Combi vs System vs Regular Boilers: Which Is Right for You?

A combi boiler heats your water on demand. Cold water enters the boiler from the mains, gets heated as it passes through a heat exchanger, and comes out the hot tap. There's no stored hot water and no cylinder — the boiler does both heating and hot water from a single wall-hung unit.

A system boiler heats water that's stored in an unvented cylinder (usually airing-cupboard sized). The boiler itself is sealed and pressurised — no header tank in the loft. When you turn a hot tap on, you draw stored hot water at full mains pressure. When the cylinder cools, the boiler refills it.

A regular boiler (also called heat-only or conventional) is the traditional setup: a boiler downstairs, a hot-water cylinder in the airing cupboard, and a cold-water tank plus feed-and-expansion tank in the loft. Gravity-fed, low pressure at the taps, lots of pipework. Most of these systems are 30+ years old and were designed for a different era.

On a combi, you get hot water the moment you turn the tap on — but only one tap at a time can run flat-out. Try to run a kitchen tap and a shower together on a 28kW combi and one of them will go cold or weak. Combi flow rates are typically 10–15 litres per minute on a good unit; an unvented cylinder on a system boiler will deliver 20–30 LPM at full mains pressure.

Heating-only performance is roughly the same across all three types — they all run at the same efficiency band (90%+ on modern condensing boilers). The differences are entirely about hot water and installation complexity.

• One bathroom, two adults — combi handles this easily. Most efficient choice.
• Two bathrooms, one used at a time — combi still works if mains pressure is good (1+ bar dynamic).
• Two bathrooms run simultaneously — system boiler with unvented cylinder. Combi will disappoint.
• Three or more bathrooms — system or regular with appropriately-sized cylinder, no exception.
• Low mains pressure (under 1 bar dynamic) — combi will struggle. Stick with stored hot water.

Combi-for-combi swaps are typically the cheapest install option in our area on most properties. Going from a regular boiler with tanks to a combi is more involved — we remove the cylinder, drain and remove the loft tanks, cap off the old pipework, and re-pipe to the new combi location, so the conversion sits in a higher band.

System boiler installations cost similar to combi swaps if you're keeping an existing cylinder, but if the cylinder needs upgrading to unvented (which it usually does for proper performance), add a separate fixed-price line for a quality stainless steel unvented cylinder plus the G3 unvented commissioning.

For a full breakdown of pricing in our area, see our guide to how much a new boiler costs in 2026.