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Asked Questions

Question:How about ASHP Efficiency Ratings?

Answer:  Answer:

The 'Efficiency' of air source heat pumps is measured by the Coefficient of performance (COP). A COP of 3 means the heat pump produces 3 units of heat energy for every 1 unit of electricity it consumes. Within temperature ranges of -3°C to 10°C, the COP for many machines is fairly stable at 3-3.5.

In very mild weather, the COP of an air source heat pump can be up to 4. However, on a 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, which for most air source heat pumps happens as outdoor temperatures approach −18 °C / 0 °F. Heat pump construction that enables carbon dioxide as a refrigerant may have a COP of greater than 2 even down to -20°C, pushing the break-even figure downward to -30 °C (-22 °F). A ground source heat pump  has comparatively less of a change in COP as outdoor temperatures change, because the ground from which they extract heat has a more constant temperature than outdoor air.

The design of a heat pump has a considerable impact on its efficiency. Many air source heat pumps are designed primarily as air conditioning units, mainly for use in summer temperatures. Designing a heat pump specifically for the purpose of heat exchange can attain greater COP ratings and an extended life cycle. The principal changes are in the scale and type of compressor and evaporator.

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

In units charged with HFC refrigerants, the COP rating is reduced when heat pumps are used to heat domestic water to over 60°C or to heat conventional central heating  systems that use radiators to distribute heat (instead of an underfloor heating array).