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Combi Gas Boilers

By Matthew Knight

Our experts tell you everything you need to know about combi boilers and heat-only boiler systems. To see which boilers are the most reliable, take a look at our boiler reviews.

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A combination or combi boiler is the most common type of boiler system in the UK, and provides heat for your radiators and domestic hot water on demand. Read our reviews to find a Best Buy combi boiler.

There are several different types of combi boiler to choose from, but they typically work by taking cold water straight from the mains supply and heating it as you need it - eliminating the need for a separate hot water cylinder or storage tank. 

We ask Which? members to rate different combi boiler manufacturers, which enables us to reveal the most and least reliable boiler brands. To find out more, check our best and worst gas boiler brands.

Combi boilers

Pros of combi boilers

  • a gas combi boiler system contains almost all components within the boiler itself, which makes the boiler system more compact and neater looking
  • there's no need for a cold water or feed and expansion tank in the loft, and you can immediately get hot water heated on demand. 

Cons of combi boilers

  • there are moving parts within the boiler unit, so there is more potential for things to go wrong than with a conventional boiler
  • a combi gas boiler system is better suited to a home with a smaller number of people, as you'll only be able to use hot water from a combi boiler system for one task at a time. For example, one person wouldn't be able to have a shower while someone else does the washing up using hot running water.

Heat-only boilers

Unlike a combi gas boiler system, an open-vented heat-only gas boiler system (with a water tank) has components housed externally from the boiler, such as a circulation pump. This type of system will also usually require cold water, as well as feed and expansion tanks in the loft.

With a heat-only boiler (also known as a conventional boiler system), the hot water is stored in a hot water cylinder or hot water storage tank. You will also normally have an on/off switch - so you can heat water in the storage tank when you know you are going to need it, and leave it switched off when not in use.

Pros of heat-only boilers

  • heat-only boilers are better suited to larger households where several people frequently need to use hot water at the same time. 

Cons of heat only boilers

  • you will have to wait for the water to heat up again once the storage tank runs out of hot water - unlike a combi boiler, which gives you hot water on demand. If you run a bath, for example, you may have no hot water for a while afterwards.

Sealed heat-only gas boiler systems

If you think you'll need a heat-only boiler but don't want lots of external components, a sealed system could be more suitable. This is a boiler system with a hot water cylinder, but the feed and expansion tank (usually in the loft) is replaced by an expansion vessel in the boiler. This also typically contains the circulation pump and may house some of the valves.


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