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New Boiler: What You Need to Know Before you Buy a New Boiler

Combi Boilers: Is a Combi Boiler the Right Boiler for You?

By Matthew Knight

Article 4 of 6

Our experts tell you everything you need to know about different type of boilers, including combi boilers, heat-only and system boilers.  

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

As well as combi boilers, there are several other types of boilers available that are more suitable for certain types of house, and certain size families. 

Choosing the right type of boiler is vital. Having the wrong type of boiler could lead to significantly higher bills or, worst yet, long delays for hot water in your home.

Read on to learn more about the different types of boilers, combi, heat only and system, and find out the pros and cons of each and what type of home they are most suitable for. 

Already know you want a combi boiler? We reveal the best combi boilers.

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.

Heat-only boilers are also known as conventional boilers.

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. 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. If you run a bath, for example, 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 gas boiler systems

System boilers, sometimes known as closed vent boilers, are the perfect choice if you think you'll need a heat-only boiler but don't want lots of external components. This is because many of these external components are built into the body of the boiler, 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 of the valves.

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, like you do with a conventional heat only boiler.  

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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