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What is a combi boiler and how does one work?

Combi boilers: everything you need to know

Combination (or combi) boilers are the most common type in the UK. These appliances supply central heating and hot water and demand without the need for an external hot water cylinder. For this reason, a combi boiler is a great choice if you’re short on space or simply want to save room in your home.

What is a combi boiler?

A combi boiler provides both central heating and hot water from the same unit. This type of all-in-one boiler heats the whole house and supplies your shower and taps with hot water almost instantly, as and when it's needed. There’s no need for storage tanks with a combi boiler, which frees up space in your home.

Without the need for a hot water cylinder or cold water tank, combination boilers are more compact than system or regular boilers. This makes them ideal if you don’t have a suitable airing cupboard or spare room in your loft. They’re also incredibly energy-efficient and cost-effective, saving you money on your bills.

How does a combi boiler work?

Combi boilers work by heating water on demand. The process works like this:

1. The combination boiler takes water directly from the mains supply.

2. A sensor inside the unit detects when you turn on the central heating or draw hot water from a tap or shower.

3. In response to this, water is then heated up inside the combi boiler via a heat exchanger.

4. If you’re using a tap or shower, the hot water travels from the boiler to the outlet through your hot water pipes.

5. For central heating, water is heated up to the temperature you’ve set on your heating control, then delivered to the radiators to heat up your home.

This process enables a combi boiler to provide hot water and heating almost instantly when you need it.

Pros and cons of combi boilers

A combi boiler could be the ideal choice for your household, but it’s important to understand the pros and cons of this boiler type before making your decision.

Advantages of a combi boiler

There are plenty of advantages to choosing a combi boiler:

  • Energy and cost efficiency – this type of boiler only heats water as and when you need it, reducing wasted energy and saving you money.
  • Near-instant hot water – as soon as you turn on a shower or tap, your combi boiler will detect the request and provide hot water almost instantly.
  • Space saving – you don’t need a separate hot water cylinder in an airing cupboard or a cold water storage tank in the loft, saving you lots of space by comparison to other boiler types.
  • Easier, cheaper installation process – a combi boiler provides all-in-one heating and hot water without any additional tanks. This makes them easier (and often cheaper) to install than other types.

Considerations before deciding on a combi boiler

In order to check a combi boiler is the right fit for you, consider the following points:

  • No backup hot water supply – other types of boiler have a hot water tank with an immersion heater that can act as a backup if the appliance fails; a combi boiler is all-in-one, so there’ll be no hot water or heating in the event that it stops working.
  • Hot water is limited to one bathroom at a time – regular and system boilers can supply hot water to multiple bathrooms at the same time. Combi boilers, on the other hand, only allow one shower to use hot water at any given point, so they’re not ideal for larger households.
  • Requires a strong mains water supply – this type of boiler needs a reasonable water pressure (or flow rate) to work properly. If your water pressure is low, a regular (heat-only) boiler would be more suitable

What size combi boiler do I need?

This depends on the size of your house and the number of radiators.

Typically, for a one- or two-bedroom property with up to 10 radiators, a small 25kW combi boiler would be suitable. Properties with 10-20 radiators would usually benefit from a 30kW combi boiler, and a high-output 35kW appliance would be the best option if you have several bathrooms and more than 20 radiators. However, these are based on average recommendations, and homeowners should always consult their local Vaillant installer to determine the best system and size for their home.

Take a look at our range of high-quality combi boilers ranging from 25kW up to 35kW

How much is a combi boiler?

A new combi boiler can cost up to £2,500 depending on the size of the appliance (although this doesn’t include installation costs).

Usually, a Gas-Safe registered boiler installer will give you a quote that includes both the price of the appliance itself and also installation costs. The total cost of installing a new average-sized combi boiler varies depending on which type of boiler you’re replacing:

  • Switching an old combi boiler for a new one is typically the cheapest form of installation;
  • Converting a system boiler to a newer combi boiler model can be slightly more expensive due to the additional labour involved;
  • Changing out a back boiler for a combi boiler is usually on the more expensive side and can cost over £3000

These are only rough figures based on the installation of an average combi boiler. The actual price may be more or less than this depending on the size of the appliance and where you live. For example, installation costs in London are usually higher than elsewhere.

How long should a combi boiler last?

You can expect a combi boiler to last between 10 and 15 years on average.

There are things you can do to make your combi boiler last longer, including having an annual boiler service and calling a boiler repair specialist if the appliance has any issues.

Reliable, energy-efficient combi boilers from Glow-worm

At Glow-worm, we manufacture high-quality, high-efficiency gas boilers that you can depend on. Explore our range of combi boilers to find out more about the different models we supply.

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Interested to learn more about the other types of boiler? You may want to take a look at these posts: