Ground-source heat-pump (GSHP) heating and cooling, often called a geo-exchange or geothermal system, is an efficient way to keep a house comfortable. The Environmental Protection Agency says a GSHP system can save 50 to 60 percent on a typical home heating bill. About 1 million GSHP systems are currently in use in the United States, and about 50,000 new systems are installed annually.
How Geothermal Works
A heat pump works like a refrigerator: It uses a fluid and a compressor to move heat from one side of a wall to the other. A refrigerator chills the air inside by warming the air outside; an air-exchange heat pump does the opposite, warming the air inside the house by cooling the air outside. Think of it as an air conditioner turned backward.
A ground-source heat pump does the same thing, but it works off the stable temperature of the soil or groundwater under your property. Depending on your geology and climate, the soil beneath your house and yard remains at a remarkably stable temperature year-round — in most parts of the country, that’s within a few degrees of 55°F (12°C). A network of pipes buried in the soil can function as a heat exchanger, keeping its working fluid at ground temperature (the heat-transfer fluid can be water, water mixed with antifreeze, or a refrigerant). The heat-pump compressor uses the transferred heat to warm the inside of the house without directly burning fuel.
A small house typically requires about 3 tons of heating/cooling capacity, equivalent to about 10.5 kilowatts. A system of this size might use about 1,500 feet (about 450 meters) of tubing buried in loops near the house (the length of the tube will vary with climate). An electric pump sends the working fluid through this loop field. A heat pump replaces the original furnace or boiler. This unit transfers heat from the loop field to the inside of the house.
Most systems use polyethylene tubing as the buried loop field and pump water through it. Modern “direct exchange,” or DX, systems use a loop field of copper tubing filled with a refrigerant. This arrangement is more expensive to purchase because of the cost of the copper and refrigerant, but the system may be less expensive to install because the buried loop is shorter. It’s also less expensive to operate because the refrigerant-to-earth heat exchange is more efficient than interposing a water-to-refrigerant heat exchanger.
The heat pump inside the house can come in two flavors: water-to-air and water-to-water. A water-to-air pump heats air for a forced-air heating duct system. A water-to-water pump heats water for circulation through a hot-water or radiant-floor heating system. A hybrid system heats both air and water — you could have both forced air and radiant heat or use the hot-water output to augment the domestic hot water supply.
Cost & Payback
The U.S. Department of Energy estimates that a GSHP system costs about $2,500 per ton of capacity, or roughly $7,500 for a 3-ton unit. Energy savings may provide payback in about five years. For installation considerations, see the Department of Energy website. Visit the Energy Star website for ratings of ground-source heat pumps.
Seth Masia is an editor of SOLAR TODAY and director of communications for ASES. Contact him at firstname.lastname@example.org. Illustration by Kurt Struve.