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.
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Operation
Air Source Heat Pumps (ASHP) function similarly to Ground Source Heat Pumps (GSHP) but utilize the ambient air temperature to produce heat for property heating. Unlike ground energy systems, the air temperature input for air source systems can vary significantly both seasonally and daily, and these systems may be less effective in cold winters.
Performance
These systems can be highly efficient delivering up to 250-400 % 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). Coefficient of Performance (COP) should be based on EN14511, which is the ratio of heat output against the power input. Additionally, ASHPs are often cited as "operating down to -15 deg C ambient temperatures." However, their output significantly diminishes at low external temperatures, reaching a point where no effective heat is produced, as the heat pump frequently enters defrost mode. Consequently, the efficiency declines markedly.
Optimising Efficiency
In heating mode ASHP typically operate at 45 °C flow and 40 °C return. Decreasing the output temperature by 5 K could increase the CoP by 10-20 %.
Avoid adding excessive safety margins to load calculations as oversized heat pumps will short-cycle and thus operate less efficiently.
Optimize ASHP sizing for part load conditions to reduce cycling frequency. Choose a unit equipped with multiple compressors for better efficiency.
Gas Boiler vs ASHP
The comparative energy cost and carbon emissions of a gas boiler using 11,500 kWh per annum against an air source heat pump are considered below. The assumption is the efficiencies are 90 % for the gas boiler and 294 % for the ASHP.
Gas and electricity prices are 6.34 and 24.86 p/kWh, respectively, making electricity 3.9 times more expensive than gas. These prices are based on the Ofgem energy price cap average as of January 1, 2025, excluding daily standing charges of 31.65 pence for gas and 60.97 pence for electricity. See the Ofgem (energy price cap )
The carbon emission factors for gas and electricity are 0.210 and 0.136 KgCO2e/kWh respectively. Carbon emission factors as listed in Table 12 of the BRE's The Government's Standard Assessment Procedure for Energy Rating of Dwellings SAP version 10.2 (11-04-2023).
Energy & Cost
The load is determined to be 10,350 kWh found by multiplying the gas load by the boiler efficiency (11,500 x 0.9). And the cost of operating the gas boiler is £ 729.10 (11,500 kWh x 6.34 p/kWh).
To match the load with a 294 % efficiency, the ASHP will consume 3,520 kWh (10,350 / 2.94) and cost £ 845.17 (3,520 x 24.86 p/kWh), £ 146.07 or 20 % more than the gas option.
The ASHP will achieve a break-even point at an efficiency of 353 %, with operating costs decreasing when this efficiency is exceeded. This is determined by dividing the gas cost of £ 729.10 by the electric tariff of 24.86 p/kWh to find a reduced input of 2,933 kWh. The efficiency is calculated as the load divided by the input.
Carbon
Based on an input of 11,500 kWh, the gas boiler will emit 2,415 kgCO2 (11,500 x 0.210).
The ASHP operating at 294 % efficiency will have an input of 3,520 kWh and emit 479 kgCO2 (3,520 x 0.136).
The carbon emission from the ASHP could range from 828 kgCO2 at 170 % efficiency to 352 kgCO2 at 400 % efficiency.
Analysis
An air-to-water ASHP operating with a flow temperature of 55 °C is unlikely to achieve an efficiency exceeding 300 %, especially when external temperatures fall below 5 °C. Only when the flow temperature is 40 °C or lower will the ASHP attain efficiency levels that provide a cost benefit in operation.
Given that the carbon emission factor for electricity is lower than that of gas, and considering that an ASHP operates with higher efficiency than gas systems, there will consistently be a reduction in carbon emissions when utilizing an ASHP.
| Term | Definition | |
|---|---|---|
| Accumulator Tank | A buffer vessel or thermal store. See Homemicro's article sizing a thermal store (buffer vessel) (here ) | |
| 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. | |
| Defrost Cycle | An operating mode required to remove ice from the evaporator formed at low ambient temperature when water vapour within the air freezes when flowing through the evaporator 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. | |
Heat Load Diversity
Understanding and accounting for heat load diversity prevents oversizing of plant which improves efficiency.
Typical Flow Temp.
Radiators: ~45 to 70 °C
Fan coil: ~45 to 50 °C
Underfloor heating: ~35 to 40 °C
Emitter size increases with lower temperatures.
Low Temp. Benefit
Heat pump efficiency in heating mode improves at lower flow temperatures.
LTHW distribution heat losses are reduced.
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Air Source Heat Pump (ASHP) id: lzc-05 (v.7.0)
