Air-source electric heat pumps are year-round space-conditioning systems capable of providing heating, cooling, and domestic hot water. Their appeal lies both in that they offer heating and cooling in a single piece of equipment—which usually means a lower capital cost—and in that they provide heat at a lower cost than electric resistance heating (in some cases, lower than gas heating as well). They can be used in most commercial applications and some industrial processes, particularly those that generate waste heat. However, most air-source heat pumps do not perform well in cold climates, because both their capacity and their efficiency decrease significantly at low temperatures.

An air-source heat pump (ASHP) transfers energy between the outside air and either air or water (for hydronic heating systems) inside a building. In cooling mode, it functions just like an air conditioner, moving heat from the inside of a building to the outside; in heating mode, the refrigerant flow is reversed so it takes low-temperature heat from the outside air and mechanically concentrates it to produce high-temperature heat, which is then delivered to a building (Figure 1). Because most of the heat is simply moved (pumped) from the outdoor air to the indoors, the amount of electricity required to deliver it is typically less than would be required from electric resistance heat.

Figure 1: Schematic diagram of an air-to-air heat pump operation

A heat pump can switch from heating to cooling functions by changing the position of the reversing valve. All common heat pumps contain two heat exchangers (one cold and one hot) plus a compressor charged with refrigerant. The hot heat exchanger delivers heat from condensing refrigerant while the cold heat exchanger absorbs heat as refrigerant evaporates. The refrigerant is forced by the motor-driven compressor to circulate and change phase from liquid to gas in the cold evaporator and back to a liquid in the hot condenser. The heat exchangers typically require a fan or pump to move air or water through them to achieve effective heat transfer from a heat source to a heat sink.

There are three applications where ASHPs are best suited:

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Efficiency. The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) defines efficiency for air-source electric units with cooling capacities larger than 65,000 Btu per hour (h) using:

Below 65,000 Btu/h, both three-phase (commercial) and single-phase (residential) units are available for use in commercial buildings. For either, AHRI uses the following metrics:

Manufacturers report data for these metrics to AHRI, which then publishes it in online directories. As part of its product certification program, AHRI audits the manufacturer information and tests a percentage of units to ensure the accuracy of the data. As of January 1, 2010, AHRI extended its program to cover equipment in the 250,000 to 760,000 Btu/h capacity range.

Federal minimum standards. New federal standards for commercial air-source heat pumps became effective on June 16, 2008, and January 1, 2010; standards for residential units became effective on January 23, 2006. These standards require manufacturers to produce equipment with specified minimum efficiencies (Table 1 and Table 2); they replace the previous standards from 1992. Note that the federal standards for commercial units do not regulate unit efficiency at 17°F, but ASHRAE does specify minimum efficiencies at this temperature in Addendum S of its Standard 90.1-2007, “Energy Standard for Buildings Except Low-Rise Residential Buildings” (Table 3).

Table 1: Smaller commercial units offer more efficiency opportunity

These are the minimum and highest available efficiencies for commercial (three-phase) air-source heat pumps. The federal standards for units larger than 65,000 Btu/h only apply to two efficiency metrics: energy-efficiency ratio (EER) and coefficient of performance (COP), measured at an air temperature of 47° Fahrenheit. A new “very large” size category was established by the Energy Policy Act of 2005. The Energy Information and Security Act of 2007 set the under-65,000 Btu/h three-phase units to the same efficiency levels as single-phase (residential) units.

Table 2: Available residential units handily beat the required minimum efficiency levels

These are the minimum and highest available efficiencies for residential (single-phase) air-source heat pumps. Residential units are available in dramatically higher efficiency levels than are required by the federal standards.

Table 3: Efficiency guidance at lower temperatures

Though the federal standards do not regulate heat pump performance at 17°F, ASHRAE Standard 90.1 provides guidance for minimum efficiencies at this temperature for commercial (three-phase) equipment.

