What are the differences between air-source heat pumps and ground-source heat pumps? - Angie's List member Mark Schinman

As their names imply, an air-source heat pump draws heat from the outside air and a ground-source, or geothermal, pump pulls heat from underground, says Stephen Nugent, owner of highly rated John Nugent & Sons in McLean, Va. "With a ground-source heat pump, you're taking energy from the earth where it stays between 50 and 55 degrees," he says. "You're extracting heat out of that constant temperature, as opposed to the air temperature, where it drops below freezing in the winter and soars in the summer." Both units typically feature a backup heater or furnace to help if temperatures swing too dramatically.

Because heat pumps extract heat from the environment and don't rely on fuel to create it, they are more energy efficient than conventional furnaces, says Tony Lucci, a comfort consultant at highly rated Van Sant Inc. in Mount Airy, Md. "Installing a ground-source heat pump is at least twice as expensive as air-source," Lucci says. "In the long run, it's 40 to 50 percent more efficient."

Both Lucci and Nugent say geothermal pumps can cost more than $30,000 due to the added cost to excavate and lay pipes. However, Nugent says the units require less maintenance and last nearly twice the life of an air-source pump. There are also tax credits available to help offset geothermal's cost, including a 30-percent federal tax credit through 2016. Some counties and cities in the D.C. area also offer incentives, Nugent says.