Boilers: How to buy the best boiler Combi gas boilers

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 or check our gas boiler brands satisfaction results to see how Which? members rate different combi boiler manufacturers.

There are several different types of combi gas 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. 

Combi gas boilers

How a combi gas boiler system works

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 potentially more that can go wrong than with a conventional boiler.
  • A combi gas boiler system is better suited to a household with a smaller number of people that tend not to need to use hot water simultaneously, 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 such as a circulation pump housed externally from the boiler. 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

  • Unlike a combi gas boiler, once the storage tank runs out of hot water, you will have to wait for the water to heat up again. 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, which also typically contains the circulation pump and may house some of the valves.

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