What size boiler do I need for a 3-bed house?

For a typical 3-bed semi with one bathroom and 8-10 radiators, a 24-30 kW combi usually fits. With two bathrooms, 30-35 kW is more realistic — or a system boiler with a cylinder.

What is the difference between heating kW and DHW kW?

Heating kW is the power needed to keep your radiators warm. DHW (domestic hot water) kW is what a combi needs to deliver instant hot water. Combi output is usually dictated by DHW, not heating.

Can I oversize "just to be safe"?

No. An oversized combi short-cycles on the heating circuit, drops out of condensing mode, and wears itself out faster. Pick the smallest boiler that meets the hot water demand.

Two bathrooms with a combi — yes or no?

A 32+ kW combi will serve two bathrooms sequentially. Run both simultaneously and flow drops by half. For reliable simultaneous use, fit a system boiler and an unvented cylinder.

Do I need a heat loss calculation first?

For an accurate answer, yes. This tool gives a fast ballpark. A proper MCS heat loss models U-values and air changes room by room — use our heat loss calculator for that.

Does the fuel type change the sizing?

Not meaningfully for gas vs LPG — both produce similar kW. Oil boilers are specified the same way. Heat pumps are sized entirely by heat loss, not by radiator count.

Is this compliant with Benchmark / installation standards?

Sizing advice here is indicative. Always follow the manufacturer installation manual and the Benchmark scheme for commissioning. BoilerTag does not replace competent design.

Is this calculator saved anywhere?

No. The calculation runs in your browser. Nothing is sent to our server.

Boiler Size Calculator (UK kW) — Free Tool for Engineers

How it works

1
Pick boiler type
Combi, system or regular. Combi sizing is driven by hot water demand; system and regular sizing is driven by heating load.
2
Describe the property
Bedrooms, bathrooms, radiator count, insulation level and shower type.
3
Read the kW figure
You get the recommended kW plus the heating and DHW splits, with notes on edge cases.

How we size the boiler

Combi boiler size is almost always dictated by hot water demand, not heating load. One bathroom typically needs 24 kW. Two bathrooms push it to 30 kW, and three to 35 kW or beyond. Power showers add another 3 kW to hit the required flow rate at mains cold temperature.

System and regular boilers are sized on heating load alone — hot water comes from the cylinder, which is independent. We approximate heating load as 1.5 kW per radiator with an uplift for larger properties, then adjust for insulation quality.

Insulation is the biggest lever. A fully retrofitted 3-bed can sometimes run on a 15 kW system boiler where a pre-1980 equivalent needs 24 kW. If in doubt, run a proper heat loss.

Common sizing mistakes

Swapping like-for-like. The old boiler was 30 kW so the new one must be too. Wrong — condensing boilers are more efficient and better-matched to the load. Size properly.

Ignoring mains flow rate. A 35 kW combi is only useful if the incoming mains can deliver 13-14 L/min. Check mains flow at the property before specifying a large combi.

Oversizing for future-proofing. Oversized combis short-cycle on heating, fall out of condensing mode and wear faster. Size for today's load; swap the boiler in 12 years if needs change.

Frequently asked questions

Radiator BTU calculator →

Size individual radiators by room.

Heat loss calculator →

Whole-house heat loss for more accurate sizing.

Boiler Size Calculator (UK kW)