Whole House Heat Loss Calculator — Boiler & Heat Pump Sizing

Heat loss has two components: fabric loss (heat escaping through walls, roof, floor, and windows) and ventilation loss (warm air leaking out). Fabric loss = Area × U-value for each element. Ventilation loss = 0.33 × Air changes × Volume. Multiply the total loss rate (W/K) by the design temperature difference (typically 25°C in the UK) to get the peak heat loss in watts.

The standard UK design temperature difference is 25°C — based on 21°C internal and -4°C external (severe winter conditions). In Scotland and northern England, -5°C to -7°C external may be appropriate, giving ΔT of 26-28°C. In mild coastal areas, ΔT of 23°C may suffice. Using -4°C external ensures the heating system can cope with all but the coldest nights.

For a gas boiler, add 20% to the calculated heat loss for hot water demand and system losses — a 10 kW heat loss typically needs a 12 kW boiler. For a heat pump, size at 1.0× the heat loss as it runs continuously at low output. Most UK 3-bed semi-detached houses need 8-12 kW of heating capacity. Oversizing wastes energy; undersizing means the house won't reach temperature in cold spells.

Heat pumps should be sized to the actual design heat loss — typically 5-12 kW for UK homes. Unlike boilers, heat pumps should not be oversized as they run most efficiently at low continuous output. An 8 kW air source heat pump suits most well-insulated 3-bed semi-detached houses. Poorly insulated homes may need 12-16 kW or fabric improvements first to reduce heat loss.

Ventilation heat loss typically accounts for 30-40% of total heat loss in modern well-insulated homes. It's the energy lost when warm indoor air escapes and is replaced by cold outdoor air. Reducing air changes from 1.0 to 0.5 ACH saves significant energy. MVHR systems recover 70-95% of heat from exhaust air (typically up to 90%+), dramatically reducing ventilation losses in airtight buildings.

No. The annual figure is an approximate estimate of the heat delivered into the dwelling, based on the UK-average heating degree-day total (~2,228 to base 15.5°C). It is not the fuel you buy: for a gas or oil boiler, divide by the seasonal efficiency (~0.9) to get the fuel input; for a heat pump, divide by the SCOP (~3-4) to get the electricity input. The estimate also ignores solar and internal gains, so treat it as indicative only — regional degree-days range from roughly 1,800 (southern England) to 2,800 (northern Scotland).