Geothermal Heating Basics

A few years ago, when “green” hadn’t quite entered the mainstream vocabulary, and natural gas prices were continuing to climb, Tom and Cindy Shepherd saw the writing on the wall. They took out a home equity loan to pay for the installation of a geothermal heating and cooling system for the family’s Indianapolis, Indiana, area home on the assumption that energy rates would quickly outpace interest on the loan. They were right.

Tom and Cindy spent several months researching the type of system best suited to their needs, interviewing installers and asking a lot of questions. An admitted “techno-junkie,” Tom’s job as a systems control technician for Honeywell became his trump card while he probed for answers.

With the help of Kris Kyler at Indiana Geothermal, a geo-loop contractor and geothermal equipment distributor, Tom and Cindy settled on a 4-ton (48,000 Btu per hour) water-to-air geothermal system to heat and cool their 3,600-square-foot, five-bedroom home. Their intent was for the new system to replace the 93% AFUE (annual fuel utilization efficiency) gas furnace and standard electric air-conditioning system.

The “geo” loops tap the earth’s abundant energy through four 150-foot boreholes. “For the most part, it was a standard geothermal install,” says Kyler. “And the benefit to the Shepherds’ utility budget was immediate.”

While many of their neighbors helplessly watch their utility bills soar higher, the Shepherds are enjoying record savings. In 2006, the Shepherds paid $3,620 for natural gas and electricity—energy used for space and water heating, air-conditioning, and pool heating. During the 12-month period following the installation, they paid $2,400 to accomplish the same thing—a 34% savings. The pool is also mostly heated geothermally, thus eliminating most gas heater operation, resulting in a further monthly savings of $100 to $400.

“Indianapolis Power and Light added to the savings by dropping our electricity rate from 4.4 cents per kWh to 3.8 cents because we installed the geothermal system,” says Tom. “IPL also added a $50 rebate, and we picked up another $300 federal tax break.”

“That played nicely into our overall savings on the loan,” says Cindy. “With the rebate, the reduction in the electricity rate, and the energy savings, a substantial part of the loan payment is covered.”

“The traditional heating and air-conditioning system that we had—considered to be high efficiency—was terribly inefficient when compared to geothermal, and rather uncomfortable,” adds Tom. “Today, we have terrific comfort year-round and an expected seven-year payback on our investment.”

Takes Little Area

The earliest geothermal systems tapped heat in the earth through pits or fissures that pushed hot water to the surface to heat homes and domestic hot water. Many of these systems used a very small pump to distribute the heat. These systems are site-specific and rare, but technology has vastly improved, permitting efficient geo-exchange from almost any plot of land. Modern systems use heat pumps to transfer heat for home space and water heating, and these systems will work in most climates. The difference in the systems is the cost of electricity to run the heat pump compressor and pumps—a good deal more electricity than used by natural geothermal systems. 

Even if you have just a small patch of land, you might be able to use a geothermal system as a hedge against an energy crisis. Modern technology extracts thermal energy with greater ease, with little disruption to the surrounding landscape, and at high enough operating efficiencies to make payback shorter than ever.

Even though the installation price of a geothermal heat pump (GHP) system can be several times that of a similarly sized air-source system, the U.S. Department of Energy states additional costs are returned in energy savings in five to 10 years. System life is estimated at 25 years for the indoor equipment and 50 or more years for the ground loop.

A ground-source unit works like a conventional heat pump to cool a home in the summer, and heat it in the winter. The key difference between an air source heat pump and ground-source is that the ground-source unit harvests the stable and renewable heat from beneath the earth’s surface, whereas air-source relies on widely varying air temperatures to do the same job. As with any heat pump, geothermal and water-source heat pumps provide space heating and cooling, and can also supply the house with hot water.

Depending on latitude, ground temperatures range from 45 to 75°F. Like a cave, this ground temperature is warmer than the air above it during the winter and cooler than the air in the summer. The GHP takes advantage of this by exchanging heat with the earth through a ground heat exchanger using a liquid transfer medium such as water or an antifreeze solution.

Because a GHP harvests energy from the earth, less fossil-fuel-based energy is used, which reduces greenhouse gas emissions and can cut utility bills by up to 70%, says Tony Landers of ClimateMaster. And very little maintenance is required because the stable heat source avoids thermal stresses to the compressor and the enclosed unit is inside, protected from the weather.

The Shepherds’ geothermal system taps the earth’s constant temperature of about 51°F in Indianapolis. From the variety of loop configurations available for a geothermal system, the Shepherds and Kyler—like the Wengers—chose the closed-loop borehole method, which disturbs the least amount of earth. For homes where ground space is limited, or for homes with mature landscaping, this configuration is ideal because all evidence of the drilling can be removed, and lines are buried.

“Most geothermal systems supply three or four units of heat or cooling for every unit of electrical energy input,” says Kyler, who has installed geothermal systems for more than 20 years.

Open- & Closed-Loop Systems

Electricity is used only to power the equipment says Tony Landers, marketing director for ClimateMaster, a manufacturer of geothermal systems. “The rest of the process uses the free, clean, and renewable energy that’s tapped just below the earth’s surface.”

There are two basic types of water-source geo systems: open-loop and closed-loop. An open-loop system typically pumps water out of a deep well, extracts heat from it, and injects it back into another well, a pond, or a river. An open-loop system tends to be more efficient because it pulls the heat out of a steady stream of water from deep in the ground. But open-loop systems are prohibited in many parts of the country because of water quality and water conservation concerns.

A closed-loop system uses a continuous loop of plastic tubing as a heat exchanger. The tubing is connected to the indoor heat pump to form a sealed, underground loop through which a glycol or alcohol antifreeze solution is circulated. Unlike an open-loop system that consumes water from a well, a closed-loop system recirculates its heat-transferring solution in the pipe. Closed-loop systems can be trenched, “bedded” (in an excavated, flat, deep bed) or drilled.

Doing the Work of Three

A geothermal heat pump system typically replaces two systems: heating and air conditioning. A geo system uses ground water or the earth as a source of heat in the winter by pulling heat from the water or ground, and using water or ground as a place to “sink” heat in the summer. The final process of thermal exchange takes place in mechanical equipment that serves both heating and cooling needs for a building. Typically, a system distributes the heat through a conventional forced-air ducted system, or through hydronic tubing in the floor like the Wengers’ system.

Many systems can also heat a home’s domestic water by either integrated water preheating or through “desuperheating.” A desuperheater reclaims heat from the air-conditioning cycle to heat water by transferring the compressor’s waste heat to a hot water storage tank, and can reduce water-heating costs in the summer by 40 to 60%, according to Landers.

Geothermal systems are not without their disadvantages, which mainly center on installation time and up-front costs. With many pieces and components to set up and integrate, the installation is more complicated and involved and, therefore, more time-consuming.

And, compared to other space-heating methods, geothermal systems are also more expensive. However, Yates points out that it’s not the equipment that eats up the budget but the preparation—outside drilling and trenching, and fusing the pipes—and connecting the many systems that take heat from the heat pump.

—John Vastyan