What is the difference between a combi, system and conventional boiler?

They differ in how hot water is produced and stored, which determines what tanks you need and how the system behaves.

Combi (combination) boiler. Heats water instantaneously on demand, directly from the mains. No hot water cylinder and no cold water tank in the loft.

Advantages: compact, no stored water to lose heat, mains-pressure hot water at every outlet, endless hot water, and no waiting for a tank to reheat. It is why combis dominate in flats and smaller homes.

Limitations: flow rate is the constraint. A combi can only heat water as fast as it flows through, so running two showers simultaneously reduces both. Performance also depends on incoming mains pressure and flow, which some properties simply do not have. Less suitable for homes with several bathrooms.

System boiler. Heats water for a hot water cylinder, usually an unvented pressurised cylinder. Most components — pump, expansion vessel — are built into the boiler.

Advantages: stored hot water supports several outlets at once at good pressure, and it works with solar thermal and immersion heaters as backup.

Limitations: the cylinder takes space, stored water loses heat gradually, and it runs out until reheated.

Conventional (regular, or heat-only) boiler. The traditional arrangement: a hot water cylinder plus a cold water storage tank and a feed-and-expansion tank, usually in the loft. Gravity-fed rather than mains-pressure.

Advantages: suits homes with old pipework that mains pressure could damage, and supports multiple outlets.

Limitations: most space required, lower hot water pressure unless pumped, and loft tanks can freeze or leak.

Which to choose depends on bathroom count, mains flow rate — worth measuring before committing to a combi — available space, and whether you may fit a heat pump later, which needs a cylinder.

All new gas boilers sold are condensing, recovering heat from flue gases.