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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 boiler types. 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.

Diverter valve boiler

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What is a diverter valve?

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

Diverter valve

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.

  1. 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.

  1. 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.

What kind of fuel does your boiler use?

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Worcester boilers installed from as little as £1,795.

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

  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.

  5. Hi just had a brand new worcester boiler the heaters are boiling but the hot water tap is running cold it will get hot if I put the heating on any advice please

  6. I have a baxi boiler, at least 15 years old, not a combi, I’m told probably divertor valve problem as no hot water but boiler working for heating. Engineer said needs system draining down to fix it, is this necessary?

  7. Hi, my boiler is wired for a Y plan, my hot water works fine on the programmer, but when the programmer changes to bring on the central heating the boiler won’t fire up. I have to knock the boiler off at the wall and back on again for the central heating to work. Could this be a wiring fault or 3 way valve issue or neither?

  8. Moved into a property with a worcester boiler last year – engineers out multiple times as initially no thermostat in the property, issues with the hot water (goes cold several times during a 15-20min shower). Never got to the bottom of it really and water issues still ongoing, just keep being told its normal… Heating randomly goes off yesterday, off for 16hrs – until once again turned off and on an hour ago, jiggled thermostat control box and it fired up, heating back on. Flame light keeps going off though, and having to give the therm control box a flick and it fires back up…eventually. Its a HA house so im likely to get completely fobbed off (emergency call out, 2 under 5’s in the house, happy to leave for 35hrs+). Any chance you can tell me what’s likely to be the issue/resolution please?
    Thanks!

  9. Hi, I have a Worcester Bosch Greenstar combi boiler, the central heating works perfect, a couple of years ago my shower/bath would start off hot and then go cold and eventually hot again, and then started working ok. The rest of the taps in the house was fine up until early this year when all of the taps would go luke warm, I changed the diverter paddle and that worked, then I had a new shower and bath fitted, last month and now the problem has started again that the shower starts off hot then drops cold then goes hot, not scalding but just how you want it

  10. Much of the time these components get blocked with chalk deposits from the water. The best remedy is to fit a water de scaling device to the water inlet pipe where it enters the house. This is an action I have taken and it works perfectly to keep water flowing where you want it flowing

  11. Hi, I have a Bosch CC1 combo boiler and the problem is that the shower water keeps going like warm and then heats up again I.e the temperature keeps changing during a shower. I have had a new shower cartridge fitted on the Mira mixer, a new plate heat exchanger, thermostat, and diverter motor fitted on the combo however the problem still exists. B gas engineer says the boiler is fine. This did not happen before therefore something must be wrong. Can you help. Thanks

  12. I have a Baxi combi 105he. I’ve changed both of the diaphragms in the diverter valve and I still need to put the heating on to get hot water. Any suggestions as to what could be the problem. Thanks.

  13. We have a 15 year old Worcester combi boiler. A few years ago we started having hot taps and showers run cold, then eventually heating up again. After testing flow rates etc the problem was diagnosed as sludge in the heat exchanger. A new exchanger was fitted and a filter installed and all was well – but it’s starting to happen again. Our showers suddenly run cold, hot taps take a long time to get got water – not all the time but it is getting worse just like last time. What is the advice? Spend money on the exchanger or other remedies or is it time for a boiler change?

  14. Hi there,

    I have a 2008 Worcester Greenstar heatslave 12/18 oil boiler running 6 zones of underfloor heating on two circuits, and a shower (and two handwashing basins). I get about 5-7 mins of hot water in the shower before it starts to fluctuate running cool then warm (never gets as hot as the first 5 mins and the warmer periods are too cool for a shower). Occasionally, we only get about 1 min of hot water before the fluctations start. It is inconsistent.

    Plumber has explained that with an oil boiler it needs a thermal store (10l tank of hot water) so it sounds like the boiler is runing out of that water then ‘cannot cope’ with continual hot water because either of the flow rate is too high and/or the coldness of the water in winter means the boiler is struggling to heat up the water.

    I have the hot water set to 5 (out of 6) on the front of the boiler, and the hot water runs about 73 degrees out of the tap and then reduces to about 40 degrees after about 5-7 mins.

    Any thoughts? It sounds like it could be diverter valve based on your explanation above. I’m expecting that a combi boiler provides hot water on demand for as long as I need it (e.g. if I want to shower for an hour and waste all my oil, that’s my decision, not my boiler’s decision’!). We have had the heat exchange replaced a fortnight ago.

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