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How to Diagnose a Faulty Diverter Valve on a Boiler – And How Much They Cost to Fix

Do you suspect your combi boiler has a faulty diverter valve? We help you figure out if this is the case and what you need to pay to fix it.

If you have a 10-year-old combi boiler, it will be much more prone to developing faults, such as a faulty diverter valve. It’s a common problem, particularly with older boilers, but this fault can occur at any time during a combi boiler’s life. 

This guide covers the common symptoms of a diverter valve fault, how to fix it, and what you can expect to pay for a replacement. 

What is a diverter valve?

A diverter valve is an essential part of your combi boiler that opens and closes to allow hot water to travel to your taps, showers or radiators. When you turn on a tap, the diverter valve will direct hot water to that tap rather than to the radiators or other taps within your home.

The valve prioritises sending water to your taps and showers over your radiators. So, if your heating is on and you turn on a hot tap, it will close off the heating supply until you turn off the hot tap. When you eventually turn off the tap, hot water is sent to your radiators again. 

Only combi boilers have a diverter valve because they heat water on demand, whereas system and heat-only boilers store hot water in a separate cylinder. 

Symptoms of a faulty diverter valve

Diverter valves in older combi boilers can break down or become damaged through wear and tear, resulting in issues that affect your boiler’s components and your heating and hot water. The tell-tale signs of a faulty diverter valve include: 

1. You can only get lukewarm water from your taps and showers

One of the most common symptoms of a diverter valve fault is lukewarm water coming from your taps and showers. As we said above, your combi boiler should prioritise hot water. If you’re getting only lukewarm water, then chances are the diverter valve is stuck open on the heating side and allowing some of the heat for your hot taps to escape. 

2. You have hot water for your taps and showers but no heating

Another sign of a diverter valve problem is if your hot taps and showers work fine, but you have no heat coming from your radiators when the heating is on. If you have hot water but no heating, this is likely similar to the previous issue, with the diverter valve stuck on the hot water side rather than the heating side.  

3. You have hot water only when the heating is on

You should still get hot water from your taps and showerheads when not using your heating during the warmer time of the year. A quick way to test for issues is to turn on your heating and let your radiators warm-up before checking whether you have hot water. If you discover that the water from your taps isn’t hot unless the heating is on, it’s a clear sign that you have a faulty diverter valve.

How to fix a faulty diverter valve

Now that we have shared the potential causes, you should have a clearer idea about whether your combi boiler’s diverter valve is faulty. 

When it comes to fixing a faulty diverter valve, you should contact a Gas Safe registered engineer instead of attempting to fix it yourself. A qualified engineer can run a fault check to determine if the diverter valve is faulty and needs cleaning or repairing. 

Your engineer may tell you that the diverter valve needs replacing if they discover the valve is badly worn or damaged. If this is the case, they will also check the other components of your boiler to see if it’s worth fitting a new diverter valve or getting a replacement boiler. 

How much does it cost to replace a diverter valve?

The cost of a replacement diverter valve can vary, depending on the brand and model of your combi boiler and how easy it is to get hold of and replace. However, we can tell you that a new diverter valve will set you back £200 on average. 

If your boiler’s diverter valve costs anything up to £120, you can expect to pay closer to £250, including parts and labour. But if your boiler diverter valve’s value is between £150 and £200, the cost will likely be £275 to £350.  

Labour costs

Labour costs can also vary depending on where you live and who you choose to fix your combi boiler. Some Gas Safe registered engineers only charge around £50 to fit a replacement diverter valve, but the overall labour cost can range from £50 to £100. If you live in an expensive area of the UK, like London, the more it will cost.

Check your warranty

If your combi boiler is still under warranty, you may find that the manufacturer will replace the diverter valve at no cost to you. The boiler warranty may say that you need to use a manufacturer-approved engineer to carry out any boiler repairs, so be sure to check the terms and conditions before contacting a Gas Safe registered engineer to fix your faulty boiler. 

Should you replace the diverter valve or get a new boiler?

Should your engineer tell you that the diverter valve needs replacing, ask them to check the rest of your boiler for problems. If they find any other major faults or your boiler is old, you should consider getting a new boiler with a generous warranty rather than replace the diverter valve on your existing one.