Air Source Heat Pump Calculator — ASHP Sizing

This calculator sizes an air source heat pump based on your property heat loss and assesses whether your existing radiators can deliver sufficient heat at the lower flow temperatures used by heat pumps. Once you know the capacity you need, use our heat pump cost calculator for UK installation prices, BUS grant deduction and payback.

Radiator assessment

• Standard rating: Radiators are rated at delta T 50°C (flow 75°C, return 65°C, room 20°C) per BS EN 442
• Heat pump flow: Output is corrected with (ΔTm/50)^1.3. On a 21°C room with a 5K flow→return spread, output falls to about 33% of the rated value at 45°C flow and about 55% at 55°C flow
• Assessment: If total radiator output at the required flow temperature exceeds the heat loss, existing radiators are adequate

COP and SCOP

The design-day COP is estimated from the required flow temperature and the outside design temperature (worst case, used for the capacity check). For annual running cost and CO₂ the calculator uses the seasonal COP (SCOP), which is higher than the design-day COP because most heating hours occur in milder weather. The SCOP figures are indicative — for a site-specific value use the manufacturer’s ErP data / MCS 026 (EN 14825) calculator.

Annual heat demand

Annual space-heating energy is the building heat loss (kW) multiplied by the equivalent full-load hours (EFLH) — typically 1,200-1,800 h in the UK — not the peak heat-pump capacity multiplied by all on-hours, since the pump rarely runs at full output. Hot water is added separately at roughly 610 kWh per occupant per year (DESNZ domestic hot-water use insights).