How does a combi boiler work?

Combi boilers are the most popular type of 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.

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 boiler might be a better option for you.

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.