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Heat Pumps – Electric – Air Source

Furnaces and chillers use a fossil fuel or electricity to create heat or provide chilled water that is used to warm or cool the building space. Heat pumps use energy to move heat—indoors on cold weather days and outside during hot weather—which in turn heats or cools the building.

Even on the coldest days, some heat can still be extracted from outdoor air. The air source heat pump provides indoor heating by pulling this heat into the building. Because the effectiveness and efficiency of the heat pump depends on the heat available in the outdoor air, areas with continuous sub-freezing winter temperatures are not usually practical applications for air source heat pumps. When outdoor temperatures fall below 40°F, back-up heat from a gas furnace or expensive electric resistance heat is required.

How It Works
A heat pumps refrigeration system consists of a compressor and two coils of copper tubing. In split systems, one coil is located indoors and the second coil is outside. Supply and return ducts connect to a central fan, which is located indoors. Packaged systems have both coils and the fan outdoors. Heated or cooled air is delivered to the interior from ductwork that protrudes through a wall or roof. Aluminum fins surround the coils and aid heat transfer.

Pressure changes caused by the compressor and the expansion valve allow the liquid refrigerant to condense at a high temperature and evaporate at a lower temperature.

In cooling mode, the air source heat pump evaporates the refrigerant in the indoor coil. As the liquid evaporates, it pulls heat from air inside the building. The gas formed during this process is compressed and moves into the outdoor coil where it releases its heat as it condenses. In heating mode, a reversing valve changes the direction of the refrigerant flow so that the refrigerant evaporates in the outdoor coil, pulling heat from the outside air, and condenses in the indoor coil, releasing its heat into the building.

Efficiency
The Energy Star® logo, a program of the Environmental Protection Agency (EPA), is awarded to those heat pumps that meet stringent energy efficiency standards established by the Department of Energy (DOE). The chart below shows the DOE recommended efficiencies for commercial air source heat pumps. A listing of Energy Star® commercial heat pumps is available from the EPA or at their website, www.energystar.gov/products.

More information about the Energy Star® program is available in the Food Service module.

Product Type and Size

Recommended Levela

Best Availableb

Air Sourcec
< 65,000 Btu/h

12.0 SEER or more
7.7 HSPF or more

13.2 SEER
8.5 HSPF

Air Source
65,000 – 135,000 Btu/h

10.1 EER or more
10.4 IPLV or more
3.2 COP or more

11.5 EER
13.4 IPLV
4.0 COP

Air Source
135,000 – 240,000 Btu/h

9.3 EER or more
9.5 IPLV or more
3.1 COP or more

10.5 EER
12.4 IPLV
3.3 COP

aEfficiency levels for air-source units sized between 65 and 240 MBtu/h meet ASHRAE 90.1 minimum efficiency requirements.
bThe best available EER and best available COP apply to different models.
cOnly units with 3-phase power supply are covered in this category.

EER (energy efficiency ratio) is the cooling capacity (in Btu/h) of the unit divided by its electrical input (in watts) at standard peak rating conditions. SEER (seasonal energy efficiency ratio) and IPLV (integrated part-load value) are similar to EER, but weight performance during the cooling season. COP (Coefficient of Performance) is the heating capacity (in Btu/h) at standard heating conditions divided by its electrical input (also in Btu/h). HSPF (Heating Seasonal Performance Factor), like SEER, weights heating performance at various conditions.

Source: U.S. Department of Energy, Federal Energy Management Program, How to Buy an Energy-Efficient Commercial Heat Pump

Some advanced heat pump technologies in the very early stages of potential commercialization are:

  • Reverse Cycle Chillers – Allows for a wide variety of heating and cooling distribution systems, from radiant floor systems to forced air systems, and multiple zones. Heat pump is connected to a large, heavily insulated tank of water that is used to heat or cool. Claimed advantages include peak efficiency even at low temperatures, free domestic hot water, and elimination of blowing cold air during defrost cycle.
  • Cold Climate Heat Pump – Two-speed, two-cylinder compressor, back-up compressor and economizer that allow for efficient operation to temperatures well below 0°F.
  • All-Climate Heat Pump – Designed with heating as its primary focus, the unit can operate efficiently without supplemental heat to temperatures below 0°F.