A system boiler is a central heating boiler that works with a hot water storage cylinder. Unlike a combi (which heats water on demand), a system boiler heats and stores a tank of hot water ready for use.
How a system boiler works
The system boiler heats water and circulates it to both the radiators (via the central heating circuit) and the cylinder (via a coil inside it). The cylinder stores typically 150–250 litres of hot water at 55–65°C. When you turn on a hot tap, you draw from the stored cylinder rather than waiting for the boiler to fire.
Advantages over a combi
- Multiple simultaneous outlets: Two showers running at once don't affect each other — the cylinder delivers mains-pressure hot water to all outlets simultaneously
- No wait time: Hot water is immediately available from the cylinder, not heated on demand
- Higher flow rates: An unvented cylinder delivers higher flow rates than any combi can match
- Better for hard water: The heat exchanger isn't exposed to DHW, reducing scale issues
Disadvantages vs combi
- Requires space for the cylinder (typically airing cupboard or utility room)
- Higher upfront installation cost (boiler + cylinder)
- Cylinder heat loss (typically 1–2 kWh/day) adds to running costs
Who needs a system boiler?
Homes with two or more bathrooms in regular simultaneous use. Households of 4+ people. Properties where combi flow rates have been inadequate. Book a free survey for a recommendation.
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