How an ASHP works
An air source heat pump extracts low-grade heat from outside air — it works even below freezing — and upgrades it via a refrigerant compression cycle to heat your home and hot water. A typical modern unit achieves a SCOP of around 3–4: three to four kilowatt-hours of heat per kilowatt-hour of electricity, against ~0.9 for a gas boiler’s best.
Most installations are air-to-water: the pump feeds underfloor heating or radiators plus a hot water cylinder. ASHPs perform best delivering lower flow temperatures into well-insulated, airtight buildings — exactly what Part L makes new dwellings.
In the SAP calculation
A strong choice: with SAP 10’s electricity carbon factor near gas parity, an ASHP at SCOP 3+ dramatically betters a gas boiler on carbon and primary energy — often enough to carry the SAP pass on its own. It’s also a common answer to planning renewables conditions, and effectively the default under the Future Homes Standard.
Practical considerations
The outdoor unit needs airflow space and sensible siting — current permitted development rules and MCS planning standards govern proximity to boundaries and noise. Cylinders need a cupboard. Compared with a GSHP, an ASHP costs far less to install but runs slightly less efficiently in deep winter. For most new builds, it’s the pragmatic choice.