Boiler Size Calculator for Homes & Buildings

Quick method: 7.2 kW. 140 m² at 52 W/m² for a modern build. It is a lookup against construction age, and it exists for the case where you do not yet have U-values.

Envelope method: 5.6 kW. Adding up the actual heat loss paths:

Walls      130 m² × 0.25 = 32.5 W/K
Windows    30 m² × 1.10 = 33.0 W/K
Roof      140 m² × 0.18 = 25.2 W/K
Floor     140 m² × 0.22 = 30.8 W/K
ΣUA                 = 121.5 W/K
Ventilation 350 m³ × 0.5/h → 0.0486 m³/s × 1206 = 58.6 W/K
Total                  = 180.1 W/K
× ΔT of 31 K = 5.58 kW, or 40 W/m²

With a 15% margin: 6.4 kW. That is the space-heating duty. Note the windows: 30 m² of glazing loses more than 130 m² of wall.

Switch the same house to a combi boiler and the required output jumps to 29.3 kW — nearly five times the space-heating duty.

12 L/min = 0.2 kg/s
× 4.186 kJ/kg·K × 35 K rise = 29.3 kW

A combi has no stored hot water, so it must raise the full flow instantaneously. The boiler is therefore sized on a two-minute shower, not on the coldest day of the year — and it will spend the entire heating season running at roughly a fifth of its rated output.

This is the single most consequential decision on the page. A system boiler with a cylinder is sized on the 6.4 kW heating load, because the cylinder absorbs the hot-water peak and recovers over half an hour. A combi serving the same house needs 30 kW. The flow rate you specify matters more than the house: dropping from 12 to 10 L/min takes the requirement to 24.4 kW, and a low-flow shower head changes the boiler you need.

Boilers are habitually oversized, and the penalty is not merely a larger purchase price.

Short cycling. A boiler whose minimum modulated output exceeds the building's demand cannot run steadily — it fires, satisfies the thermostat, shuts off, and repeats. Every cycle carries a purge and light-off loss, and cycling wears the ignition and fan. In mild weather a 30 kW combi in a 6 kW house may cycle several times an hour.

It defeats condensing operation. A condensing boiler only recovers latent heat when the return water is below the flue gas dew point — about 55 °C for natural gas, lower as excess air rises. Oversized emitters running at low flow temperature achieve this; an oversized boiler driving high flow temperatures through undersized radiators does not, and the unit runs several points below its rated seasonal efficiency while still being labelled "condensing".

Turndown ratio is what to check. A boiler modulating 1:10 from 30 kW still has a 3 kW floor, which is workable. One modulating 1:4 has a 7.5 kW floor and will cycle badly in the shoulder seasons. When a combi must be large for hot water, turndown ratio — not rated output — is the specification that determines whether it will heat the house well.

Steady-state, no solar or internal gains. The calculation is a design-condition heat loss. It ignores occupants, appliances and solar gain, which is the conservative choice for sizing but means the boiler will rarely see this load.

No thermal bridging allowance. Plane-element U-values alone understate real loss. EN ISO 14683 handles junctions through linear thermal transmittance; a typical allowance is 10–20% on the fabric total for ordinary construction, more for poorly detailed junctions.

Infiltration is estimated, not measured. The 0.5 ACH default suits a reasonably airtight modern dwelling. A draughty older house can exceed 1.0 ACH, which here would add another 58 W/K — a third of the total load. Where a blower-door result exists, use it.

No room-by-room distribution. A whole-house figure sizes the boiler. It does not size radiators, pipework or pump, all of which need the load broken down per room.

No warm-up allowance. Intermittently heated buildings need extra capacity to recover from setback. EN 12831 provides a reheat power term; continuously heated homes do not need it, but a building heated only at weekends does.

Heat load: EN 12831-1 is the European method for design heat load, including thermal bridging and reheat terms. CIBSE Guide A gives UK design temperatures and fabric data. In North America, ACCA Manual J is the residential equivalent, with Manual S for equipment selection.

Hot water: BS EN 806 and BS 8558 for domestic water service design; CIBSE Guide G for demand estimation. A cylinder is sized on daily volume and reheat time, not on instantaneous flow.

Efficiency: the Ecodesign regulation (EU 813/2013) sets minimum seasonal efficiency and effectively requires condensing boilers; UK Building Regulations Part L references it.

Next steps: do a room-by-room load before buying radiators — the whole-house figure cannot tell you whether the back bedroom is served. Check that the emitters can deliver the load at a flow temperature of 50–55 °C, because that is what keeps the boiler condensing; if they cannot, the fix is larger radiators rather than a hotter boiler. Confirm the gas meter and supply pipe can carry the peak input, which for a 30 kW combi is not trivial. Then check the boiler's minimum modulated output against the 6.4 kW heating load, not against its maximum.

Only for a rough first estimate. Final sizing should be based on design heat loss from walls, roof, windows, floor and ventilation/infiltration.

Instantaneous domestic hot-water production can require much more power than the building heat loss, so the DHW flow rate often governs combi-boiler input.

No. Excessive boiler capacity can cause short cycling, poor seasonal efficiency and control problems. Check minimum modulation as well as maximum capacity.