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What size boiler do I need?

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What boiler type is right for my home?

Before choosing the size of boiler, it is important to select the ideal boiler type for your needs. There are three main types of boiler:

Combi boilers

Combi boilers combine all components of a traditional central heating system into one compact unit. They use condensing technology and provide hot water on demand, unlike regular boilers and system boilers which store hot water in advance.

Combi boilers are popular because of their compact size and efficiency. They are a great choice for most small-to-medium sized homes.

Open vent boilers

Open vent boilers, otherwise known as heat-only boilers, work by heating water and distributing it throughout a home using a system of tanks and heat exchangers. A cold water tank stored in the loft will take water from the mains and feed it into a hot water cylinder.

Open vent boilers are usually better suited for larger homes with significant demands for heating and hot water because they can store large quantities of hot water ahead of time.

System boilers

System boilers also store hot water in a separate cylinder, making them suitable for larger homes. They can supply a constant level of hot water throughout a home, even when multiple showers and taps are open simultaneously.

Contrastingly to regular boilers, system boilers use a pressurised heating circuit, topped up by mains water. They also occupy less space because the expansion vessel is internal in a system boiler, whereas it is external in a regular boiler.

What factors affect boiler size?

Once you have chosen the type of boiler you want, the next step is to select the correct size according to the heating requirements of your home. Alongside boiler type, the factors that affect boiler size include:

  • Number of radiators

  • Number of bedrooms

  • Number of bathrooms

What size combi boiler do I need?

Combi boilers typically have a larger power output than system and heat-only boilers because they have to heat water on demand. The most suitable boiler size can be worked out according to the following categories, and once you have taken a look you can browse our range of combi boilers to find a suitable solution.

Number of bedroom
Number of radiators
Number of bathrooms
Recommended size
More than 4
More than 3

What size system boiler and regular boiler do I need?

System and regular boilers store hot water ahead of time, meaning they are built with a smaller power output than combi boilers. The table below showcases the best boiler sizes depending on the heating requirements of a home. If you're interested in having one of these types of boilers installed in your home, you can browse our range of system and heat onlyboilers to find the perfect solution for your heating and hot water demands.

Number of bedrooms
Number of radiators
Number of bathrooms
Recommended size
More than 420+More than 327-40kW

Please be aware that these are just general guidelines into what size boiler would fit within certain properties. We always recommend consulting with a professional heating engineer before making your decision. Why not get in touch with a Vaillant Advance installer.They will be able to give you a free no-obligation quote after expertly surveying your home.


  • The first step to finding the right size boiler is choosing the appropriate boiler type for your needs, whether combi, system or regular.
  • Combi boilers are a popular choice of boiler for most homes, while system and regular boilers are suited to larger homes.
  • The factors that affect boiler size include the number of radiators, the number of bedrooms and the number of bathrooms.
  • Combi boilers have a larger power output than system and regular boilers because they are designed to heat water on demand.

Find the right boiler for your home