Faulty Diverter Valve [Causes and Fixes] Re July 8, 2022

Faulty Diverter Valve [Causes and Fixes]

Radiator being turned off by thermostatic radiator valve

If you’ve had your trusty boiler for a long period of time, at some point inevitably you’ll start to run into issues and wear and tear. In terms of parts, a diverter valve boiler fault is one of the most common issues you might face with older combis.

If your combi is on its last legs (+10years old), you might notice that your radiators aren’t heating as they used to, and your so-called hot shower is getting slightly colder.

The boiler’s diverter valve acts like a Traffic officer; it directs your hot water to your taps, radiators or both, to make your home warm and cosy.

So, what’s a diverter valve? How does it work? How do I know it’s failed, and how can I fix it? All these questions will be answered below.

What is a boiler diverter valve?

A boiler diverter valve is an essential part of a combi boiler heating system, deciding 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 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 the 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.

Have I got a faulty diverter valve?

If you’re unsure that you’ve got a faulty diverter valve, these are the signs to look out for;

1. Your hot water isn’t hot anymore (luke-warm water in the taps and showers)

This is a textbook case; if you find that the hot water coming out of your taps is lukewarm, or even cold, there’s a possibility that you’ve got a faulty diverter valve. Why is that, you may ask? 

It’s because your boiler is designed to prioritise hot water instead of heating when they’re both on. Should the valve get stuck on the heating side, some of the heat which is supplied to the taps escape.

2. Hot water won’t come out of the taps unless the heating is on

If the hot water only comes out of the tap when the heating is on, that might be another sign that you have a faulty diverter valve. But how do you check?

Turn on the heating and, shortly after, turn on the hot water tap. If the water temperature goes up after turning on the heating, this might be caused by a faulty diverter valve.

3. Hot water in taps but no heating

Another common sign of a faulty diverter valve is having hot water on demand but no heating which could be caused by the problem in the water flow.

In this case, the diverter valve might have got stuck on the hot water side, meaning most of the heat has gone to the hot water and not enough to the radiators leaving your room cold.

How do I test a boiler diverter valve?

If you suspect that your diverter valve is faulty, we recommend that you call a Gas Safe registered engineer 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.

Faulty diverter valve fixes

If there’s a problem with your boiler, don’t attempt to fix the diverter valve yourself. You should always call a Gas Safe heating engineer to take a look at it. 

Any work carried out on a gas appliance is extremely dangerous. It can damage your boiler further and, even worse, put you and your family at risk.

During a visit from the heating engineer, they’ll run tests to diagnose the problem (which could be related to a faulty diverter valve or other boiler-related issues with similar symptoms).

If the problem is a stuck valve, they might be able to clear it. The engineer might be able to fix it by cleaning the valve using a commercial chemical cleaner and then refitting it. 

On the other hand, if your valve can’t be repaired, the engineer may recommend that you need a new valve. 

If the boiler is still under warranty, you may be able to get the diverter unit replaced free of charge. Plus, if the warranty has expired, you’ll need to consider the cost of a new diverter unit and the engineers’ time.

You might want to consider your options at this time. Is it worth getting the boiler fixed or buying a new boiler?

The cost of replacing a boiler diverter valve

If your boiler needs a new diverter valve, the new unit will set you back between £180 and 350*. To break that down;

A new diverter valve retails for £80-200*. It will differ based on your boiler make and model. For example, the Vaillant 0020132682 diverter valve brass with adaptor retails for £126, while the Worcester 87161567460 diverter valve v37 costs £124 while the Alpha 3.012752 diverter valve CB24,28,25,33, CD24,32 can cost £172*.

Then, there’s the cost of labour. The labour cost is a bit trickier to estimate because it differs based on location and the complexity of the job. Generally speaking, the labour cost for replacing the boiler diverter valve could range between £80-150*.

Should I repair or replace my boiler?

If your boiler keeps breaking down, it may be cheaper in the long run, to cut your losses, and get a new one, just come to us at WarmZilla and we’ll sort you out!

If your boiler is older than 8-10 years, you’re better off getting a new one.

When shopping around for a new boiler, we’d advise using the WarmZilla boiler comparison tool. Just select up to 3 boilers you want to compare, and you’ll be able to compare key stats side by side to see which boiler is best for your home.

Not prepared to waste money on fixing your knackered old boiler?

Then it’s time for a new boiler! 

When you buy a new or replacement boiler with WarmZilla, your boiler package will also include;

  • Your chosen boiler, along with the necessary flue kit needed for your central heating system
  • Installation by a qualified Gas Safe Engineer 
  • Removal of your old boiler and parts
  • A FREE system cleanse of your central heating system
  • Chemical Inhibitor for added protection for your new boiler
  • 10-year warranty, which will be registered by WarmZilla after installation
  • A FREE magnetic system filter for ongoing protection
  • The Neomitis RF Digital Wireless Room Thermostat

*Average boiler prices were taken from an online heating supplier on 7/7/22. Prices are for the charging unit alone and may be subject to change.


If there’s a problem with your boiler, don’t attempt to fix the diverter valve yourself. You should always call a Gas Safe heating engineer to take a look at it. 

Any work carried out on a gas appliance is extremely dangerous. It can damage your boiler further and, even worse, put you and your family at risk.

  • Hot showers but cold radiators
  • You can only get lukewarm water from your taps
  • Your hot water only turns on when central heating is on

It can cost between £180 and 350* (including parts and labour).

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