What is a combination boiler (combi)?

What is a combination boiler (combi)?

combi boiler is one of three boiler types (combi, regular and system). You can tell if you have a combi boiler as they don’t use water storage tanks - most often found in your loft or airing cupboard. If you have a combi boiler or are thinking of getting one, take a look at our quick guide to find out if they are right for your home.

How does a combi boiler work?

Combi boilers heat water on demand directly from your mains supply and send it to your hot taps and central heating. When you turn on a hot tap your boiler will start burning fuel (usually gas). This heats the cold water in a heat exchanger inside the boiler. Once you turn the hot tap off the boiler will also stop. As combi boilers heat water on demand they don’t need a hot water cylinder to store pre-heated water. They also don’t use a cold water storage tank as they are supplied by your mains water supply, and so use your mains water pressure.

Watch this helpful video on 'how combi boilers work'.

5 advantages of a combi boiler.

  • Compact. They don’t require much storage space unlike other types of boiler, and can often fit easily in cupboards.

  • Never run out. As they heat your water on demand they don’t run out, unlike boilers that store pre-heated water.

  • Instant hot water. Many new combi boilers keep a small reservoir of heated water to instantly supply hot water to your taps.

  • High water pressure areas. They are great for high water pressure areas as they draw from your mains supply.

  • Energy efficient. They only heat water to meet your precise needs so you aren’t wasting energy heating water you’re not using.

5 disadvantages of a combi boiler.

  • Low water pressure areas. Due to using your mains water pressure they’re not well suited to low pressure areas.

  • High demand. As they don’t store pre-heated water they struggle to heat enough to supply more than one hot tap or shower at a time.

  • No back up. Combi boilers don’t use immersion heaters, meaning you have no back up if they break down.

  • No more power showers. As they don’t have a hot water cylinder they cannot be used with power showers or most solutions to improve water pressure.

  • Could damage older pipework. In high water pressure areas they may be incompatible with older pipework, which could be damaged by the high mains pressure.

Learn more about boiler problems and solutions

Could you save money with a new boiler?

Heating and hot water make up over half of the average heating bill, so an efficient boiler can make a big difference. Upgrading to a new A-rated boiler could save you up to £475 a year on your fuel bills1. As an E.ON Next customer, enjoy an exclusive £150 discount on a new boiler. Plus, feel confident knowing that your professional installation will be carried out by a Which? Trusted trader at no extra cost. So, don’t wait for the inconvenience of a breakdown, get more out of your energy. Get a quote online.

Relax with boiler cover from HomeServe.

Sometimes things don’t go according to plan. Be ready for the unexpected with boiler cover from HomeServe - there to help you 24/7 in the event of a boiler breakdown. Whether your boiler needs repairing or replacing you’re covered. Plus, with a free boiler service in your first year we can help you spot any problems and keep your boiler ticking along. Want to protect more than just your boiler? HomeServe can provide cover for your heating, electrics, and plumbing too.

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1 An average household could save up to £475 on their annual gas bill. Savings information for boilers in the UK provided by the Energy Saving Trust, based on a detached house with a boiler rating of G.

Published 28/08/2023