HVAC: Air-Source Electric Heat Pumps

Efficiency. The Air-Conditioning and Refrigeration Institute (ARI) 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.
• IPLV (integrated part-load value). A measure of the part-load cooling efficiency in units of Btu/watt-hour. This metric only applies to units that are capable of capacity reduction.
• 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.

For three-phase air-source heat pumps with cooling capacities below 65,000 Btu/h, the ARI 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.

The cooling efficiencies of heat pumps under 250,000 Btu/h are certified according to standards published by the ARI. ARI standards also apply to heat pumps of 250,000 Btu/h and over, but the ARI currently has no certification program and does not publish efficiency data for this size range. However, because the federal standards that will become effective in 2010 include a new size category that goes up to 760,000 Btu/h, the ARI has said that it will develop a certification program for these units before the new standards become effective.

Federal minimum standards. The current U.S. federal standards for commercial air-source heat pumps were established in 1992 and require manufacturers to produce equipment with specified minimum efficiencies (Table 1). New federal standards, established by the Energy Policy Act of 2005, will become effective January 1, 2010. Standards for three-phase equipment under 65,000 Btu/h are under consideration for revision. Note that the federal standards do not regulate IPLV or COP at 17°F.

Highest available efficiency. Manufacturers of heat pumps continue to offer higher-efficiency units. As of 2006, the highest-efficiency air-source heat pumps on the market in sizes ranging from 65,000 to 135,000 Btu/h have EER values as high as 11.5 and COPs as high as 3.6; units from 135,000 to 240,000 Btu/h have EER values as high as 11.0 and COPs as high as 3.3.

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 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 wont be able to provide sufficient cooling, but if a unit is oversized (the more frequent occurrence), it not only costs more but will 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.

As manufacturers raise the efficiency of their products to comply with the new federal standard that takes effect in 2010, its likely that they will introduce new high-efficiency options as well. In turn, the CEE and Energy Star will also likely raise the efficiency levels that units must meet in order to comply with their definitions of high efficiency.