Articles on LZC, HVAC, PC & CAD too!

Air Source Heat Pump (ASHP)

Air Source Heat Pump (ASHP)

Air Source Heat Pumps absorb heat from the outside air using a refrigerant circuit. A compression cycle turns this low grade heat into a higher temperature heat capable of heating water for property heating and hot water circuits.

A message from a sponsor


These systems can be highly efficient delivering up to 250% seasonal efficiency. However, as the external temperature falls the operating efficiency falls too.

The performance of an ASHP is often quoted with "A7" (this being the air at 7°C). Performance should be based on EN14511, which will demonstrate their true performance. Furthermore, ASHP's are often quoted as "operating down to -15 deg C ambient". The output drops markedly at low external temperatures to the point where there is no effective heat at all since the heat pump is almost permanently in defrost mode. As a result, the efficiency declines significantly.


If an ASHP is replacing an existing system, the carbon factor of the fuel must be considered. Converting from a conventional gas system could actually be worse since the change from gas to electricity as a primary fuel source means an increase of CO2 concentration per kWh based on UK Building Regulations 2016 data. Gas consumption emits 0.194kg of CO2 for every kWh used, whereas electricity generation emits 0.43kg per kWh, more than 200% increase! The system improvements have to counter this before any savings can be made.



Air Source Heat Pumps (ASHP) operate in a similar fashion to Ground Source Heat Pumps (GSHP) but use the ambient air temperature to generate heat for use in heating properties. Unlike ground energy systems the air temperature input for air source systems can vary greatly both seasonally and daily and the systems are not suited to cold winters.

Term Definition
Accumulator Tank A buffer vessel or thermal store
Condenser Coil The outdoor unit of a heat pump that has a condensing coil heat exchanger that either releases or collects heat. Also comprises the compressor. Also known as outdoor unit or outdoor coil.
Evaporator Coil The indoor unit of a heat pump that has an evaporator coil heat exchanger that either heats or cools the space. Also known as indoor unit or indoor coil.
Heat Pump Equipment that heats or cools by moving heat. During the winter, a heat pump draws heat from outdoor air and circulates it through your home's air ducts. In the summer, it reverses the process and removes heat from your house and releases it outdoors.
SEER The Seasonal Energy Efficiency Ratio is an energy efficiency rating for air conditioners. The higher the SEER, the greater the energy performance and operating cost saving.

Suggested Reading

BSRIA: BG7/2009 Heat Pumps

CIBSE: TM51 Ground Source Heat Pumps (here )

A message from a sponsor

2009-2022 All Rights Reserved -    Based upon an original design by w3.css with a little help from Bootstrap & Pixaby / free images Pixaby free images