Combi vs system vs heat-only boiler

The three boiler types heat water differently, and the right one for a given property is decided almost entirely by how many bathrooms are in use and whether there is space for a cylinder — not by brand, and rarely by budget alone.

Combi boilers

A combi heats water directly from the mains, on demand, with no cylinder or loft tank needed anywhere in the house. That makes it the simplest and most space-efficient option, ideal for a flat or a house with one bathroom. Its real limitation is simultaneous demand: run two showers at once on a combi and both will suffer, because it can only heat water as fast as one flow allows.

System boilers

A system boiler heats water into a hot water cylinder, which stores a supply ready to use, but needs no cold water tank in the loft because the components that manage expansion and circulation are built into the boiler itself. This suits a home with two or more bathrooms where several taps or showers might run at once, since the stored hot water is not competing for the boiler’s instant heating capacity the way a combi’s would.

Heat-only (regular) boilers

The traditional setup: a boiler, a separate cold water tank in the loft, and a hot water cylinder. More components, more places for something to need attention, but it remains the right choice where a property already has that setup and a full system change is not worth the disruption, or on very large properties with heavy demand throughout.

What actually decides it

Number of bathrooms likely to be used at once, whether there is loft space for a tank, and whether an existing cylinder and pipework can be reused rather than replaced, all matter more than which specific boiler brand or model gets chosen afterwards. See boiler installation for how we size and specify a new system, and boiler replacement if the question is really about swapping an ageing one rather than starting from scratch. If the property currently has no cylinder and you are weighing up whether adding one to move away from a combi makes sense, our page on hot water cylinders covers that decision directly.