Combi or system? It's the first question most homeowners face when looking at a new boiler — and the wrong choice can mean cold showers or wasted money.
What is a combi boiler?
A combi (combination) boiler heats water directly from the mains on demand. There's no hot water cylinder or cold water tank — the boiler is the entire system. Turn on the tap and the boiler fires up instantly.
Pros: Space-saving, cheaper to install, instant hot water, no heat loss from a cylinder.
Cons: Can struggle to supply two showers simultaneously, lower flow rate than stored hot water.
What is a system boiler?
A system boiler heats water and stores it in a separate cylinder (usually in an airing cupboard). Hot water is available immediately from the cylinder, but the cylinder needs time to reheat after it's been used up.
Pros: High flow rate — multiple showers at once. Better for large families. Works with solar thermal.
Cons: Requires cylinder space, slightly higher installation cost, heat loss from cylinder overnight.
The honest answer
For most UK homes — 3 bedrooms, 1–2 bathrooms, family of 2–4 — a combi boiler is absolutely fine and the best value choice.
If you have 3+ bathrooms, 5+ people in the house, or regularly need 2 showers running simultaneously, a system boiler with a large cylinder is the better choice.
Still unsure? Use our 2-minute calculator — it asks the right questions and recommends the right boiler type for your home.