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The 1,500-foot pipes (above, left) used by the Trinity Church are most unusual. But the basic principle of the geothermal system (below) is no different from one you could have in your home.

“The only place to go is down.” That’s what the congregation of Boston’s Trinity Church decided. The landmark building, built in 1877, needed a new heating and ventilation system. Surrounded by public squares and huge high-rises (including the famed John Hancock Tower), the church has no backyard where builders could hide the unsightly equipment. After investigating a variety of technologies, the historic congregation-which has been meeting in various buildings since about 1735-decided to go with a geothermal heat pump (GHP).

Typically, such a pump is built by running numerous pipes through the soil encircling the building. But the church had almost nowhere to dig. So instead of drilling numerous holes 200 to 300 feet deep, as other large buildings have required, the church will run only six pipes 1,500 feet straight down to provide enough cooling and heating water to keep the building well tempered.

GHPs are efficient and environmentally friendly. So why don’t more builders opt for them? “It’s still not a well-known technology,” says Sara Quinn of the Geothermal Heat Pump Consortium. Currently, some 650,000 GHPs are in use in the U.S.-about 1 percent of the market. “But it has enormous potential,” Quinn goes on. “It can cut heating and cooling costs by 25 to 40 percent.”

In Winter (illustration, left)

1. Water in the pipes absorbs heat from the soil.
2. Cold refrigerant in the heat exchanger absorbs the water’s heat.
3. A compressor pressurizes the refrigerant and raises its temperature.
4. The unit sucks in cold air that absorbs heat from the refrigerant.
5. A fan blows the warm air into the building’s ducts.

In Summer

A. The unit sucks in
warm air.
B. The refrigerant absorbs its heat.
C. A fan blows the cooled air back into the building.
D. Cool water absorbs heat from the refrigerant.
E. The warmed water returns to the ground.