Highest available efficiency. Though manufacturers continue to offer higher-efficiency commercial units, the highest available efficiencies for units above 65,000 Btu/h are not much higher than the minimums now required by the standards. Also, these highest levels are almost the same as those available in 2006. However, the new standard that became effective January 1, 2010, eliminates several units with efficiencies as low as 9.0 SEER from the market. For units below 65,000 Btu/h, more high-efficiency options are available—one unit even has almost twice the SEER rating than is required by the federal standards.

Configuration. Air-source heat pumps are available in two different configurations:

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Select a type. When retrofitting an existing building with a heat pump, it is best to consider what heat distribution system the building already has in place. If it’s got air ducts, an air-to-air heat pump is likely to be most cost-effective; if hydronic piping is in place, an air-to-water system will likely be less expensive. New construction requires a full cost-effectiveness analysis of the HVAC system to choose the type of heat pump that best complements your other choices.

Select the right size. Just like an air conditioner, an undersized heat pump won’t be able to provide sufficient cooling, but if a unit is oversized (the more frequent occurrence), it not only costs more, but will also lead to higher costs for associated ductwork and other auxiliaries. Operating costs increase too, because oversized equipment spends more time at less-efficient part-load conditions. Specifiers and designers commonly overestimate loads because they fail to take into account the reduced air-conditioning loads that result from energy-efficient lighting, and they overestimate plug loads by using inflated nameplate ratings of office equipment in the building.

It is also critical to use diversity factors when calculating internal loads. For example, consider a school: Peak load for the classrooms occurs when the classrooms are full, peak for the auditorium happens during an assembly, and peak for a gym occurs during a basketball game with the stands full. However, peak load for the school is not the sum of these loads, because they do not all occur simultaneously.

Consider high-efficiency units. Two organizations offer high-efficiency recommendations for air-source heat pumps: Energy Star—a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy—and the Consortium for Energy Efficiency (CEE), a nonprofit that aims to accelerate the adoption of energy-efficient technologies. The Energy Star program allows manufacturers to apply Energy Star labels to equipment that meets the program specifications so that consumers can readily identify qualified products. The CEE’s specifications are used by utilities that offer rebates for equipment that meets its efficiency levels. Consumers can check with their local utility for rebates that this equipment qualifies for or just use the CEE’s criteria to help guide the selection of high-efficiency equipment. Recommendations are available for both commercial and residential units:

To find energy-efficient heat pump products, use the AHRI Directory of Certified Product Performance. These directories include products from all AHRI member-manufacturers. For commercial systems, search under “unitary large equipment”; for residential systems, search under “heat pumps and heat pump coils.” The CEE also maintains a directory of AHRI-certified products under 65,000 Btu/h cooling capacity (both single- and three-phase power) in its Directory of Energy-Efficient HVAC Equipment.

Evaluate high-efficiency models by performing a cost-effectiveness calculation. There are several calculators available that can help to evaluate energy costs for ASHPs and other equipment options. For applications where electricity is the only fuel source available, use the calculators or approaches from the first two equipment comparison methods below. If heating fuel is also available, then use the calculator in method 3 for residential equipment. For commercial equipment, or to derive a more accurate energy savings estimate than the calculators can provide for any equipment, use the simulation tools in method 4.

Pay attention to design, commissioning, and maintenance. No matter what equipment you choose, it’s also important to make sure that the overall system is designed to be efficient, that it’s commissioned to operate as planned, and that it’s properly maintained. Comprehensive testing, adjusting, and balancing of commercial units and their controls will maximize installed efficiency and comfort. For all units, conducting regular tune-ups, cleaning and adjusting the system to correct airflow and improve heat transfer, and repairing major duct leaks can yield surprising energy savings at low cost. Also, a low-static-pressure duct system will reduce control problems, noise, and the fan power required. For more best practices, see the CEE’s Guidelines for Energy-Efficient Commercial Unitary HVAC Systems (PDF).

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