Boilers are one of the most used appliances in the world. They assist in the supply of hot water to your household so that you can undertake everyday tasks such as cooking, cleaning and keeping your home warm. Our reliance on them can often lead to frustration when they break down – and this inconvenience can impact us even beyond the winter months. 

When looking to replace or upgrade a boiler, it is important to understand the differences between each type so that you can pick or recommend a unit that meets the needs of the property. To help develop your knowledge around combi boilers, we’ve created this helpful guide explaining what they are and how they work.

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Combi Boilers Explained

A combi boiler, otherwise known as a combination boiler, is a highly efficient and compact boiler that acts as both a water heater and a central heating unit. This makes them the perfect choice for smaller households where space may be an important consideration. 

They are a very cost-effective solution due to the water being heated instantly rather than being heated and then stored in a separate unit. As a result of their space-saving features, combi boilers account for half of the total boiler installations in the UK.


Advantages of a Combi Boiler

Combi boilers offer the following advantages over other units:

  1. Compact – no additional water tank required
    A combi boiler acts as a water heater and a central heating unit without the need to store water in a separate tank.
  2. Excellent water pressure
    Combi boilers are known for offering better pressure than other types of unit, offering a great showering experience without the need to install a pump.
  3. Instant hot water
    Combination boilers heat water on demand, so the owner will have an unlimited supply of hot water.
  4. Energy- and cost-efficient
    Water is heated instantly rather than stored separately, meaning you only use energy as and when you need hot water. This in turn saves the owner money!
  5. Fast, easy and relatively cheap to install
    As combi boilers don’t require an additional tank, they are typically the most straightforward to install.


Disadvantages of a Combi Boiler

There are, however, a few disadvantages to installing a combi boiler:

  1. Can only support one shower or bath at one time
    For households with multiple bathrooms, it is important to note that combi boilers can only support one shower or bath at one time.
  2. No backup water heater
    Due to the absence of a hot water cylinder with an immersion heater, there won’t be a backup supply of hot water if there is a problem with a combi boiler.
  3. Requires a good level of mains pressure
    It is important to check that your mains water supply can deliver good levels of flow rate and pressure to support a combi boiler.
  4. Incompatible with power showers
    Combi boilers rely on the pressure that is dictated by the mains supply and so they cannot support power showers.



How Do Combi Boilers Work?

Combi boilers provide the owner with access to instantaneous hot water and central heating. The two systems have independent heat exchangers – one that links to your radiators and another that connects to your hot water supply.

Combination boilers work using a sensor that detects when you’ve requested hot water, which then tells the boiler to burn fuel (whether that is electric, gas, or oil). The heat exchanger then gets hot enough to heat the water that flows over it.

Inside a combi boiler, you’ll find that the control valves work in different directions to one another, so the system will only deliver hot water to either your radiators or your hot water tap at any one time. Combi boilers come with heating controls that you can use to set your heating to the desired temperature.



This guide has explained the fundamentals of what a combi boiler is and how one works. To learn more about the other types of boiler, check out our guides to system boilers and heat-only boilers.

If you’re interested in becoming a heating engineer or developing your existing career further, take a look at our range of new entrant gas engineer courses or get in touch today for more information.

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