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How to fix a boiler diverter valve

A boiler diverter valve is an essential part of a combi boiler heating system, determining where hot water is sent throughout the home. Over time a diverter valve can become faulty and consequently cause issues with sending hot water to your taps or radiators.
a boiler diverter valve

What is a diverter valve?

A diverter valve is a mechanism in a combi boiler that opens or closes to direct hot water either to radiators or taps and showers. Diverter valves are not found in system or conventional boilers because the valve is only needed in boilers where water is heated on demand, whereas system and conventional boilers store hot water in a storage cylinder.

If a specific hot water tap is turned on, the combi boiler will heat water and the valve will direct the hot water to the tap rather than to the home’s radiators or other taps. If the heating is turned on, the combi boiler will do the same for the radiators. If the heating is on and someone turns on the hot tap in the shower, the diverter valve will prioritise sending hot water to the shower instead of the radiators until the shower is turned off.

How do I know if my diverter valve is faulty?

The first step to fixing a faulty diverter valve is ensuring that it is faulty in the first place. The most common symptoms for a faulty diverter valve are:

  • I have hot showers but my radiators stay cold

    A diverter valve may be faulty if the hot water taps still work while the radiators do not turn on.

  • I can only get lukewarm water from my taps

    Another sign of a faulty diverter valve is if you only can only get lukewarm water from your taps. In this scenario it is likely that the diverter valve is stuck and allowing some hot water to pass through to your radiators.

  • My hot water only turns on when central heating is on

    It is common for a faulty diverter valve to only become apparent in the warmer months once the central heating is not needed. If hot water only comes out of your taps when the central heating turns on, it is likely that the diverter valve is not diverting hot water properly.

How do I test a boiler diverter valve?

If you suspect that your diverter valve is faulty, we recommend that you call an engineer registered with the Gas Safe Register to take a look at it for you. While certain checks can be as simple as removing the boiler cover and looking for the diverter valve, it is illegal to carry out any maintenance on your boiler.

How do you free a stuck diverter valve?

Instead of attempting to fix the diverter valve yourself, we recommend that you call a qualified heating engineer to come take a look at it for you. Any work carried out on a gas appliance is dangerous and can damage the boiler further if not handled carefully.

A boiler engineer will be able to run checks to see if a faulty diverter valve is the cause of the problems, or if there is another underlying issue. It is likely that the valve has simply become stuck over time and needs to be cleaned using a commercial chemical cleaner, after which it can be refitted.

The engineer may also advise you to replace the valve depending on the severity of the problem. We recommend checking to see if your boiler is under warranty. If it is, you may be able to replace your diverter unit free of charge. If not, you will have to factor in the cost of the engineer’s time and the replacement valve itself, which can cost anywhere between £55 and £200.

In some cases it may be more economical long-term to replace the entire boiler system, which can be the case for combi boilers over 10 years old. Your heating engineer will be able to advise you on this following their checks on your system.

Summary

A diverter valve is a mechanism in a combi boiler that opens or closes to direct hot water either to radiators or taps and showers.

A diverter valve may be faulty if your water is lukewarm, if your hot water taps work but your central heating does not or if your hot water taps only work when the central heating is on.

You should always call a qualified engineer to free a stuck diverter valve, at which point you may need to clean the valve or replace it entirely depending on the age of the model.

If you need to book a boiler repair, visit our myVAILLANT web online portal which gives you control over your boiler’s guarantee, repair and service information.

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