BASI Guide: to a combi boiler


Combi boilers are now the most popular boiler system installed in homes across the UK, replacing conventional or heat-only heating systems that require a cylinder and a tank.

So, what makes combi boilers so suited to our homes?

The name combination comes from the ability to simultaneously heat your home and your water supply from one place. Modern combination boilers are a high-efficiency water heater and central heating boiler rolled into one.

They work by taking cold water from your mains supply, heating it within the boiler itself and distributing it throughout the house on demand.


Diagram from Worcester Bosch page


  • Combi boiler systems are compact and simple to install as they don’t require additional space for a hot water tank or cylinder
  • Because they only heat the hot water you actually use, combi boilers are cost-effective and energy efficient to run
  • Combi boilers heat your water straight from the mains, meaning you don’t have to plan in advance what hot water you are going to use – or run out unexpectedly
  • Because water comes straight from the mains, your hot water should always maintain a good, strong pressure
  • All combi boilers installed by BASI are from leading manufacturers and come with up to a 10 year manufacturer warranty


  • As water is delivered straight from the mains, combi boilers struggle to run two hot water sources at once, which may be an issue if you have a large household with multiple bathrooms
  • Combi boilers contain moving parts and can be more prone to breakdowns (although this can be easily combatted with an annual service)
  • If you have a pre-existing power shower, this will be made redundant by a combi boiler unless professionally adjusted

Think a new energy efficient boiler could benefit your home? Get in touch now to arrange your free survey with a BASI engineer.

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