Air Source Heat Pump Calculator

Lower flow temperatures give much higher efficiency. SCOP (Seasonal COP) is the average over the heating season.

Underfloor heating or oversized / low-temperature radiators are strongly recommended for heat pump compatibility.

A formal heat loss survey (to MCS or SAP) gives the most accurate figure - use the manual override if you have one.

• ASHP output is sized to cover the design heat loss at the design outdoor temperature.
• A 1.25× oversizing factor is applied to handle defrost cycles and brief cold snaps without using backup electric resistance heating.
• Oversizing too much reduces part-load efficiency - avoid more than ~1.5× unless the climate is severe.
• Common residential sizes: 5-16 kW. Larger properties or cold climates may need 20 kW+.

• Underfloor heating (35°C): Ideal. Large surface area means very low flow temperature and maximum SCOP.
• Upgraded / fan radiators (45°C): Standard radiators upsized by 1.5-2× or low-temperature fan convectors. Good efficiency.
• Standard radiators (55°C): Works but significantly reduces SCOP. Best reserved where refurbishment is not possible.

Moving from 55°C to 35°C flow can improve SCOP by 35-50%, typically adding years off payback.

• SCOP - Seasonal COP: average efficiency across the heating season. A SCOP of 3.5 means 3.5 kWh heat per 1 kWh electricity.
• Design heat loss (kW) - the rate of heat loss from the property at the coldest outdoor design temperature. This drives the ASHP unit size.
• Full-load hours - the equivalent number of hours per year the ASHP runs at full capacity. Multiplied by heat loss to give annual heat demand.
• MCS - Microgeneration Certification Scheme. MCS-certified installers are required for UK grant eligibility (Boiler Upgrade Scheme).