Combi or system boiler: which is right for your home?
For most one to three-bed homes with a single bathroom, a combi boiler is the right choice: hot water on demand, no cylinder, no loft tank, less to go wrong. For larger homes, or where two showers can run at once, a system boiler with a hot water cylinder is the better fit. Neither is better overall, they suit different houses.
That is the answer in one paragraph. Here is the reasoning so you can sense-check it against your own home.
How a combi boiler works
A combi (combination) boiler heats your radiators and produces hot water on demand from one unit. There is no separate hot water cylinder and no cold tank in the loft. It only heats the water you actually use, which saves space and avoids heat lost from a stored tank.
How a system boiler works
A system boiler works with a separate hot water cylinder, usually in an airing cupboard. It heats a tank of water ready for use, so several outlets can draw hot water at the same time without the flow dropping away.
Which one suits your home
Use this as a rough guide:
- One to three beds, one bathroom: a combi is usually ideal. Lower install cost, no cylinder taking up a cupboard.
- Four-plus beds, or more than one bathroom in regular use: a system boiler with a cylinder copes better when two showers run together.
- Poor mains water pressure: a combi depends on incoming mains pressure, so a system boiler with a cylinder can be the safer choice.
- You currently have a cylinder and like strong simultaneous flow: keeping a system setup may make more sense than forcing a combi in.
The two-showers test
The simplest way to decide: do two hot taps or showers ever need to run at the same time? A combi delivers hot water well to one outlet at a time, then flow drops if a second is opened. If simultaneous demand is a real part of daily life in your house, lean towards a system boiler. If it is not, a combi is simpler and cheaper.
Cost difference
A combi is generally cheaper to install because there is no cylinder. Switching from a system setup to a combi takes longer, typically two days, because the old cylinder and tank are removed and pipework is re-routed, so that conversion costs more than a straight like-for-like swap. Our full breakdown of what a new boiler costs in Yorkshire covers the figures in detail.
Running cost
For a small to medium home, a modern combi only heats water on demand, so it usually runs more efficiently than an older system boiler losing heat from a stored cylinder. In a larger home with high simultaneous demand, a well-specified system boiler can be the more sensible long-term setup. Savings always depend on the house and how it is used, so be wary of anyone promising a fixed figure.
Space and disruption
A combi frees up a cupboard because there is no cylinder and no loft tank, which is worth something in a smaller house. Converting from a system setup to a combi means losing nothing useful but gaining that space back, though the conversion itself is a two-day job rather than a one-day swap. If you are tight on storage, that reclaimed airing cupboard can be the deciding factor on its own.
Hard water and your area
In hard-water parts of Yorkshire, limescale builds up faster in any boiler. It is not a reason to pick one type over the other, but it is a reason to have the system flushed and protected properly at install and serviced every year, whichever you choose. A system that is looked after lasts its full life regardless of type.
Get it sized properly
A rule of thumb is fine for a first idea, but boiler sizing depends on bedrooms, bathrooms, radiators and how many taps run at once. The wrong size either struggles or wastes gas. We size it at the survey rather than guessing.
To see the options for your home and have it sized correctly, compare combi and system boilers with us and we will recommend honestly based on your house, not on margin.