How Does a Central Heating System Work

How a combi boiler heats your home and your water with no need for a separate not water storage tank?

The heat source for a central heating system is the boiler. 70 per cent of UK households now use a combination, or combi boiler to heat their homes and their water-all in one compact unit. Inside the boiler, water is heated by a metal heat exchanger and pumped into a closed system of pipes that loop around the home to radiators in each room. By using a combi boiler, if you turn on your hot tap, water from the mains is heated as needed, with no need for a large and obtrusive storage cylinder.

All modern boilers fitted in the UK are condensing boilers, which are more fuel-efficient because they re-use the heat energy that in a non-condensing boiler would be expelled through the flue. The water is heated as it flows through the pipes inside one (or sometimes two) heat exchangers, which are suspended in hot gases over the burner’s flames, maximizing the heat transfer from the burner. In a combi, the temperature of the flue gases is reduced to 50-60”C (rather than the 120-180”C in a non-condensing boiler), and most of the gas leaves the flue as water vapour while the rest is drained away as condensate, or water.

Central Heating System WorkCentral Heating System Work


The combi boiler can be programmed to switch the central heating on and off at desired times and at a specific temperature. A pump inside the boiler transports water through the radiators.

Hot water

Hot water is delivered at mains pressure. As soon as the hot tap is turned on, the water flow is detected by the gas burner, which uses electric sparks to ignite the gas that heats the water as it runs back and forth over a heat exchanger.

Cold mains

The combi heats water direct from the mains as and when it’s required. There’s no need for a coldwater storage tank.


The temperature of a room’s individual radiator can be controlled using a thermostatic radiator valve.


A room thermostat is used to control the temperature of the water in the system.

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