Introduction to Air Source Heat Pumps


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.


Efficiency, Cost & Carbon Comparison - Homemicro article on operating cost and carbon emissions

Sizing a Buffer Vessel for use with an ASHP details here
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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.

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

The significant drawback with ASHP is that you must consider what fuel it is replacing. If it is in place of a conventional gas system it could actually be worse since the change from gas to electricity as a primary fuel source means an increase of CO2 concentration per kWh. 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.

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". At such temperatures, the output is very low indeed 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.

View the article on thermal store buffer vessels

Accumulator Tank A buffer vessel or thermal store
Condenser Coils 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.

BSEE magazine article on ASHP: ASHP's compared to condensing boilers
Energy Saving Trust article: Generate your own energy with Air Source Heat Pumps
Heat Pump Schemes BSRIA heat pump testing schemes article

ASHP - last updated 13th March, 2016 by Corny