Boiler buying guide
Combi vs system boiler: which one is right for your home?
A combi boiler is best for flats and small-to-medium homes with one bathroom — it provides hot water on demand, takes up less space, and doesn't need a cylinder. A system boiler is better for larger homes with two or more bathrooms running simultaneously, because it stores hot water in a cylinder and can supply multiple outlets at once without a drop in pressure. For most London terraced houses with a single bathroom, a combi is the right call. For larger semis and detached homes with multiple bathrooms, a system boiler usually wins.
By MAMOSS Engineering Team — Gas Safe registered engineers
How a combi boiler works
A combi (combination) boiler heats your central heating and provides hot water on demand from a single unit. There's no hot water cylinder and no cold water tank in the loft — just one wall-hung unit, usually in the kitchen or utility cupboard.
When you turn on a hot tap, the boiler fires up and heats mains-pressure water as it passes through. That makes it efficient (no stored hot water cooling down) and powerful at the tap.
How a system boiler works
A system boiler heats your central heating and a separate hot water cylinder. The cylinder stores a tank of hot water, ready to be drawn from any tap or shower at any time.
Because the hot water is stored, you can run two showers or a shower and a bath simultaneously without the temperature dropping — something a combi struggles with.
You don't usually need a cold water tank in the loft (those are for the older 'regular' or 'heat-only' boilers).
When a combi is the right choice
Choose a combi if: you have one bathroom, you have limited space (especially in a London flat or terraced house), you want to free up an airing cupboard, you have decent mains water pressure (at least 1.5 bar), and you don't run two hot taps simultaneously very often.
Combis suit the vast majority of London flats and small-to-mid terraces. They're typically cheaper to install and take up less space.
When a system is the right choice
Choose a system boiler if: you have two or more bathrooms used at the same time, you have a larger home (4+ bedrooms), your mains pressure is poor and a combi would underperform, or you want the option to add solar thermal or a heat pump in future.
Larger London semis and detached homes — and any property where teenagers tend to overlap shower times — benefit from the stored hot water of a system boiler.
Cost comparison
A combi swap is usually cheaper to install than a system boiler swap because there's no cylinder. A regular-to-combi conversion is also cheaper than a regular-to-system conversion overall.
Long-term running cost is similar for both modern A-rated units. The bigger drivers are insulation, controls and the size of the boiler relative to your home — not the combi/system choice itself.
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