Lukewarm Water from Combi Boiler: The Diverter Valve

The Diverter Valve is a mechanical valve used in combination boilers to divert the flow of plasma between the domestic hot water (DHW) heat exchanger and the radiator.

The "plasma" consists of water, heated by the primary heat exchanger and is extremely hot. The diverter valve controls the flow of this hot fluid. When the central heating system starts, the plasma passes through the radiators, however, when the domestic hot water taps open, the flow of this hot plasma diverts to the DHW heat exchanger, which heats the cold water.

If this valve is faulty or not operating correctly, then the hot plasma will divert into the radiator circuit instead of the DHW heat exchanger. It is common for these valves to fail, but luckily easy to spot the symptoms from simple basic signs.

The radiator pipes -- whether inlet or outlet, it does not matter -- should not heat up when you turn on the hot water taps. If you find either of these pipes heating up and eventually, the radiators getting warm then you have just diagnosed the fault. When the diverter valve fails, the domestic hot water supply from the tap will be cold. Typically, it is not even lukewarm but completely cold. This presents a very different set of conditions to when the heat exchanger has failed because then the domestic hot water supply from the tap will be warm. These tiny little clues are very important for correct fault diagnosis.

Get Qualified Engineers

You should never attempt to repair a boiler yourself. There are many boiler repair companies on Google and it is always a good idea to find a fully qualified and certified Corgi repair engineer to repair your boiler.

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