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How combi boilers work

Combi boilers are designed to provide instant hot water, as well as heating the central heating system. 

If the temperature in the home drops below the pre-set level, the room thermostat sends a signal to the boiler to fire up so that heat can be directed into the central heating system. The opposite happens when the thermostat detects that the required room temperature has been reached.

When there is a request for hot water - triggered by someone turning on a hot tap - water is heated inside the combi boiler which then flows through to the relevant tap.

Control valves inside combi boilers operate in different directions, either letting the water flow through the central heating system or diverting it to a hot water tap, as required, but never both at the same time.

Important factors affecting operation

Combi boilers require sufficient mains water pressure in order 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 conventional system boiler might be a better option for you. Try our title="Find a Baxi boiler" target="_blank">Find a boiler tool 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 hence more heat is required to produce hot water than to heat a home through the radiators, so it is usually the hot water output that determines your choice of combi 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.

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