Diagnosing a Boiler Faulty Diverter Valve & The Cost To Fix It

Diverter valves are an integral part of your heating system and without it working correctly, you’ll have problems with your hot water and / or central heating. 

In this guide we’ll explain:

• What a diverter valve is
• Symptoms of a faulty diverter valve
• How to free a valve that’s stuck
• The likely repair and replacement cost
• Whether it makes sense to repair or replace your boiler

Boiler Diverter Valves in Summary: What They Are, Fixes & Costs

Boiler diverter valves are fitted in combi boilers and control when to push hot water or central heating demand to taps, showers and radiators in your home. If you have a conventional or system boiler (using a hot water cylinder or cold water storage tank), they don’t use diverter valves so this won’t be the issue.

A faulty diverter valve is likely the cause of your hot water and heating problems if your central heating system is acting unusually, including scenarios such as:

• Taps and showers producing lukewarm water
• Central heating works, but hot water doesn’t
• Hot water works fine, but your central heating doesn’t.

The typical repair cost of a diverter valve is £200, whereas a replacement could cost up to £500. 

Before investing this kind of cash in an old and unreliable boiler, we’d suggest comparing repair / replacement costs with a new boiler; you can get a fixed price on screen here within 2 minutes. 

What is a Boiler Diverter Valve?

Combi boiler diverter valves work by switching supply between central heating (radiators) to hot water (taps and showers) in your property. As an example if your central heating is on but you then run a bath, the diverter valve is the component that tells your bath tap it needs hot water. 

Combi boilers will always prioritise hot water over your central heating. So in the example above, you may find that radiators start to cool down whilst you run a bath. 

This isn’t necessarily a problem, it’s simply the diverter valve prioritising hot water demand over your central heating. As soon as you turn off your bath tap, the valve will then divert energy back to the central heating to get your radiators hot again.

‍A diverter valve is a component found only in combi boilers. Combi boilers heat water “on demand” which is why they need a diverter valve to switch between hot water and central heating. Central heating systems which use conventional or system boilers have a hot water storage tank / cylinder instead, removing the need for a diverter valve.

Put simply, if you have a conventional or system boiler and are having heating problems, a diverter valve isn’t going to be the cause of the fault.

3 Symptoms of a Faulty Boiler Diverter Valve

Now you know what a diverter valve is, how do you know if your boiler diverter valve is broken? 

Below are the 3 most common symptoms that suggest there’s an issue with your boiler’s diverter valve.

1. Lukewarm Water From Taps or Showers

If you’re only able to get lukewarm water from taps and showers, the likely cause is a sticking boiler diverter valve. 

Assuming your central heating is working to some extent, when you demand hot water the valve isn’t able to open fully, meaning it’s not working at full capacity.

2. Central Heating Works, But No Hot Water

Another symptom that might suggest a broken boiler diverter valve is when your radiators are getting hot, but you can’t get any hot water from taps or showers. 

In this case, your diverter valve is stuck on the heating side. Not being able to switch to hot water means even when you turn on hot taps or showers, cold water will come out.

3. Hot Water Works, But Central Heating Doesn't

Similar to above but in reverse, if you can get hot water but your radiators won’t get hot, your diverter valve is either stuck on the hot water side, or broken altogether.

How to Free a Stuck Diverter Valve

Unfortunately, freeing up a stuck diverter valve isn’t a DIY fix. Until it’s been tested, you won’t be able to determine whether it’s stuck or broken completely. 

If you suspect a faulty diverter valve is causing problems with your combi boiler, you’ll need to call a qualified engineer. Gas Safe registered engineers are the only people legally qualified to work on gas boilers. 

Regardless of what brand your boiler is (Worcester, Baxi, Ideal, Vaillant, Viessmann etc), a Gas Safe engineer will be able to diagnose the problem, fix it, or in some cases they may suggest you replace your boiler.

Every Gas Safe engineer will have a number registered to their name or company. You can search this number to check they’re qualified using the Gas Safe Register tool here. 

Costs to Fix or Replace a Boiler Diverter Valve

Whether you’re looking at getting the valve on your boiler repaired or replaced, the costs associated with both are usually substantial. 

If you’ve got a boiler from a leading brand such as Alpha, Ideal, Vaillant, Viessman or Worcester, there’s a chance you could get the part fixed or replaced under warranty. If not, it may be worth considering a replacement boiler rather than sinking money into an old unreliable one.

Want a price for a new combi boiler instead of repairing your old one? 

Get a fixed price on-screen using our quick (it takes less than 2-minutes) boiler quote form here. All boilers come with a 10-year plus warranty and we only use leading brands such as Alpha, Ideal, Vaillant, Viessmann and Worcester.

Repair Cost

So how much does a boiler diverter valve cost to repair? If you’re looking at a case of a stuck valve there’s a chance a Gas Safe engineer could free it up, avoiding the need for a replacement valve or even worse; replacement boiler!

Typical costs associated with diagnosing and freeing up a stuck diverter valve are likely to be in the region of £150-250 depending on how long it takes and where you are in the country (labour rates vary depending on where you live).

Replacement Cost

If your Gas Safe engineer can’t free up a stuck diverter valve, you’ll need a replacement. 

Expect costs to be in the region of £100-200 for the valve, and £150-300 for labour. Meaning the overall cost of a replacement valve could be anywhere up to £500. 

If you’re lucky enough to have a reasonably new boiler that’s under parts and labour warranty, then that’s the route you should take. Typically, there will be a sticker on your boiler which offers contact details for the warranty team. 

Repair Cost Versus New Boiler Cost

As you can see above, fixing a broken boiler isn’t exactly cheap. Unless your boiler is new and under a parts and labour warranty, there’s little point sinking big money into an old, inefficient and unreliable boiler. 

We install boilers from leading brands such as Alpha, Ideal, Vaillant, Viessman and Worcester that come with a 10-year plus warranty. The cost of a diverter valve fix could pay off around 20% of a new boiler, with prices for a new boiler installation starting at £2,250 (property dependant). 

Here are a few resources to help you decide between repairing and replacing your current combi boiler: 

• Our 2-minute clickable boiler quote form (a fixed price on a boiler installation tailored to your property)
• A detailed boiler installation cost guide
• Dwellow’s guide to the best and most reliable boilers. 

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