Geothermal Heat Pump: How It Works
For the ultimate in comfort and energy conservation, start by digging a hole
An electrically powered, geothermal heating and cooling system transfers heat between your house and the earth using fluid circulated through long loops of underground pipe.
How It Works
Given all the attention being paid to solar power these days, you might be surprised to learn that one of the most promising solutions to high energy costs isn't up in the sky but buried deep under your lawn. Superefficient geothermal heat pumps provide clean, quiet heating and cooling while cutting utility bills by up to 70 percent. "With this technology, everybody could be sitting on top of their lifetime energy supply," says TOH plumbing and heating expert Richard Trethewey.
In principle, a geothermal heat pump functions like a conventional heat pump, by using high-pressure refrigerant to capture and move heat between indoors and out. The difference is that conventional systems gather their heat—and get rid of it—through the outside air. Geothermal systems, in contrast, transfer heat through long loops of liquid-filled pipe buried in the ground.
As our cave-dwelling ancestors discovered long ago, if you go far enough underground, the earth's temperature stays at a constant 50 degrees or so, no matter how hot or cold it gets outside. So while a conventional "air-source" heat pump struggles to scavenge heat from freezing winter air or to dump it into the summer swelter, its "ground-source" counterpart has the comparatively easy job of extracting and disbursing heat through the 50-degree liquid circulating in its ground loop. That's why it takes only one kilowatt-hour of electricity for a geothermal heat pump to produce nearly 12,000 Btu of cooling or heating. (To produce the same number of Btus, a standard heat pump on a 95-degree day consumes 2.2 kilowatt-hours.) Geothermal systems are twice as efficient as the top-rated air conditioners and almost 50 percent more efficient than the best gas furnaces, all year round.
Another advantage is that there's no need for a noisy outdoor fan to move air through the compressor coils. Geothermal units simply pump liquid, so they can be parked indoors, safe from the elements. Most come with 10-year warranties, but they can last much longer. In the 29 years since Jim Partin, one of the technology's earliest adopters, installed one in his Stillwater, Oklahoma, house, he's replaced only two contact switches.
Heat Pump Parts
As with ordinary heat pumps, the refrigerant in a geothermal heat pump runs in a loop through a compressor, condenser, expansion valve, and evaporator, collecting heat at one end and giving it up at the other. The direction of refrigerant flow, which is controlled by the reversing valve, determines whether heat is moving into the house in winter (shown) or being pulled out of it in summer. With the addition of a desuperheater, residual warmth from the system can also supplement a conventional water heater, further reducing energy bills.
Costs & Tax Incentives
Despite these benefits, only 47,000 geothermal units were installed last year in the U.S. That's just a tiny blip compared with the approximately one million conventional heat pumps sold during the same period, even though ground-source heat pumps cost about the same to buy. Here's the rub: You have to bury a lot of pipe—about 1,500 to 1,800 feet for a typical 2,000-square-foot home. (The actual length should be calculated by an expert, based on the optimal heating and cooling loads for the house.) A setup that size could cost as much as $20,000 to install, depending on soil conditions and how much digging and drilling is involved. A house on a big lot, for instance, might be able to use pipes laid horizontally in long, 4-foot-deep trenches. Houses on small lots or rocky ledges could require three or four holes drilled about 300 feet straight down, a much more costly process.
Even with this significant front-end investment, geothermal systems are so energy-stingy that the payback period is remarkably brief. A study by the Air Force Institute of Technology calculated that it takes on average just seven to eight years to recoup costs. Your actual break-even point depends on local utility rates, excavation/drilling costs, how well your house is insulated, the efficiency of the model you choose, and what incentives your state or utilities provide. A good installer who's knowledgeable about heating and cooling as well as your local geology will be able to make those calculations for you.
The current federal incentive is limited to the standard $300 tax credit for Energy Star HVAC installations. (Canadians retrofitting an existing home with geothermal qualify for a $3,500 federal grant.) Some forward-thinking utilities have offered low-interest loans to homeowners willing to adopt the technology. "It's a win-win arrangement," says Steve Rosenstock, energy solutions manager at the Edison Electric Institute, an association of utilities. "The utilities reduce peak demand for heating and cooling as their customers dramatically lower their electric bills." And because the plastic ground loops should last 50 years or more, the payoff for homeowners, and for the environment, can last for generations.
What it is
An electrically powered heating and cooling system that transfers heat between your house and the earth using fluid circulated through long loops of underground pipes.
How it works
An indoor heat pump uses a basic refrigeration cycle—evaporation, compression, condensation, and expansion—to capture and disburse heat from and to the ground to warm the house in winter and cool it in summer.
Why you'd want one
Cuts home heating and cooling bills by 30 to 70 percent. Eliminates noisy outdoor compressors and fans. Reduces greenhouse gas emissions by the equivalent of planting 750 trees or taking two cars off the road.
What to look for
For federal tax credits, pumps must meet Energy Star efficiency standards. For closed-loop systems, you need an EER of 14.1 and a COP (coefficient of performance) of 3.3.
Where to get it
To find manufacturers, visit the Geothermal Heat Pump Consortium website. To find trained installers and designers who know the local geology and how to size systems for maximum efficiency, go to the International Ground Source Heat Pump Association's website.
What it costs
$15,000–$20,000 installed for the system, including ground loops, heat pump, and controls. The Database of State Incentives for Renewable Energy (dsireusa.org) provides up-to-date information on state incentive programs.
Can I Retrofit One?
Retrofitting a ground-source system is not difficult, as long as burying the ground loop is feasible. A house will need ducts to distribute cool air on hot days. Those same ducts can provide warm air in winter. Some geothermal heat pumps can hook up to an existing air handler, other units come with their own integral air handler. Houses with hot-water heating can use geothermal systems, too, although additional radiators may be needed because these systems do not reach the higher temperatures of fuel-fired boilers. (That's not a problem for radiant floor heat, which operates at lower temperatures.)