LEW PRATSCH (United States Department of Energy): You're referring to a geothermal heat pump, which cools as well as heats your house. It's incredibly efficient compared with the usual cooling and heating systems — at least 25 percent more efficient, and often much more. That could mean substantial savings on utilities.
So how does it work?
The system relies on a natural phenomenon. If you dig six feet into the earth, you'll discover that the temperature is stable — between 40 and 70 degrees, depending where you live — no matter what the season, no matter what the temperature above ground. By taking advantage of this more constant, even temperature, you can heat or cool your home.
I'm not sure I follow you.
It's pretty simple, really. Hundreds of feet of pipe are buried five to six feet underground, usually in horizontal ditches in your yard. If your yard isn't wide enough, they can be buried vertically, down to 300 or so feet. The pipes are filled with water and antifreeze, sealed, and connected to the geothermal heat pump in your basement or attic. To cool your house, the pump draws the heat from inside the house and transfers it to the water, which is then circulated through the underground pipes. The heat is absorbed by the earth, and the water is cooled by the lower underground temperature before it circulates back indoors. It's a continuous cycle.
What about heating in winter?
You simply flip a switch and the process is reversed. Relatively warm water in the underground pipes — remember, it may be zero degrees above ground, but 50 degrees below — is transferred inside and compressed, which heats it more. The heated air is then circulated through the house.
Why is that more efficient? You're still using pumps and compressors and so on.
Well, the big issue is that you're heating without burning any fossil fuels. And it takes far less energy to boost 50-degree underground temperatures to 70 degrees than to boost zero-degree air temperatures to 70. Also, in the summer the excess heat from your house can be routed for use by your water heater, so you can save even more.
How much does it cost to install?
Costs vary across the country, but in a market where contractors regularly install systems, it will probably cost 50 percent more than a normal HVAC [heating, ventilating, and air conditioning] system. In areas with less competition, it could be double. But over the life of the system you'll save money, and it may even add value to your house. Because all the hardware for a unit is underground and inside the house, it's not susceptible to corrosion from weather. So while a typical air conditioner or heater may need replacement after 15 to 20 years, a geothermal pump can last 30 years, and usually requires less maintenance.
Wow, I'm sold! Where do I start?
Before replacing your current system, we recommend you make your home as energy-efficient as possible. I'm talking about making a few big changes like installing more or better insulation, or replacing old windows. Doing so typically means reducing the size of the geothermal heat pump system you'll need, and thus lowering the installation cost by as much 35 percent. And of course, it lowers the cost of running it. By the way, when it is running, it's so quiet, you hardly know it's on.
Lew Pratsch, Zero Energy Homes Project Manager for the U.S. Dept. Of Energy; .