Combination Boilers-Heating Services

Combination gas boilers

A combination ‘combi’ boiler is the most common form of boiler system in the UK, and provides heat for your radiators and domestic hot water on demand.

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.

How a combi gas boiler system works

How a combi gas boiler system works

Pros and cons 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 – but as there are moving parts within the boiler unit, there is more that can potentially go wrong than with a conventional boiler.

On the plus side, a combi gas boiler means there’s no need for a cold water or feed and expansion tank in the loft, for example, and you can immediately get hot water heated on demand. That does mean 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.

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.

If you’re interested in getting a combi boiler, see our boilers review for the latest models, including test scores and Best Buys. Our gas boiler brands satisfaction results also show how Which? members rate different combi boiler manufacturers.

Heat only boilers

Unlike a combi gas boiler system, an open-vented heat only gas boiler system (with a water tank) has components like 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 and cons 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.

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.

If you’re interested in getting a heat only boiler, see our boilers review for the latest models, including test scores and Best Buys. Our gas boiler brands satisfaction results also show how Which? members rate different boiler manufacturers.

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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