Geothermal heat pumps work on a different principle than ordinary furnace/air conditioning systems, and they require little maintenance or attention from homeowners. Furnaces must create heat by burning a fuel–typically natural gas, propane, or fuel oil. With geothermal systems, there’s no need to create heat, hence no need for chemical combustion. Instead, the Earth’s natural heat is collected in winter through a series of pipes, called a loop, installed below the surface of the ground or submersed in a pond or lake. Fluid circulating in the loop carries this heat to the home. An indoor geothermal system then uses electrically-driven compressors and heat exchangers in a vapor compression cycle–the same principle employed in a refrigerator–to concentrate the Earth’s energy and release it inside the home at a higher temperature. In typical systems, duct fans distribute the heat to various rooms.
In summer, the process is reversed in order to cool the home. Excess heat is drawn from the home, expelled to the loop, and absorbed by the Earth. Geothermal systems provide cooling in the same way that a refrigerator keeps its contents cool–by drawing heat from the interior, not by injecting cold air.
Geothermal systems do the work that ordinarily requires two appliances, a furnace and an air conditioner. They can be located indoors because there’s no need to exchange heat with the outdoor air. They’re so quiet homeowners don’t even realize they’re on. They are also compact. Typically, they are installed in a basement or attic, and some are small enough to fit atop a closet shelf. The indoor location also means the equipment is protected from mechanical breakdowns that could result from exposure to harsh weather.
Geothermal works differently than conventional heat pumps that use the outdoor air as their heat source or heat sink. Geothermal systems don’t have to work as hard (which means they use less energy) because they draw heat from a source whose temperature is moderate. The temperature of the ground or groundwater a few feet beneath the Earth’s surface remains relatively constant throughout the year, even though the outdoor air temperature may fluctuate greatly with the change of seasons. At a depth of approximately six feet, for example, the temperature of soil in most of the world’s regions remains stable between 45 F and 70 F. This is why well water drawn from below ground tastes so cool even on the hottest summer days.
In winter, it’s much easier to capture heat from the soil at a moderate 50 deg F. than from the atmosphere when the air temperature is below zero. This is also why geothermal systems encounter no difficulty blowing comfortably warm air through a home’s ventilation system, even when the outdoor air temperature is extremely cold. Conversely, in summer, the relatively cool ground absorbs a home’s waste heat more readily than the warm outdoor air.