If you visited any of the caverns around Texas during the blazing hot summer, you must have noticed it was nice and cool underground. That’s because several feet below the Earth’s surface, the temperature stays between 60 to 70 degrees, regardless of how hot or cold it is above. Wouldn’t it be nice if you could use the Earth’s constant, moderate temperature to efficiently heat or cool your home or business?
You can. Read about Libby and David Sartain in this month’s Texas Co-op Power magazine. The couple restored Ancient Oaks, a historic plantation home in Bastrop County. Libby Sartain is a descendant of the home’s original owner. The restoration included installing a ground-source heating and cooling system.
Almost all heat-pump systems essentially work the same way. In the summer, they cool our homes by moving heat from inside to outside. In the winter, they heat our homes by moving heat from outside to inside. Conventional heat pumps use the outside air to regulate the temperature in your home or business. How hard they work depends on how hot or cold it is outside.
Most heating and air-conditioning systems use those air-source heat pumps. But now, ground-source heat pumps that use the Earth’s temperature for heating and cooling are, well, gaining ground.
Here’s how they work: Ground-source heat pumps have three loops. The first loop is the air that circulates through an air-handling unit inside your home. The second loop is in the air handler and contains a refrigerant, which flows through a coil. The air passes over the coil and is heated or cooled, depending on the season. So far, that’s the same as air-source systems.
It’s the third loop that’s different. In an air-source unit, the refrigerant flows through the compressor and fan unit outside your home, where it releases heat from inside into the air. In a ground-source unit, the third loop contains water that flows through the air-handling unit, outside your home and then circulates through a series of underground pipes. During the summer the water removes the heat from inside your home and transfers it to the Earth. During the winter, the process is reversed and the water is heated underground and warms the air in your home.
Jim Bose, professor and director of Oklahoma State University’s engineering technology division and executive director of the International Ground Source Heat Pump Association, said since the Earth’s temperature below the surface is more moderate than the air above it, ground-source units don’t have to work as hard and are more efficient.
“They consume about 30 percent less electricity, which pays for the system,” Bose said. “In a new house, it’s a no-brainer.”
The cost of a ground-source system can be built into the cost of a new house and spread over the life of the mortgage. The new homeowner would see an immediate cost savings. But existing homes, such as the Sartains’ historic one, can benefit from these high-efficiency systems, too.
Though more efficient than conventional heating and air-conditioning systems, ground-source systems cost more to install. For each ton of air conditioning, a 200- to 300-foot hole must be drilled to install pipes to circulate water. A high-efficiency, 3–ton variable-speed unit would cost about $30,000, according to Stan Johnson, of Stan’s Heating and A/C. Johnson has been installing heating and air conditioning systems for nearly 40 years.
By comparison, a high-efficiency, 3-ton variable-speed conventional unit would cost $8,000 to $10,000.
Johnson told me his company installed what was then called geothermal systems back in the ’70s but stopped because the technology wasn’t perfected. “We got back into it in 2008 and it took off because of the 30 percent tax credit,” Johnson said. “We’ve had a lot of people take advantage of that tax credit.”
In some cases, Johnson said — depending on factors such as the size of the unit and the house, and if the house is all electric, electric and propane, or electric and natural gas — the payback can be as short as six years.
The federal tax credit for geothermal heat pumps expires in 2016. For more information, go to energystar.gov.