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.
There are three applications where ASHPs are best suited:
- Where electricity is the only fuel source available. Since ASHPs can provide heat up to almost four times more efficiently than electric resistance heaters, their operating costs will be signficantly lower.
- Where heating loads are small and the capital cost of an ASHP is less than that of a separate air conditioner and furnace. Even if cheap heating fuel produces low energy costs for a furnace, this may not outweigh the lower capital cost of buying just one piece of equipment where heating loads are small.
- Where heating loads are large and the difference in price between electricity and heating fuel is great enough to produce lower energy costs for the ASHP. ASHPs can be highly cost-effective when they have both a lower capital cost (from buying one piece of equipment instead of two) and a lower energy cost.
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:
- EER (energy-efficiency ratio). The ratio of the rate of cooling (Btu/h) to the power input (watts) at full-load conditions. The power input includes all inputs to compressors, fan motors, and controls.
- IEER (integrated energy-efficiency ratio). A measure of part-load cooling efficiency. Note that this is a new metric that began to be applied to unitary equipment on January 1, 2010, and it supersedes the previous measure, integrated part-load value (IPLV). IEER was developed to provide a more representative measure of part-load performance than IPLV—the two are not directly comparable. Also note that, because IEER is calculated by summing the EER at different load factors, it uses the same units as EER. However, the AHRI does not include units in its definition of IEER.
- COP (coefficient of performance). The ratio of the heat output in kilowatt-hours (kWh) to the energy input in kWh. Some air-source units are tested and rated at two standard air temperatures: 47° Fahrenheit (F) and 17°F.
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:
- SEER (seasonal energy-efficiency ratio). The seasonally adjusted ratio of the rate of cooling (Btu/h) to the power input (watts).
- HSPF (heating seasonal performance factor). The ratio of the rate of heating (Btu/h) to the input power (watts), adjusted for seasonal outdoor temperature fluctuations and part-load effects.
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).
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:
- Air to air. The most common type of heat pumps, air-to-air units are used widely for heating (and cooling). In heating mode, outdoor air is the source of heat. Heat is delivered to the building as hot air, either through ducts or through air handlers mounted in the heated space. Fans force both the outside and inside air through the heat exchangers. Most air-to-air heat pumps do not perform well at low outdoor temperatures and must rely on an additional heat source to maintain heating capacity.
- Air to water. This type of heat pump is usually used in larger buildings, such as offices or multifamily dwellings, where hydronic heat distribution and zonal control are necessary. Water (or antifreeze) is pumped through the hot heat exchanger, and the resulting heat is distributed inside the building through fan coils, baseboards, or radiant-heat tubing in the floors. This type of heat pump can also be applied to heat domestic hot water, using fan-forced indoor air, outdoor air, or heated exhaust air as the heat source.
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:
- Commercial. Equipment that meets the Energy Star program’s Light Commercial Heating and Cooling Key Product Criteria is awarded the Energy Star label, which helps consumers and others readily identify high-efficiency products. The CEE encourages the use of high-efficiency air-source heat pumps through its High-Efficiency Commercial Air-Conditioning & Heat Pumps Initiative, information on which is available on its Commercial HVAC page. The CEE Unitary Air Conditioning and Heat Pump Specification (PDF) provides two tier levels of efficiency for equipment less than 65,000 Btu/h of cooling capacity, but just one tier for units over this, reflecting the unavailability of higher-efficiency equipment in this size range.
- Residential (under 65,000 Btu/h, single phase). Energy Star offers its Air-Source Heat Pumps and Central Air Conditioners Key Product Criteria. The CEE offers the High-Efficiency Specification, Central Air Conditioners and Air-Source Heat Pumps (PDF); its Tier 1 specification matches that of the Energy Star program.
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.
- ASHP versus a high-efficiency ASHP. There are three calculators available that can estimate energy savings from choosing one air-source heat pump over another. The Life Cycle Cost Estimate for Energy Star Qualified Air-Source Heat Pumps spreadsheet (XLS) can be used to estimate savings for units in cities in the United States, but only for systems under 65,000 Btu/h cooling capacity. The user must enter the SEER and HSPF for the unit being evaluated as well as for a baseline unit (which could just be one that meets the standards), as well as a few other specifications. Energy Star currently has no plans to develop a calculator for units over 65,000 Btu/h, but the U.S. Department of Energy’s (DOE’s) Federal Energy Management Program has an Energy Cost Calculator for Commercial Heat Pumps that can estimate lifetime energy costs for units between 65,000 and 240,000 Btu/h. Unlike the Energy Star calculator, this one does not use default-listed cities, so it can be used for any region. However, the user must obtain local annual heating and cooling hours of operation for the facility, in addition to the EER and COP for the units being compared.
The third tool is from Energy Star Canada, which provides a Simple Savings Calculator for both residential and commercial units on its Procurement—Purchasing Toolkit that can be used to estimate savings for units in cities within Canada. Units under 65,000 Btu/h cooling capacity can be evaluted by selecting the “air-source heat pump” option from the drop-down menu, and units over 65,000 Btu/h can be evaluated by selecting the “commercial heat pump” option. The user must enter the SEER and HSPF for the former and the EER and COP for the latter for the unit being evaluated as well as for a baseline unit, as well as a few other specifications. - ASHP versus an air conditioner used in conjunction with electric resistance heat. Use the calculators described in method 1 for residential or commercial equipment, entering the cooling efficiency for the air-conditioning component. However, instead of using the heating efficiency from a second ASHP, use an approximate efficiency for an electric resistance heater. For residential equipment, use a HSPF of 3.413 for the heating efficiency, and for commercial equipment, use a COP of 1.
- ASHPs versus gas-fired equipment (residential). To estimate the energy savings that come from choosing between a residential ASHP and a gas furnace or boiler, use the DOE’s Heating Fuel Comparison calculator (XLS). Note that unlike the other calculators listed above, this one will not directly produce the energy use or savings from high-efficiency options based on the climate. Rather, it will produce the net cost to generate the same amount of heat using different types of fuel and equipment. This will show which fuel/equipment combination has the least energy usage for a given load. To estimate the magnitude of the energy usage or savings compared to other equipment, the user must multiply the cost per Btu from the calculator by the actual heating loads (in Btu) for an application.
- ASHPs versus gas-fired equipment (commercial). Use a simulation tool such as eQUEST (from DOE-2) or the DOE’s EnergyPlus to model different equipment options. In calculating the annual cost of operation, use heating and cooling loads, the local cost of electricity, the efficiency and capacity of the equipment, and the part-load operation of equipment, if applicable.
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).