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Understanding Different Boiler Types:

Which One is Right for You?

Everything you need to know about different boilers, including combi boilers, heat-only and system boilers.

  Combi is the most common type of boiler in the UK. It will provide hot water on demand and is a great choice for many homes. But it's not the best option for all. Heat-only and system boilers are much better suited than combi boilers in various situations. The wrong type could lead to significantly higher bills or long delays for hot water. Read on to learn more about combi, heat-only and system boilers, and find out the pros and cons of each.

  Unlike a combi boiler, an open-vented heat-only boiler (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 and feed and expansion tanks in the loft. The hot water is stored in a hot water cylinder or hot water storage tank with a heat-only boiler. You will also usually 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.

  A system boiler is sometimes known as a closed vent boiler, and they are a perfect choice if you think you'll need a heat-only boiler but don't want any external components. This is because many of these external components are built into the boiler's body, as with a system boiler. 

A system boiler still has 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 valves.

Combination or combi boiler

Pros of combi boilers

  • a gas combi boiler contains almost all components within the boiler itself, which makes the boiler system more compact and neater-looking 

  • there's no need for cold water or feed and an 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 also known as a Regular boiler

Pros of heat-only boilers

  • heat-only boilers are better suited to larger households where several people frequently need to use hot water simultaneously. They also require space for a cold water feed tank, usually in the loft.


Cons of heat-only boilers

  • They don't give you hot water on demand, so you will have to wait for the water to heat up again once the storage tank runs out of hot water. For example, if you run a bath, you may have no hot water for a while afterwards.

  • Because you don't get hot water on demand, they are a bit less efficient than combi boilers, simply because you will gradually lose heat over time from the hot water cylinder.

System boiler

Pros of system boilers

  • System boilers are good for large homes where hot water may be required at more than one tap at a time.

  • You don't need the space for a large cold water tank in the loft as you do with a conventional heat-only boiler.  

  • Brilliant if you live in an area with low mains water pressure.


Cons of system boilers

  • You still need space for a hot water cylinder.

  • System boilers can cost more to install than a basic combi boiler

  • Like conventional heat-only boilers, they are not quite as efficient as combi boilers as you will lose heat from the stored hot water in the hot water cylinder. 

  • Once the hot water has run out from the hot water cylinder, you will need to wait for it to heat up again before you can use the hot tap.

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