Open-loop GSHPs are also referred to as groundwater or surface-water heat pumps, depending on the water source. Water is pumped from the ground or a body of water, circulated through the heat pump, where heat is either extracted from it or added to it, then returned to the source. If the water is extracted from a well (most common), it is usually returned to the ground through a separate injection well. Open-loop heat pumps present a greater risk of contaminating either the aquifer (groundwater) or the surface water from which the water is drawn, and are prohibited in some states.
Rather than circulating an antifreeze solution through the ground, direct-exchange (DX) GSHPs circulate the refrigerant directly. These heat pumps are simpler and more efficient because they don’t use a secondary heat exchanger in the heat pump cabinet to transfer heat between the ground-contact antifreeze and refrigerant loops. But DX heat pumps require copper tubing, which is more expensive than polyethylene. They require a lot more refrigerant, and the copper may corrode and eventually leak.
Some GSHP manufacturers claim COPs as high as 6 for their systems under certain conditions, and COPs of more than 3 are common. High COP claims may be exaggerated for real systems in the field—the performance of GSHPs drops as the heat-exchanger loop warms or cools the soil over time.
One of the only long-term studies of GSHP performance is a field study of 83 heat pump installations in the United Kingdom by The Energy Saving Trust, with support from government agencies, utility companies, and manufacturers. Published in 2010, the study examined 54 GSHP installations and 29 air-source heat pump installations.
Only a handful of the GSHPs (13%) performed at a COP of 3 or higher. About the same number had measured COPs of less than 2. The largest number performed with COPs in the 2.2 to 2.4 range—below the expected performance of GSHPs.
One of the problems appears to be poor installation or lack of commissioning (inspecting, testing, and tweaking the installation after completion). Operation was another problem, with some homeowners telling researchers that they did not understand the operating instructions.
GSHPs also alter the ground temperature, causing a drop in performance. “You end up cooling the ground a lot in the winter and warming it up a lot in the summer,” says Temple. In climates such as New England, where heating loads dominate, cooling of the ground from GSHPs may accumulate over multiple years—dropping the GSHP performance from year to year. In cooling-dominated climates, a similar drop in performance may occur over time as the ground warms up from year to year. More testing is needed to understand these seasonal and year-to-year performance issues with GSHPs.
Air-source heat pumps have long been popular in some parts of the United States. Until recently, they only made sense in milder climates where winter temperatures rarely drop below about 40°F.
Unlike their ground-source cousins, ASHPs have separate indoor and outdoor units where different parts of the vapor-compression refrigerant cycle take place. But like GSHPs, they can be switched from heating mode to cooling mode seasonally.
ASHPs rely on the outside air as the heat source in winter and as the heat sink in summer. When outside temperatures are low, it’s harder to extract heat from the air, and efficiency drops. In fact, at temperatures below 30°F, most older U.S. ASHPs would automatically switch to electric-resistance heating, so the benefit of the refrigerant cycle was lost.
In the past 10 to 15 years, there has been a revolution with ASHPs, as Japanese companies have brought variable-refrigerant-flow (VRF) or “minisplit” heat pumps into the U.S. market. Most VRF heat pumps can operate down to 0°F (or even lower) without significant loss in performance. In southwestern New Hampshire, for example, these systems have been successfully heating houses without any other heat source, even with outside temperatures as low as -18°F.
VRF heat pumps vary the flow of refrigerant in ways that significantly boost performance, while the older, standard heat pumps operate at a constant flow—either on or off. VRFs also benefit from improved refrigerants and sophisticated electronic controls.
The indoor unit can be wall- (most common), floor-, or ceiling-mounted. Small-diameter refrigerant lines connect the indoor and outdoor units. They are relatively easy to install and elegant in their simplicity. They cost a lot less than GSHP systems, because trenching or well drilling isn’t required, nor are long lengths of tubing .