What size boiler do I need? A plain-English sizing guide

The two numbers that matter

A combi boiler has a heating output (radiators) and a hot water output (taps and showers). The hot water figure is usually the one that decides the size — it sets how many outlets can run at once.

Rough guide by home

• 1–2 bed flat, one bathroom — 24–25kW combi
• 3-bed house, one bathroom — 28–30kW combi
• 4-bed house, two bathrooms — 35–40kW combi, or a system boiler with a cylinder
• Two+ bathrooms in constant use — system boiler with a cylinder beats any combi

Why bigger is not better

An oversized boiler short-cycles: it fires, hits temperature, stops, and repeats. That wears components, wastes gas and gives you temperature swings. The right size runs steadily. Our quote builder recommends a size from your home's details and the survey confirms it — nobody should sell you a 40kW boiler for a two-bed flat.

System and heat-only sizing

For boilers feeding a cylinder, the heating output is sized to the radiators and the cylinder recovery — typically 12–25kW for most homes. The cylinder, not the boiler, decides how many showers can run at once.