General boiler info

How Does a Combi Boiler Work?

All about combi boilers

Combi, or combination boilers are the most popular type of domestic boiler sold in the UK. Water is heated instantly when a hot tap is turned on, so there is no need for a hot water storage cylinder, making them the perfect choice for homes where space is at a premium. In this guide, we explain how combi boilers work and answer some frequently asked questions.

How does a combi boiler work?

As its name suggests, a combination boiler combines a water heater and central heating boiler in one compact casing. If the room thermostat sends a signal to the boiler to fire up because the temperature has fallen below the set point, the boiler will fire up so that heat can be pumped into the central heating system. As the thermostat set point is reached, the boiler will ‘modulate’ or turn itself down, keeping the heating at the right temperature and saving energy.

When someone turns on a hot water tap, a valve inside the boiler diverts the heat into the domestic hot water supply, providing hot water until the tap is turned off, when the valve returns to the ‘central heating’ position. The heat can be directed to either the central heating system or the hot water taps but never at the same time.

Is a combi boiler suitable for my home?

Combi boilers require sufficient mains water pressure to deliver a good water flow rate; low mains water pressure means hot water will merely trickle. If your mains water pressure is low or you have more than one bathroom, a System Boilers might be a better option for you. Try our Product selector to find out.

It is also important to ensure that the heat output of the boiler is correct for your needs. Combi boilers have two heat outputs: one for Domestic Hot Water and the other for Central Heating. More effort and therefore more heat is required to produce hot water than to heat the radiators, so it is usually the hot water output that determines your choice of boiler.

A Gas Safe registered installer will be able to help you determine what boiler you require to meet your needs; you can locate your nearest installer by using our Find an Installer tool.

Benefits of combi boilers

  • Easy to use – most combi boilers have simple controls. With time and temperature controls, you can programme your heating to come on when you need it, at your chosen temperature, enhancing your comfort, giving you more control and reducing annual fuel bills.
  • Everything needed to deliver heat to your radiators or hot water to your taps is included in the boiler casing, so you don’t need to find room for a hot water storage cylinder or central heating pump – perfect for homes where space is at a premium. 
  • As there is no need for a tank in the loft or a hot water storage cylinder, an added benefit of having a combi boiler is that less pipework is needed, saving time and money during installation.

Will I need a separate hot water cylinder?

No, because a combi boiler only heats water when a tap is turned on, so there is no need to store the hot water.

What models of combi boilers does Baxi offer? 

Can I have a combi boiler if I have LPG?

Yes, we have some dedicated LPG models of combi boiler so that homes off the mains gas grid can enjoy all the benefits of our high-efficiency combi boilers.

What controls or accessories do I need with a combi boiler?

When the Building Regulations changed in April 2018 (commonly known as Boiler Plus), timer and controls now need to be fitted as part of every boiler installation, along with extra energy efficiency measures for combi boilers

1) Seasonal space heating efficiency of the new boiler no less than ErP 92%

2) Time and temperature control required for all installations

3) All combi installations must also include one of the following:

  • Weather compensation
  • Load compensation
  • Flue Gas Heat Recovery
  • Smart control with automation and optimisation

You can find out more about Boiler Plus and a description of these extra measures here.

How can I prevent the hot water supply from my combi from fluctuating?

If you turn on more than one hot tap at the same time when you have a combi boiler, the flow of hot water will be reduced. If the water is not hot enough, you may need to turn the tap down to reduce the flow and increase the temperature. If you have a high demand for hot water, such as more than one bathroom, a system boiler with hot water storage cylinder may be a better option for you.

Who can install my combi boiler?

Combi boilers should only be installed by a Gas Safe registered engineer. You can find your nearest Baxi Gas Safe registered installer here.

How often should my combi boiler be serviced?

Your boiler should be serviced every year by a Gas Safe registered installer, who can check it is working safely, efficiently and reliably. An annual service is also a requirement of your boilers warranty. You can find your local Baxi installer here or call Baxi Customer Support to book a service.

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