Overview
A combi boiler heats water on demand from the mains — no storage cylinder needed. A system boiler stores hot water in a separate cylinder for higher-volume delivery.
Cost comparison
Combi installation: from £999. System boiler + cylinder: from £1,499. Combi wins on upfront cost.
Hot water performance
System boilers with a large unvented cylinder deliver mains-pressure hot water to multiple outlets simultaneously. A combi's flow rate (10–13 L/min) can struggle with two showers running at once. System wins for larger households.
Space
Combis need no cylinder — ideal for smaller homes where an airing cupboard is needed for storage. System boilers require an airing cupboard. Combi wins on space.
Our verdict
Up to 3 beds, 1–2 bathrooms: choose a combi. 4+ beds or 3+ bathrooms: system boiler is usually better value over time despite higher upfront cost.