How do I calculate my home's heat loss?

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.

What is the design temperature difference for the UK?

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.

What size boiler do I need?

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.

What size heat pump do I need?

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.

Why is ventilation heat loss important?

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 85-95% of heat from exhaust air, dramatically reducing ventilation losses in airtight buildings.

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Whole House Heat Loss Calculator — Boiler & Heat Pump Sizing

Calculate total house heat loss from fabric and ventilation losses. Determines boiler or heat pump size in kW for UK design conditions.

House TypeDetermines exposed wall area

Total Floor Area

All floors combined

Number of Floors

Ceiling Height

2.4m typical

Wall U-Value

0.35 cavity, 1.5 solid uninsulated

Roof U-Value

Floor U-Value

Window U-Value

Window % of Wall

20% typical

Air Changes per Hour

0.5 modern, 1.0 draughty

Heating System

How We Calculate This

This calculator determines total dwelling heat loss by summing fabric losses through each building element and ventilation losses, then sizes the heating system.

The formula

Fabric loss (W/K) = Σ(Area × U-value) + Thermal bridging

Ventilation loss (W/K) = 0.33 × ACH × Volume

Peak heat loss (kW) = Total loss (W/K) × ΔT ÷ 1000

Boiler size = Peak heat loss × Sizing factor

For example, a semi-detached house 85 m² over 2 floors: Total fabric + vent loss = ~350 W/K. At ΔT 25°C: 350 × 25 = 8,750 W = 8.75 kW. Gas boiler: 8.75 × 1.2 = 10.5 kW. Heat pump: 8.75 kW.