In December 2010, Leesburg residents Geff Garnhart and Margie Lang-Garnhart faced a problem. It was cold outside, their furnace was broken beyond repair, and they wanted one that would be less harmful to the environment than their oil-burning unit. But the one they were considering would cost about three times as much as a conventional furnace.

On the advice of a friend, they decided to take the “green” path and invest in a geothermal heating unit, hoping that, over time, the energy savings would help offset the higher upfront costs.

“We love it,” Garnhart said. Not only have the family’s energy costs dropped sharply but the new heat pump also heats and cools their house better, he said.

“We had heard a lot about geothermal through some friends of ours,” Lang-Garnhart said. “We looked into it, and we were surprised at what [a geothermal system] could do for a house our size.”

The Garnharts are among a growing number of homeowners who are taking advantage of government incentives to reduce their homes’ greenhouse gas emissions — and their energy bills. The U.S. Department of Energy estimates that 50,000 geothermal heat pumps are installed in the United States each year.

Geothermal heat pumps use underground pipes to move the Earth’s natural heat into the house from hundreds of feet below the Earth’s surface, where the temperature is relatively constant, about 55 degrees Fahrenheit. The Energy Department estimates that geothermal heat pumps use 25 to 50 percent less energy than standard, air-to-air pumps that heat or cool outside air, where temperatures can vary by 100 degrees or more over the course of a year.

Geothermal systems, which resemble a traditional heat pump, tend to last longer and require less maintenance than conventional heat pumps, according to the Energy Department. Internal components of a geothermal system typically last 25 years, while the ground loop has a life of more than 50 years, the department estimates.

Although the Garnharts have seen savings from the geothermal system, the choice to replace their furnace was not easy.

Garnhart, 45, a project manager with Hewlett-Packard Enterprise, estimated that the total cost of the geothermal system was $23,000.

“It was triple the cost of a conventional furnace,” he said. “We were at the point where we had to go one way or the other. We couldn’t repair our existing furnace anymore, so we had to buy something new.”

Their start-up costs were reduced by a federal program that offers a tax credit of 30 percent of the cost of the geothermal system, and they took out a loan to pay for the system.

Lang-Garnhart, 45, a social worker, said, “We now spend zero on oil,” resulting in an annual savings of more than $1,200. She said that electricity bills are slightly higher in the winter than they were before but that the family’s electricity usage is much lower in the summer, when their power bills used to be the highest.

Garnhart said that he is hopeful that the geothermal unit will, in effect, pay for itself in about seven years and that the low energy costs will someday make the house more appealing to potential homebuyers.

“We plan to be in the house for quite some time,” he said. “But the resale value will be huge.”

The Garnharts’ conversion to geothermal energy caused a small sensation in their neighborhood when a truck pulled a drilling apparatus — towering twice as high as their house — onto their front lawn. This was necessary to bore holes hundreds of feet deep and bury a piping system that would extract heat from underground.

Piping for a 2,000-square-foot home such as theirs has to extend 800 feet to extract sufficient energy from the ground, Garnhart said. The original plan was to drill one hole about 800 feet deep to bury the piping.

“But they hit water at about 600 feet,” Garnhart said, so a second hole was drilled, 200 feet deep.

The Garnharts said that their lawn suffered temporarily because of damage from the truck and the drilling of the holes.

“For four or five months we just had to say, ‘Excuse me, don’t even look,’ ” Garnhart said with a laugh. Fortunately, their garden and sidewalk were spared.

The switch to geothermal energy was also an opportunity to set an example for their 14-year-old son and 12-year-old daughter.

“We used this as a teaching opportunity, but they already knew a lot about it already,” Lang-Garnhart said. “They’re part of a generation that is concerned about taking care of the world around them. They were very excited to see what they were learning about applied in a practical way.”