James Elston

Published April 9, 2024

Do you get very little hot water when you turn on your taps? Or only hot water when your central heating is on? Well, that brass knob, known as the diverter valve, that connects pipes in your combi boiler might be the culprit. 

But you won’t be able to free a stuck diverter by yourself; a qualified heating engineer will need to be called in to save the day.

This guide isn’t a DIY instruction manual on how to fix your boiler – but it does include DIY tests on how to confirm that it’s stuck. Apart from that, the goal of this article is to help homeowners understand their boiler diverter valve and address the issue correctly, without getting their hands too dirty.

It’s important to note that according to the Gas Safety Regulations of 1998, it’s illegal to perform any diverter valve maintenance (or any type of boiler maintenance, for that matter) on your boiler if you’re not a professional Gas Safe engineer.

With that in mind, Eco Happy is here to help you get to know your diverter valve a bit better, the common signs to look out for when it malfunctions and how you can address this problem.

Understanding The Diverter Valve

Understanding why diverter valves are crucial mechanisms in combi boilers is the first step to figuring out why there’s no hot water coming out of the hot water taps.

The diverter valve is a vital component in combi boilers that is responsible for directing hot water to either the radiators or the domestic hot water tap. Its unique function allows for the efficient multitasking combi boilers are known for. However, it’s essential to note that diverter valve functions may differ across various boilers.

What causes a stuck diverter valve?

There are various reasons why a diverter valve may get stuck. Some of the most common ones are:

  1. General wear and tear: Over time, the valve’s internal components, like springs and seals, can wear out, losing their ability to move smoothly, causing it to get stuck.
  2. Mineral deposits: Many areas in the UK have hard water. This can leave mineral build-up inside the valve, hindering its movement and leading to jams.
  3. Debris or corrosion: If dirt or rust particles enter the valve mechanism, they can block its operation and cause it to get stuck.
  4. Faulty actuator: Some diverter valves use electric actuators to control their movement. If the actuator malfunctions, it can leave the valve stuck in one position.

Common Symptoms Of A Faulty Diverter Valve

Here are a few common symptoms of a faulty diverter valve:

  1. Lukewarm water from taps: This is probably the most common sign of a faulty diverter valve. While both the heating and hot water systems are on, a combi boiler with a faulty valve may prioritise directing the water to the central heating. This causes the hot water supply to get cut off.
  2. Hot water only works when the heating is on: Try turning off your heating system and then open the hot water taps. If the water is cold coming out, but then gets hot again once you turn the central heating back on, it’s a clear sign that it’s the boiler diverter valve causing issues.
  3. The hot water is working but the heating system isn’t: This problem is triggered by the water flow. Your diverter valve might have gotten stuck on the hot water side. This means that most of the heat has gone to warming up the water, instead of your house.
  4. The heating in your house is irregular: Your radiators seem to be heating inconsistently or patchy and the central heating is not functioning properly throughout the house.

Testing your boiler’s diverter valve

After you’ve experienced one of the common symptoms, like no hot water, the next step is confirming that it is in fact due to a faulty diverter valve.

Is it necessary to contact a certified technician for boiler repair yet? Luckily, you can perform a few boiler mechanism tests yourself without risking your boiler warranty or the combi boiler breaking. However, it’s strongly advised not to perform these tests if you’re not 100% comfortable, as it can be dangerous and impact the warranty if not done correctly.

Here’s how to check if your diverter valve is stuck: 

  • Open the combi boiler: If your combi boiler has a removal lid or cover on its front, you can remove this. However, you’ll need to check your boiler manual to ensure the cover is safe to remove as you need to be very careful while doing this.
  • Locate the diverter valve: Next, you need to look for the diverter valve. Because this brass valve is responsible for the flow of hot water used for the heating and taps, it will have various cold and hot water pipes connected to it.
    The exact positioning of the valve will differ from boiler to boiler, so if you can’t identify it, it’s best to wait for a qualified heating engineer to diagnose the problem. 
  • Inspect the valve: Finally, you’re going to open any hot water tap in your house, if the diverter valve doesn’t move, it’s stuck.

Remember, it is not recommended that you carry out this test on your own if you haven’t done anything like it before. If you’re uncertain, contact a professional.

How To Address A Stuck Diverter Valve

You probably don’t want to end your warranty early, destroy your central heating system, or break the law. That’s why you have to hire a qualified heating engineer to diagnose and solve any issues you’re experiencing with your combi boiler.

A heating engineer will: 

  • Assess and confirm the problem: A qualified heating engineer will first confirm if it’s the diverter valve causing an issue. They might even discover other boiler issues that will require a different solution.
  • Clean the diverter valve: The faulty diverter valve will likely be removed and cleaned using commercial chemical cleaners. 
  • Valve replacement: If the heating engineer discovers that the valve is damaged beyond repair, they might have to replace it or install a new combi boiler.

Costs and financing options

You’re probably wondering how much this faulty diverter valve is going to cost you. If the boiler is still under warranty, you might be able to get the diverter unit replaced free of charge.

Either way, you will have to hire a professional. The average costs associated with fixing a stuck diverter valve are as follows:

  1. Cleaning and refitting a stuck diverter valve: £40 to £130
  2. Replacing the diverter valve: £80 to £200
  3. Replacing a combi boiler costs: £800 to £2,800

If this is a bit above your price range, don’t stress! There are a few boiler financing options out there for you to consider. There are government grants available as well as warranties if your whole diverter unit might need to be replaced. Contact Eco Happy today – we will be happy to help you go over the Boiler Upgrade Scheme and the heat pump grants option!


How do I know if my diverter valve is failing?

A malfunctioning diverter valve within a boiler can significantly disrupt the proper flow and distribution of hot water. This disruption can manifest in several ways, including:

  • Reduced hot water temperature
  • Irregular heating performance
  • Complete loss of hot water

Can I fix a diverter valve unit myself?

Unless you’re a qualified heating engineer or have experience with a diverter valve, it’s not recommended to take on this DIY project. It is also illegal to do so, according to the Gas Safety Regulations of 1998. It’s best to find a qualified Gas Safe engineer for the job.

Do diverter valves go bad?

Yes, an older boiler diverter valve is more prone to failing. The life expectancy of a diverter valve is around 10 years.

Final Thoughts

While the urge to tackle repairs yourself is understandable, tinkering with a diverter valve is best left to professionals. This crucial component controls your boiler’s hot water flow, and tampering with it can cause more problems in your heating system than it solves.

Your faulty diverter valve will be assessed by the professional you’ve appointed, and they will come up with the best solution.

You should have your combi boiler serviced regularly to prevent unnecessary labour costs and damage in the future.

James Elston

Boiler Expert

James Elston is the top boiler replacement and heating expert at Eco Happy. He has over 20 years of experience in the industry, focusing on Gas Safe boiler installations and offering home-heating and energy-saving solutions to homeowners across the UK. From sourcing the most energy-efficient combi boiler to providing specialist heating advice, James ensures that Eco Happy maintains the highest standards and best customer service.

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