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What Is a Combi Boiler?

Traditionally called a combination boiler because they were originally designed to combine all components of a traditional central heating system into one compact unit. A combi boiler is a type of boiler that can provide both hot water and central heating.

Summary of topics:

How does a combi boiler work?

Advantages of combi boilers

Points to bear in mind

How long do combi boilers last?

How much does it cost to install a combi boiler?

How does a combi boiler work?

Modern combi boilers use condensing technology to efficiently heat the system water by extracting latent heat from the flue gases. When you require heating or hot water, a gas burner in the combi boiler is ignited. The heat from the flame is transferred via the main heat exchanger to the primary circuit, at which point the heat is moved to either directly to the radiators or to your hot water taps via a plate heat exchanger.

Many modern combi boilers also have a pre-heat system which allows the combi boiler to store a small amount water which can drawn from the boiler. The water is held inside the boiler and maintained at your desired temperature throughout the day, ready to be sent to the hot water tap as quickly as possible.

What are the advantages of a combi boiler?

Combi boilers are a popular choice for many homeowners due to their various benefits. These include, but are not limited to:

Compact size

One of the benefits of a combi boiler is its size. Given that external storage tanks are not required for a combi boiler, the entire system is controlled through the same appliance. This includes heating and hot water.


Another advantage of a combi boiler is its efficiency. Some modern combi boilers are built with a flue gas heat recovery system, which involves collecting the gas created by burning gas or oil and using it to heat water from the mains. This allows the system to burn less fuel and achieve an efficiency of over 90% in the process, which consequently reduces a household’s heating bills.

Saves money

Purchasing a single unit (Combi Boiler) can be cheaper than buying multiple system components compared to installation of a conventional system which usually require, tanks, cylinders, pumps etc..

Provides hot water on demand

Combi boilers are also a great choice because of how quickly they can heat water when needed. Unlike system boilers and regular boilers, which heat and store specified quantities of water in advance, combi boilers heat water when it is needed. A combi boiler can therefore provide a continuous supply of hot water.

Points to bear in mind

Unsuitable for larger homes

One disadvantage of a combi boiler is that the amount of water it can provide at any given time is limited to the size of the boiler, this typically means that on the average combi boiler, only one tap/outlet can be used effectively at a time. While many homes will be perfectly served by a combi boiler, larger homes with multiple bathrooms will require hot water at a rate that a combi boiler cannot meet. In these cases, a system boiler may be more appropriate, due to its external hot water tank and mains pressure hot water supply options available.

Requires good mains pressure

Another downside is that because combi boilers take water directly from the mains, the water pressure will drop rapidly if multiple taps and showers are turned on at the same time.

Larger homes are therefore often more suited to regular or system boilers that receive their water from a hot water cylinder.

How long do combi boilers last?

Combi boilers last anywhere between 10-15 years, depending on their model and how regularly they are serviced. Regular boiler servicing can extend the lifetime of a combi boiler considerably.

How much do combi boilers cost to install?

The price of a combi boiler will vary depending on the manufacturer, model and size. Combi boilers can cost between £500 and £1700, with larger and more efficient models sometimes twice, or even three times as expensive as cheaper models. It is also important to factor in the cost of installing the boiler. Different sizes and boiler types suit different needs and property types, and it is always best to seek advice from a local expert to find the most suitable option.

If you're interested in having a combi boiler installed, we have a range of Which? Best Buy awarded combination boilers that deliver excellent performance whilst remaining efficient. Talk to your local Vaillant installer to help find your perfect solution and benefit from extended guarantees.


  • A combi boiler is a type of boiler that can both provide hot water and central heating in the same system.
  • The advantages of a combi boiler are its size, efficiency and its ability to provide hot water on demand.
  • Combi boilers last between 10-15 years.

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