What is a Combi Boiler?

Combi boilers combine the functions of central heating units and hot water heaters in one compact unit. Combination boilers are the most popular choice for new boiler installations in the UK, thanks to their small size and efficiency.

The Combination Boiler

The combination boiler is a single condensing boiler that can provide hot water for both use in the home and for heating. Another significant difference from other boilers is that the combi boiler heats water from the mains as it is needed, rather than storing hot water for later use. Since the boiler is only active when needed and doesn’t produce more hot water than is actually used, it is very energy efficient.

Combination boilers can also help you to save a lot of space. All you will need when you have a combi boiler is one compact unit, which will usually be placed in your kitchen or an airing cupboard. You won’t need separate units for hot water and central heating, and since you will be getting water straight from the mains without storing it in your home, you won’t need a cold water tank or hot water cylinder for storage. This is particularly useful if you live in a smaller property or if you want to make space in your home for a loft conversion.

How It Works

Combi boilers work just like other condensing boilers, but they only activate when they are needed. As soon as you turn the hot tap or switch your heating on, the boiler will fire up and begin producing hot water for you. You shouldn’t have to wait more than a few seconds for the water to reach the right temperature.

Since the water is coming straight from the mains, without being stored in a tank, it will be supplied at mains pressure. As long as you have good water pressure in your neighbourhood, this means that you should have adequate pressure for most uses, unless you have a very large house with lots of occupants.

Benefits of a Combi Boiler

Combi boilers are a popular choice for some very good reasons:

  • Space Saving: Combining hot water and central heating into a single unit while eliminating the need for cold water tanks and hot water cylinders can save a lot of space.
  • Energy Efficiency: Combi boilers are very efficient and only produce hot water as it is needed, so they are good for your finances and for the environment.
  • Easy Installation: Combi boilers are quicker and easier to install as they are simpler than other boilers, which means that installation costs tend to be lower too.
  • Ease of Use: A combi boiler can produce unlimited hot water quickly, when you need it and you only need to work one system for heating and hot water.
  • Steady Temperature: Since most combination boilers are fitted with thermostats, you shouldn’t experience any sudden changes in water temperature when you are showering.

Disadvantages of the Combi Boiler

Although combi boilers can work well in most homes, there are some potential problems that might make them a bad choice in some cases:

  • Water Pressure: Combination boilers must have a reliable supply of cold water. If there are any issues with your water pressure or if there are a lot of people using your hot water all at once, a combi boiler could struggle.
  • Shower Compatibility: Combi boilers work well with most showers, but they cannot always provide enough hot water for powerful multi-jet showers. Pump assisted showers are also incompatible with combi boilers as they should be fed from a storage tank.
  • Delays: Although combi boilers are very quick, there will be a slight delay while the boiler fires up. However, similar delays can occur even when you are supplied through a hot water tank as there can still be a little cold water in the pipes.
  • Slower Baths: Baths can take a bit longer to fill when you have a combi boiler as the water needs to be heated rather than just supplied from the cylinder.
  • No Backup: If something goes wrong with a combi boiler, you won’t have a backup as there will be no immersion heater. You will be without hot water and heating until the boiler is repaired.


Wallis Plumbing Ltd provide installations of combi boilers for both domestic and commercial properties in Woking. If you need a professional Plumber in Woking for the installation of a new boiler please contact us for more information and a quotation.