The Guide to Home Geothermal Energy

• geothermal energy
• heating
• cooling
• alternative energy
• energy
• geology

“You’re not making heat, you’re moving heat,” Colorado geothermal installer Jim Lynch says. Installations like Lynch’s tap into the earth below the frost line—which always stays around 50 degrees Fahrenheit—to reduce a home’s heating and cooling loads. All HVAC systems require energy-intensive heat movement, a task responsible for over half of the average house’s total energy demand. Geothermal works more efficiently because the system’s mild starting point creates an efficient shortcut to the target temperature. Imagine a 100-degree Florida day or a 0-degree Michigan night: Spot the system 50 degrees, and it doesn’t work so hard to get the house comfortable.

Unlike wind and solar, geothermal’s power source never varies.

Bob Brown, vice president of engineering with equipment maker Water­Furnace, says, “The ground’s there all the time. It’s great for heating and it’s great for cooling. All I’ve got to do is bury a plastic pipe, put fluid in and, lo and behold, I’ve got a great system.”

How Geothermal Happens

• In the ground: A water-filled, closed loop of 1-inch high-density polyethylene (HDPE) pipe ferries heat between the earth and the house. Pipes descend 4- to 6-inch-diameter vertical wells—the number and depth depend on the house’s site and size—before ganging together in a header and bringing lukewarm water in through the basement walls. Drillers backfill each hole with bentonite grout (or new enhanced grouts, engineered with fly ash) to maximize thermal conductivity.

• In the house: Pumps cycle water through the pipe loop to the heart of the system: the geothermal unit, which acts as furnace and air conditioner. This machine uses refrigerant and the temperate water from the underground pipes to heat or cool air. The air is then circulated through standard ductwork. With a device called a desuperheater, the unit uses excess heat to warm up domestic hot water at no added cost. The results feel the same as those from any standard forced-air HVAC system.

The Flow

Air in the ducts (1), refrigerant in the geothermal unit (2), and water in pipes (3) flow past each other like interlocking gears. Water brought from underground transfers heat to the refrigerant, or absorbs heat from it, depending on the season. Like an air conditioner, the unit compresses or expands the refrigerant to raise or lower its temperature. Finally, the refrigerant, now heated to 180 F or chilled to 40 F, fills condenser/evaporator coils. Air in the ducts blows across the coils to be cooled or warmed, then flows through the house.

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