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What is a combi boiler?

combi boiler on a wall in a kitchen

A combi boiler is a type of boiler that can provide both hot water and central heating. They also known as 'combination boilers' as they were originally designed to combine all the components of a traditional central heating system into one compact unit.

How does a combi boiler work?

Contemporary combination boilers uses condensing technology to efficiently heat the water in the system 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 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 biggest benefits of a combi boiler is that it's small in size. Given that external storage tanks are not required for a combi boiler, the entire system is controlled through the same appliance, including heating and hot water. This allows them to easily fit into kitchen cupboards, bathrooms and airing cupboards.
  • Efficient

    Another advantage of a combi boiler is its efficiency. Some modern combi boilers are built with a flue gas heat recovery system, which works by 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, helping to 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 open vent boilers, which heat and store specified quantities of water in advance, combi boilers heat water when it is required. A combi boiler can therefore provide a continuous supply of hot water.
two men looking at the boiler in the kitchen

Points to bear in mind

Unsuitable for larger homes

A combi boiler has a limited water capacity due to its small size. This is a disadvantage because it can only provide a limited amount of water at a time. This means 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 a open vent or system boiler as they store hot water in a cylinder (often stored in the airing cupboard).

How long do combi boilers last?

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

How much do combi boilers cost to install?

The price 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 a gas 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 at your home, we have a range of combination boilers including the new ecoTEC plus 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 both hot water and central heating in the same system.
  • The advantages of a combi boiler is its size, efficiency and the ability to provide hot water on demand.
  • Combi boilers can last between 10-15 years if looked after well with annual servicing.