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Diagnosing a Faulty Diverter Valve on a Boiler [And What It Costs to Fix]

A faulty diverter valve is a common problem with older combi boilers. Over time they can wear or break, causing havoc on central heating systems. Today, we’ll cover:

  • What is a diverter valve?
  • Symptoms that highlight a faulty diverter valve on your boiler
  • What should you expect from an engineer?
  • The costs to fix a faulty diverter valve

It might be a simple fix, but diverter valves are expensive.

The cheapest way to get your boiler repaired is using local engineers or an online boiler quote site like HEATABLE.

Fed up of paying out for repair bills due to boiler breakdowns, you can get a brand new Worcester Bosch boiler fitted by HEATABLE from £1,770 or a Viessmann Vitodens from £1,670. They’re typically much cheaper than local installers, due to the fact they operate online and have streamlined their sales and acquisitions.

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What is a diverter valve?

Here is what a diverter valve looks like (Worcester Bosch).

The diverter valve on a combi boiler works much like a lock on a canal would. It opens and closes to allow heat up both hot water and the water in the heating system which provides heat to radiators and towel rails.

In theory, the valve should always prioritise your hot water (water from taps and showers) over your heating system.

So, when you switch on a tap, the diverter valve will close off the heating supply until you have turned the tap off.

Since this is a moving part, a lot of diverter valve faults relate to the valve sticking.

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Symptoms that highlight a faulty diverter valve on your boiler

#1 – Luke-warm hot water (taps and showers)

This is one of the most common symptoms of a faulty diverter valve. Luke-warm water will be supplied to showers and taps. The boiler should prioritise hot water, but if the valve is slightly stuck open on the heating side, some of the heating power (that is usually supplied to the taps and showers) is escaping.

#2 – You have to leave the heating on otherwise you have no hot water

If it’s a warmer time of the year and your heating is off, try turning it on and letting it warm up. Once it has, see if there is any hot water. If the temperature of the water increases after switching on the heating, this is another sign of a faulty diverter valve.

#3 – Lots of hot water but no heating

This is a common fault linked to a diverter valve. The diverter valve is likely stuck on the hot water side. So even though your hot water is a priority, the valve can’t open to allow the heating to warm up.

What should you expect from an engineer?

We never advise trying to fix a boiler, instead use a Gas Safe engineer.

An engineer will run a fault check to ensure that it is a faulty diverter valve, and not something else.

There are other boiler issues that can show similar symptoms.

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If this is a new property, or the problem has always been present, it could be something as simple as valves (on the pipework) not being open.

This fault check will usually include checking to see that all controls are working correctly.

The engineer will likely replace the whole diverter unit. In many cases, it’s only the diaphragm that needs replacing, but labour costs mean it’s worth getting the whole unit replaced.

The last thing you want is to replace the diaphragm and then have to call an engineer back in the near future because the valve itself is sticking.

Whilst the boiler is in bits, it’s certainly worth having the engineer assess the other visible parts for wear and tear.

If there are not major issues, then the replacement of the faulty diverter valve is all that is needed.

When the diverter valve is faulty, and other expensive parts are on their way out, we’d suggest investigating the cost of a new boiler. For that, we’d recommend HEATABLE — they offer exceptional value for money. You can check prices on their site here.

Costs to fix a faulty diverter valve

Expect to pay £250-350 to have a faulty diverter valve replaced.

Like every other aspect of a heating system, there’s a lot that affects the cost.

The cost of the valve itself

First, the cost of the diverter valve.

If we take a quick look over at Google, we can see that valves fluctuate from around £80-200.

If your boiler has a diverter valve that costs £80-120, it’s likely £250 will cover the cost of the repair (including parts and labour)

If your boiler has a diverter valve costing £150-200, it’s much more likely to cost £275-350 to repair (including parts and labour).

The cost of labour

Hourly rates throughout the country vary. Being in a less expensive part of the UK, compared with say London, could make £50-60 difference to the overall repair cost.

Also, consider the ease of replacement. Some faulty diverter valves are easy to get to and easy to replace.

The quicker and easy the replacement is, the less it’s going to cost.

When you shouldn’t replace a diverter valve

Boilers that are 8-10 years old are certainly not as advanced as new boilers. We already know there is a diverter valve fault.

Get the engineer to check the rest of the boiler parts thoroughly.

If there is anything else majorly wrong with the boiler, it’s worth considering a new boiler. Find out the cost of a new combi boiler here.

Boiler manufacturers tend to offer warranties in the 5-10-year range, so you know you won’t have any problems during that time.

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  1. I have Vaillant Combi Bolier Turbomax Plus 828 E, GMC No.. 4704409

    It was installed in 2004.

    Occasionally F22, appears which does not allow Hot water or Central Heating together.

    Independently Hot water or Central Heating work satisfactorily without any problem.

    How much would it cost to rectify it ?

    How much would it cost to replace it?.

    1. A new Worcester boiler (which is a top brand with 7-10yrs warranty) would start from £1,795 with next day install if you order before 3PM.
      Personally, given the deals you can get online, I would opt for a new install considering the other boiler is 16 yrs old.

  2. My Worcester boiler needs topping up every second or third day the service man said that’s normal he checked for leaks and found none it never used to do that yes it needed to be topped up but it is happening a lot lately any Help Please

    1. Did the service man check the expansion vessel, too?

  3. I’ve had diverter valve changed but the problem still remains my hot water comes on and even though heating is turned off due to weather the heating is still coming on? Can you please advise.

    1. Basically, it looks like this:
      A fault diverter or for example one that is blocked with debris, will fail to send hot water only into the hot water circuit, and will send some to the radiator circuit as well. That gets you hot radiators when you only want hot water. I think your engineer should know more about this as he installed the new diverter.

  4. Ive just moved into a property, VOKERA UNIVA HE boiler is heating radiators but there is zero hot water coming from all taps. This issue isnt covered in your site. Any advice?

    1. It could be a case of a faulty diverter, but I cannot say for sure. A service man should know better upon inspecting your installation.

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