A heat pump uses the same refrigeration cycle technology as your home’s refrigerator or air conditioner pooling heat from one environment and dumping it in another. Because a heat pump is equipped with a reversing valve, it can both heat and cool your home. During the heating season, heat pumps move heat from the outdoor air to warm your house; during the cooling season, the cycle is reversed.
About 85% of the installed residential heat pumps are air-source heat pumps, which transfer heat between the house and the outside air. These can be either large central units that distribute heat and cooling through ducts or smaller ductless units that are installed in one or more areas of the home for zoned heating and cooling. Ground-source and water-source heat pumps are other options. More information on these and other types of heat pumps, including absorption or gas-fired heat pumps and reverse-cycle chiller heat pumps, can be found at www.energysavers.gov
Central Air-Source Heat Pumps
Most heat pumps installed in U.S. homes are central air-source heat pumps that distribute heated and cooled air through ducts, just like a central forced-air furnace. Most of these air-source heat pumps are split systems meaning that the air handler (which houses the blower fan) is indoors and the compressor is outdoors. The air handler contains the blower and the inside coil for the heat pump.
Current federal law requires that air-source heat pumps have a minimum heating efficiency, or Heating Season Performance Factor (HSPF), of 7.7 and a minimum cooling efficiency of Seasonal Energy Efficiency Ratio (SEER) of 13. Higher efficiency central air-source heat pump models are available with up to 9.6 HSPF and SEER 23.
Because the heating efficiency of standard, central air-source heat pumps drops when outside temperatures drop below about 35°F,
a backup heat source is often needed, especially in cold climates. Air-source heat pumps can be all-electric or dual-fuel systems. All-electric air-source heat pumps come equipped with electric- resistance strip heaters for supplementary heat if needed. Dual-fuel systems combine the air-source heat pump with another source of supplementary heat, such as a gas furnace. Another type of central air-source heat pump developed in Canada for cold climates is the bivalent heat pump; it uses a gas- or propane-fired burner to increase the temperature of the air entering the outdoor coil, allowing the unit to operate at lower outdoor temperatures with less frost buildup on the outside coil (NRCan 2009).
If you heat with electricity, a heat pump can trim the amount of electricity used for heating by as much as 30% to 40% compared
to electric resistance heat. The efficiency of today’s air-source heat pumps is one to two times greater than those available 30 years ago due to technical advances such as thermostatic expansion valves, variable-speed blowers, improved coil design, two-speed compressors (instead of single-speed compressors), and improved motor designs. Variable-speed compressor designs that better match refrigerant flow to load are in development and will make heat pumps more effective at lower outside temperatures. Variable-refrigerant-flow, ductless “mini-split” heat pumps are already available that can heat at 100% capacity at outdoor temperatures as low as 5°F.
If you plan to install a heat pump, ask your HVAC installer to confirm that the existing ducts are appropriately sized for the heat pump. Ducts may need to be larger for a heat pump than for a gas or oil furnace because furnaces generally deliver air to the living space at between 130°F and 140°F. Heat pumps provide air at about 80°F to 115°F so more air needs to be delivered to provide the same amount of warmth. Your HVAC contractor should confirm that the supply air registers achieve a “throw” appropriate for a heat pump. Choosing the right register design can be important for minimizing comfort complaints because heat pumps blow more air at cooler temperatures than gas- or oil-fired furnaces.
Some researchers suggest central-air-source heat pumps may need to be slightly oversized, to enable the system to provide enough warmth without turning on the backup heat.
Ductless Heat Pumps
High-performance ductless heat pumps are an efficient alternative to central ducted heat pump systems. Ductless heat pumps are sometimes referred to as mini-split heat pumps because they consist of a single outside compressor/condenser unit connected to one or more wall- or ceiling-mounted indoor air handler units. They can provide zone heating and cooling without ducts. The outdoor units are mounted on the wall or on a concrete or stone pad outside the house; refrigerant tubing connects the inside and outside units through a small hole in the wall.
Ductless heat pumps have been used in Asia and Europe since the 1970s and they comprise 80% to 90% of the residential HVAC market there. They have been used in U.S. commercial buildings since the 1980s, but they still comprise less than 3% of the U.S. residential market. They are 25% to 50% more efficient than electric baseboard or wall heaters (NEEA 2009). Ductless heat pumps provide increased energy savings over standard heat pumps in several ways. Because they are ductless and mounted inside conditioned space, they avoid the distribution losses of a central furnace that has leaky ducts installed in an unheated attic or crawlspace. Ductless heat pumps provide zoned heating because units can be turned off or not installed in rooms that aren’t being used. They use a much smaller blower than central units; however, more than one inside unit is typically needed to serve the whole house. Up to eight inside units can be connected to one outside unit.
