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Air Source Heat Pump Calculator — ASHP Sizing
Size an air source heat pump for your existing property. Assess radiator compatibility, COP, running costs, and savings versus gas heating.
From heat loss survey or EPC
Total radiators in the property
Drives hot-water demand and cylinder sizing
Ofgem cap Jul-Sep 2026: 7.33p/kWh
Ofgem cap Jul-Sep 2026: 26.11p/kWh
Old boiler 70-80%, condensing 90%+
UK typically -3°C
EFLH × heat loss = annual space-heat demand. UK typical 1,200-1,800 h
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How We Calculate This
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).
Frequently Asked Questions
ASHP capacity should match your property heat loss (typically 6-12 kW for UK homes) plus hot water demand (typically 2-3 kW). A common mistake is oversizing, which leads to short-cycling and reduced efficiency. A properly insulated 3-bed semi-detached house typically needs an 8-10 kW ASHP.
Not necessarily. Heat pumps run at lower flow temperatures (35-45°C vs 70-80°C for gas boilers), so radiators produce less heat. If your radiators are already oversized for the rooms they heat, they may work fine. Otherwise, you may need to upgrade some radiators to larger models or add fan convectors for rooms with the highest heat loss.
It is important to separate two figures. The design-day COP is the worst-case value on the coldest day (around -3°C outside) — typically 2.5-3.0 at a 45-55°C flow temperature. The seasonal COP (SCOP) is the average across the whole heating season and is higher, because most heating hours happen in milder weather. UK SCOPs are typically 3.0-3.5 at 45°C flow and 2.5-3.0 at a fixed 55°C flow; the MCS minimum is 2.8 (measured at 35°C flow per the MCS 026 / EN 14825 method). This calculator uses the design-day COP for the worst-case capacity check and the SCOP for the annual running-cost and CO₂ figures.
It depends on your SCOP and tariff. At the Ofgem price cap for 1 July to 30 September 2026 (gas 7.33p/kWh, electricity 26.11p/kWh), an ASHP at a SCOP of 3.0 delivers heat at about 8.7p/kWh, compared with about 8.6p/kWh for an 85% efficient gas boiler — so on the standard cap a 55°C fixed-flow system is roughly line-ball with gas, and can be slightly dearer. The economics tip clearly in the heat pump’s favour with a higher SCOP (low flow temperature / weather compensation, SCOP 3.5+) or a dedicated heat-pump electricity tariff with cheap off-peak units. The £7,500 Boiler Upgrade Scheme grant covers a large share of the install cost.
Size the cylinder to the number of occupants — roughly 45-60 litres of stored hot water per person for a heat-pump coil cylinder. A 1-2 person home is usually fine on 150-180 L, a 3-4 person home on 200-250 L, and 5+ occupants on 250-300 L. Heat pumps reheat water more slowly than gas boilers, so the cylinder must have a large, heat-pump-rated coil and good insulation. Follow your installer’s DHW sizing for your actual usage pattern.
The Boiler Upgrade Scheme (BUS) gives £7,500 towards an air source or ground source heat pump in England and Wales, with the scheme funded to 31 March 2030. From 21 July 2026 there is a temporary uplift to £9,000 for off-gas-grid homes currently heated by oil or LPG (air-to-water and ground-source only), expected to run until the end of March 2027 before reverting to £7,500. The installer must be MCS-certified to claim the grant on your behalf.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.