Combi boilers are the UK's most popular type of boiler, installed in around 70% of homes. But how do they actually work?
The basic principle
A combi boiler is called a "combination" boiler because it combines two functions in one unit: central heating and domestic hot water. Unlike traditional boilers, a combi heats water directly from the mains supply on demand — there is no hot water storage cylinder.
How it heats your home
When your thermostat calls for heat, the combi fires its burner, which heats a primary circuit of water. This hot water circulates through your radiators via the central heating pump, giving up its heat to each room, then returning to the boiler to be reheated.
How it heats your hot water
When you turn on a hot tap, the boiler detects the flow of water and switches from central heating mode to domestic hot water mode. Cold mains water passes through a secondary heat exchanger inside the boiler and is heated almost instantly before flowing to your tap.
This "on demand" approach means you never run out of hot water — but the flow rate is limited by the boiler's DHW output (typically 10–13 litres per minute for a good combi).
Limitations of combis
The one limitation: if two people try to use hot water simultaneously (e.g., one person showers while someone else washes dishes), the flow rate is split between both outlets. For larger households, a system boiler with a cylinder is usually the better choice.