An air source heat pump uses outside air as a heat source or heat sink. A compressor, condenser and refrigerant system is used to absorb heat at one place and release it at another.


Outside air, necessarily existing at some temperature above absolute zero, is a heat container. An air-source heat pump moves ("pumps") some of this heat to provide hot water or household heating. This can be done in either direction, to cool or heat the interior of a building.

The main components of an air-source heat pump are:

  • a heat exchanger, over which outside air is blown, to extract the heat from the air
  • a compressor, which acts like a refrigerator but in reverse and raises the temperature from the outside air
  • a way to transfer the heat into a hot water tank or heating system, such as radiators or under-floor heating tubes

How air source heat pumps workEdit

Heating and cooling is accomplished by moving a refrigerant through the heat pump's various indoor and outdoor coils and components. A compressor, condenser, expansion valve and evaporator are used to change states of the refrigerant from a liquid to hot gas and from a gas to a cold liquid. The refrigerant is used to heat or cool coils in a building or room and fans pull the room air over the coils. An external outdoor heat exchanger is used to heat or cool the refrigerant. This use of outside air has led to the term "Air Source" Heat Pump. The overall operation uses the concepts described in classic vapor compression refrigeration.

When the liquid refrigerant at a low temperature passes through the outdoor evaporator coils, the temperature of the outside air causes the liquid to boil. This change of state from liquid to a vapor requires a considerable amount of energy or "latent heat" which is provided by outside air passing over the coils.

This vapor is then drawn into the compressor where the temperature of the vapor is boosted to well over 100 degrees Celsius. At this point we have used heat from the outside air to change the liquid refrigerant to a gas and added an amount of compression "work" to raise the temperature of the vapor. The vapor now enters the condenser heat exchanger coils where it begins to transfer heat to the air being drawn across the coils. As the vapor cools, it condenses back to a liquid and in so doing releases and transfers considerable latent heat to the air passing over the condenser unit coils. We have used the heat energy of outside air to change the phase of the refrigerant and then released this heat for heating, a typical heat pump operation.

At this stage we now have a very cold liquid refrigerant compressed to a high pressure. The refrigerant is next passed through an expansion valve which turns it back to a low pressure cold liquid ready to re-enter the evaporator to begin a new cycle.

The heat pump can also operate in a cooling mode where the cold refrigerant is moved through the indoor coils to cool the room air.


The 'efficiency' of air source heat pumps is measured by the Coefficient of performance (COP). In simple terms, a COP of 3 means the heat pump produces 3 units of heat energy for every 1 unit of electricity it consumes. In mild weather, the COP of an air source heat pump can be up to 4. However, on a very cold winter day, it takes more work to move the same amount of heat indoors than on a mild day. The heat pump's performance is limited by the Carnot cycle and will approach 1.0 as the outdoor-to-indoor temperature difference increases at around −18 °C (0 °F) outdoor temperature for air source heat pumps. However, heat pump construction methods that enable use of carbon dioxide refrigerant extend the figure downward to -30 °C (-22 °F). A Geothermal heat pump will have less change in COP as the ground temperature from which they extract heat is more constant than outdoor air temperature.

Seasonally adjusted heating and cooling efficiencies are given by the heating seasonal performance factor (HSPF) and seasonal energy efficiency ratio (SEER) respectively.

Advantages and disadvantagesEdit


  • Typically draws approximately 1/3 to 1/4 of the electricity of a standard resistance heater for the same amount of heating, reducing utility bills.[1] This typical efficiency compares to 70-95% for a fossil fuel-powered boiler[citation needed].
  • Few moving parts, reducing maintenance requirements. However, it should be ensured that the outdoor heat exchanger and fan is kept free from leaves and debris. Morover, it must be borne in mind that a heat pump will have significantly more moving parts than an equivalent electric resistance heater or fuel burning heater.
  • As an electric system, no flammable or potentially asphyxiating fuel is used at the point of heating, reducing the potential danger to users, and removing the need to obtain gas or fuel supplies (except for electricity).
  • May be used to heat air, or water.
  • The same system may be used for air conditioning in summer, as well as a heating system in winter.
  • lower running costs, the compressor being the thing that uses most power - when in comparison with traditional electrical resistance heaters.[citation needed].


The following disadvantages are associated with all air source heat pump designs:

  • Air source heat pumps require electricity for operation. Electricity generation accounts for nearly 40 percent of emissions pollutants and greenhouse gases in the United States.
  • External space needs to be found for the outside condenser unit which can be somewhat noisy[citation needed] and possibly unsightly.
  • The cost of installation is high (though less than a Ground Source heat pump because a ground source heat pump requires installation of a ground loop).

The following disadvantages are associated with units charged with HFC refrigerants:

  • Usually marketed as low energy or a sustainable technology, the HFCs have the potential to contribute to global warming[citation needed]. The effect the refrigerant could have is measured in global warming potential (GWP) and ozone depletion potential (ODP).
  • Air source heat pumps lose their efficiency as external temperatures fall. In colder climates the system needs to be installed with an auxiliary source of heat to providing heat at low temperatures or if the heat pump should require repair.
  • The COP is reduced when heat pumps are used to reach over 55°C for heating domestic water or in conventional central heating systems using radiators to distribute heat (instead of an underfloor heating array).
  • Retrofit is difficult when used with conventional heating systems using radiators or radiant panels. The lower Heat Pump output temperatures would mean radiators would have to be increased in size or a low temperature underfloor heating system be installed instead.


Air source heat pumps can provide fairly low cost space heating. A high efficiency heat pump can provide four times the heat compared to an electric heater.

Air source heat pumps are sometimes used to provide hot water from a pressurized system up to temperatures of 55°C. To minimize the risk from Legionellosis it is advised that hot water is heated to above 60°C.

The overall lifetime costs for using air source heat pumps should be considered carefully as mains gas (where available) may be cheaper than electricity (although is has higher carbon emissions).

If saving energy and lowering carbon emissions are important, then other low-carbon technologies should be considered, such as superinsulation, ground source heat pumps, boilers fueled by biomass (where cheap local biomass is available), and solar water heating.

Air source heat pumps should last for over 20 years with low maintenance requirements.[2][3]


  1. "Georgia State University Heat Pump Notes". 
  2. "Frequently Asked Questions by Danfoss Heat Pumps". 
  3. "How air source heat pumps work from". 


Summer, John A. (1976). Domestic Heat Pumps. PRISM Press. ISBN 0-904727-10-6.

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