Combination Boilers - Explaining Combi Boilers and Diagnosing Faults

Summary: An explanation about how a Combination Boiler works. Combination boilers fault finding and repair

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The combination boiler description below applies to an indirect combination boiler where the domestic hot water is heated in a secondary calorifier. See below for more about a direct combination boiler. Our projects section also contains lots of information about central heating of all kinds including oil fired.

Combination boilers combine the functions of a central heating boiler and an instantaneous multi point water heater. They give priority, in the main, to the supply of domestic hot water (dhw). The combination boiler has all its operating components contained within the casing and is typically designed for use within a sealed central heating system.

Installation is made easier by incorporating an expansion vessel, thus eliminating the need for a feed and expansion cistern in the roof space. Like an ordinary boiler it will have a gas fired burner and a heat exchanger. The waterways of both form part of the central heating system. In most of the early designs of combination boiler, hot water from the heat exchanger passes to a diverter valve, which directs the flow to a domestic hot water calorifier or central heating circuit. The calorifier is tightly packed with small copper tubes, through which secondary water flows the moment the hot water draw off is opened. As the water flows it picks up heat from the surrounding primary water in the calorifier. The central heating system is pressurised and needs a relief valve to protect against any build up of pressure in the system.

A pressure gauge on the appliance indicates the operating pressure. A by-pass, connected across the primary flow and return, ensures that an adequate flow of water is maintained. This by-pass may, or may not be a built in feature of the appliance.

The flue arrangements for combination boilers may be open, room sealed, fan assisted and room sealed, fan assisted open, miniature flue/condensing or se-duct. The primary heating circuit of combination boilers should be pre-commissioned, cleansed and treated with a corrosion protector in the same way as any other heating system. In hard water areas, the secondary domestic hot water should be treated with an in-line scale reducer or water softener (See projects section for info on both of these) This will prevent the build up of limescale on the instantaneous water heater and the accompanying reduction in flow rate and water outlet temperature.

With a direct combination boiler the secondary water is heated in one of two ways. The water may pass through pipes which run down the inside, or next to the primary pipes in the heat exchanger, or by passing through a separate heat exchanger which is bolted on top of the main heat exchanger. A direct combination boiler does not need a diverter valve or a secondary calorifier.

In a domestic hot water system where the (condensing) boiler is a replacement for an existing storage cylinder system or a conventional boiler, it is essential that all redundant pipework is removed to eliminate "dead legs" which can hold air and cause air locks. These can also cause cold spots and slow burner shut off.

The diagrams below identify most parts of a combination boiler.

Cross section of a combi boiler - left hand side

Cross section of a combi boiler - left hand side

Cross section of a combi boiler - right hand side

Cross section of a combi boiler - right hand side

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