Combi vs system boiler: which is right for your home?

The short answer

Small-to-medium home, one bathroom, decent mains pressure: combi. Large home, two or more bathrooms, multiple showers running at once: system boiler with unvented cylinder. Everything else falls into the middle and depends on specifics.

How each one works

Combi boiler: heats your hot water on demand, straight from the mains. No tank, no cylinder. You turn on the tap and within seconds it's hot. When it stops, it stops, no stored reserve.

System boiler: heats water in a hot water cylinder (unvented, pressurised, mains-fed in modern setups). Hot water is stored, so you can run two showers and the kitchen tap simultaneously without anyone losing pressure. But when the cylinder's empty, you wait for it to reheat.

Combi pros

• Saves space, no cylinder, no cold water tank in the loft.
• Endless hot water while the boiler's running.
• Cheaper install, typically £1,950–£2,450 fitted.
• Simpler system, fewer parts, fewer things to go wrong.

Combi cons

• Single-outlet, run the shower and the kitchen tap at once and both get cold.
• Mains-pressure dependent, poor mains pressure means poor showers.
• Flow rate matters more than output, check your incoming mains before committing.

System boiler pros

• Multiple outlets, two showers at once, no drama.
• Consistent pressure regardless of kitchen tap activity.
• Works with heat pumps later, cylinder can stay when you upgrade.
• Good for large families or homes with 3+ bathrooms.

System boiler cons

• Space for cylinder, typically airing cupboard (150–250 litres).
• Install cost is higher, typically £3,250+ fitted.
• Wait time if cylinder empties, though with a decent system this is rare.
• Building Control notification required for unvented cylinders.

The flow-rate question

Before you commit to a combi, test your mains flow rate. Fill a bucket from the kitchen cold tap for 30 seconds, measure the litres. Double it for the flow rate in l/min. Under 12 l/min and a combi will struggle to run a decent shower, a system boiler is probably the right answer.

If you're in BT9, BT7, Holywood or Bangor and have a larger house, installers often recommend system. If you're in a BT15 two-up-two-down with good mains, combi is almost always right.