Advances in technology in recent years have increased performance to the point that units are now available with heating efficiencies as high as 12 HSPF and cooling efficiencies as high as SEER 26. The most efficient ductless heat pumps use a variable-speed compressor that can vary the refrigerant flow. They also have linear expansion valves rather than open/close valves, and multi-speed rather than single-speed fans to continuously match the heating or cooling load. Unlike conventional air-conditioning and heating systems that stop and start repetitively, the inverter technology adjusts the motor speed, allowing the system to adapt more smoothly to shifts in demand with less temperature variation and much lower energy use. When maximum capacity is not needed, compressor revolution and power decreases, increasing energy efficiency. For example, one model reports a capacity range of 3,100-24,000 Btus in heating mode and 3,800-14,500 Btus in cooling mode.
The best performing ductless heat pump models perform at a much wider temperature range than standard heat pumps. Some models can operate at an outdoor temperature range of -5°F to 75°F for heating and 14°F to 115°F for cooling, eliminating the need for backup heat sources in most locations.
Ground-Source Heat Pumps
A ground-source heat pump is an electric heat pump that exchanges heat with the ground or ground water, instead of air. The temperature of the earth below the surface remains fairly constant at a U.S. average of 55°F throughout the year (cooler in the north, warmer in the south) with less than 20 degrees variation over the year at 5 feet below the surface.
Because heat is exchanged with the ground rather than the outside air, which has more erratic temperatures, ground-source heat pumps remain a very efficient source of heating and cooling all year. Additional efficiency is gained by using water rather than air as the heat-exchange fluid. (The ground-source systems we are describing here do not include the geothermal systems that use high below-ground temperatures associated with volcanic activity for heat and power production.)
Ground-source heat pumps may be closed-loop or open-loop systems. A “closed-loop” ground-source heat pump circulates water (or a mixture of water and anti-freeze) from the heat pumps to horizontal or vertical pipes that are buried in the ground in contact with the earth, which serves as a heat source in winter and heat sink in summer. After exchanging heat with the ground, the water is circulated back to the heat pumps in a closed loop. Closed-loop configurations include piping laid in horizontal rows or loops in trenches 5 to 10 feet deep, or vertical loops inserted in boreholes that are 75 to 500 feet deep and filled in with bentonite or other grout materials to aid heat transfer to the soil. Closed loops can also be laid in a private pond to exchange heat with the pond water. Another much less common type of closed-loop system is the direct exchange heat pump, which circulates refrigerant rather than water or antifreeze directly through the ground in a single closed loop of copper tubing. This system uses more refrigerant and copper tubing, which are expensive but are more efficient at heat transfer so less tubing length and thus less digging is required.
An “open-loop” ground-source heat pump uses groundwater from a well as the heat source and heat sink. The water circulates through the heat pump(s) once and is returned to the ground through a separate injection well or through surface discharge.
In heating mode, the heat is transferred from the ground loop to the refrigerant loop in the heat pump, then distributed to the home via a second heat exchanger, by warming either air, which is blown over the heat exchanger and through ducts just like a central furnace, or fluid, which flows through tubing installed in the floors to provide radiant heat to the rooms.
Because the compressor for the ground-source heat pump is located inside the home, it is subject to much less wear and tear than the outdoor compressor fans of air-source heat pumps, and as a result, ground-source heat pump equipment lasts longer and maintains its efficiency better than air-source heat pumps. Two- speed compressors that more effectively match demand and scroll compressors with fewer moving parts have dramatically increased efficiency since the 1990s.
Most ground-source heat pumps are equipped with a desuperheater, which is an auxiliary heat-recovery system that can be connected to the home’s water heater tank to provide up to 25% to 50% of the home’s domestic hot water. Because they use extra heat from the cooling process they are more effective in hot climates where the heat pump is in cooling mode most of the time.
Ground-source heat pumps can have high installation costs because they require drilling or trenching (CEC 2011). If there is a pond on the property, the loops can be laid on the pond bed, a less costly installation than digging trenches, as long as the tubing is covered by 8 feet of water year-round.
Ground-source heat pumps have risen in popularity in the United States from 35,600 units shipped in 2000 to 115,400 ground-source heat pumps shipped in 2009. The ground-source closed-loop units shipped in 2009 had an averaged rated heating efficiency of 4.1 Energy Efficiency Ratio (EER) Btus/hr/W and an average rated cooling efficiency of 20.4 EER Btus/hr/W.
Ground-source heat pump efficiencies of 300% to 600% have been reported, compared to 175% to 250% for central ducted air-source heat pumps. Pump power consumption is not usually included in the rated efficiency of the system and should be taken into account when considering a ground-source heat pump installation (Sherwin et al. 2010). Good thermal connectivity between the loop and ground is essential for high efficiency and soil irregularities can affect performance. System life is estimated at 25 years for the inside components and 50+ years for the ground loop.
While ground-source heat pumps can save more energy than central ducted air-source-heat pump systems, studies are still being done to determine whether their additional costs justify their installation over variable refrigerant flow ductless heat pumps. One option that has been proposed for increasing ground-source heat pump efficiency is combining the ground-source heat pump with the variable refrigerant flow technology of ductless heat pumps. In a modeling study of multifamily housing (using energy savings data that were confirmed by field studies), ground-source heat pumps combined with variable refrigerant flow technology cut energy use by 36% compared to an air-source central heat pump system, while ductless heat pumps cut energy use by 32% and a regular ground- source heat pump alone cut energy use by 28%.