for the solar-geothermal energy plants they contain, they have low ceilings, wall-to-wall carpeting and triple panes of glass on the smaller-than-normal windows.

Insulation, of course, is very important. Walls have 2 1/2 inches of sprayed-on urethane between the siding and sheathing.

The smaller Oak Creek South homes have six collector panels on the roof; the larger houses, seven. Each panel measures 35 square feet and has an output of 5,500 Btu. Panels are mounted flat and never need adjusting, since they are manufactured for the specific latitude and longitude of the area.

In a typical house, the Jones system costs $9,000 installed. A conventional heat-pump installation for the same-size house would cost about $5,000; a gas furnance, $2,500.