Boiler Size Calculator
Find the right boiler output for your home. An undersized boiler can't heat your home efficiently; an oversized one short-cycles and wastes fuel. Get the right size first time.
Combi vs System vs Regular
Combi boiler: no hot water cylinder, heats water on demand. Best for homes with 1–2 bathrooms, up to 15 radiators. System boiler: pressurised cylinder in airing cupboard, no cold water tank. Better for 2–3 bathrooms. Regular / heat-only: separate cold tank and hot cylinder. Best for older systems or multiple bathrooms.
Output Sizing Rules
Each radiator needs approximately 1–1.5 kW. Add 3 kW per bathroom for hot water demand (combi boilers). Typical 3-bedroom semi: 24–28 kW combi. 5-bedroom detached: 35–40 kW. Adding 20% for poor insulation and heat loss is sensible.
Efficiency Ratings
Modern A-rated condensing boilers achieve 90%+ efficiency. Replacing a G-rated boiler (60% efficiency) with an A-rated model typically saves 20–30% on gas bills. Annual gas savings of £300–500 are common for poorly-insulated homes upgrading from older boilers.
Energy Efficiency Priorities
Home energy improvements should be prioritised by payback period. Quickest payback: draught proofing (£50-200 cost, saves £60-100/year — under 2 years). Loft insulation (£300-500 DIY, saves £150-200/year — 2-3 years). Switching to LED lighting (under £100, saves £40-60/year — under 2 years). Longer payback: cavity wall insulation (3-5 years). New efficient boiler (5-10 years). Solar panels (7-15 years). Heat pump (10-25 years at current energy prices). For homes eligible for ECO4 funding, the pa
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