Is a combi boiler better than a system boiler?

It depends on your home. A combi is great for smaller homes and one bathroom, giving instant hot water and saving space. A system boiler with a cylinder suits larger homes with several bathrooms used at once. Paul gives you an honest recommendation.

Do combi boilers run out of hot water?

Because they heat on demand, they do not run out the way a cylinder can. But running two hot outlets at once can reduce the flow, so a very high-demand household may be better on a system boiler.

Can I switch to a combi boiler?

Often yes. Converting from an old system with a cylinder and tank to a combi is a common job. Paul will assess your home and mains pressure and tell you honestly if it is the right move.

How Does a Combi Boiler Work? | Explained

How does a combi boiler work?

In short: A combi (combination) boiler heats water on demand directly from the mains: when you turn on a hot tap or the heating, it fires up and heats the water there and then, so there is no hot water cylinder or loft tank taking up space, and you get hot water instantly without waiting for a tank to fill.

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Combi boilers are the most common type fitted in UK homes today, and for good reason, but it helps to understand how they work so you can decide whether one suits your home. Here is the plain-English version from a Gas Safe engineer.

How a combi heats your home and hot water

A combi does two jobs from one compact unit. For heating, it warms water and pumps it round your radiators. For hot water, it heats water straight from the mains on demand, the moment you open a hot tap, rather than storing it in a cylinder.

Because it heats on demand, there is no need for a hot water cylinder in the airing cupboard or a cold water tank in the loft, which frees up space and means you are not paying to keep a tank of water hot. When a hot tap and the heating are both called for, the combi prioritises the hot water.

The pros and cons of a combi

A combi suits a lot of homes, but not every home. The honest trade-offs are:

01

Instant hot water, no waiting

You get hot water on demand without waiting for a cylinder to heat up, and no risk of running out mid-shower, as long as demand is not too high.
02

Saves space

No cylinder and no loft tank means a combi frees up cupboard and loft space, which is why they suit flats and smaller homes so well.
03

Best for one bathroom at a time

A combi heats water for one outlet strongly, but running two showers at once can drop the flow. For a big house with several bathrooms used together, a system boiler with a cylinder may suit better.
04

Depends on mains pressure

A combi relies on good mains water pressure and flow. If your incoming mains is weak, Paul will tell you honestly whether a combi is the right choice.

Still stuck? Give Paul a bell.

Forty years on the tools, Gas Safe registered, and the fella who quotes it is the fella who does it. No call centre, no pushy sales.