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.
Choosing the Right Boiler
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.
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