How to detect and fix a faulty Vaillant diverter valve

Company type: Manufacturer

HQ: Remscheid, Germany

Founded: 1874

CEO: Norbert Schiedeck

If the water in your taps only gets lukewarm or your radiators aren’t getting hot enough, you may have a problem with your Vaillant diverter valve. In this article, we will tell you what a diverter valve is, what it looks like and how to get it replaced to get hot water running as soon as possible.

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What is a diverter valve?


A diverter valve is a mechanism that allows the hot water to be guided to the central heating (radiators) or to the domestic hot water system (taps, showers, etc.). The diverter valve is only needed in boilers where water is heated on demand. Therefore, this mechanism is only found in combi boilers, as the system and conventional boilers store hot water in a storage cylinder.

When a hot water tap is turned on, the combi boiler heats the water and the valve directs it to the tap rather than the home’s radiators or other taps. When the heating is turned on, the combi boiler will do the same for radiators. If the heating is on and you turn on the hot tap in the shower, the diverter valve will close off the heating supply, prioritising sending hot water to the shower until you turn the tap off.

What are the symptoms of a faulty Vaillant diverter valve?

1. The hot water taps barely reach a lukewarm temperature

The most common sign of a defective diverter valve is lukewarm water. If your shower or tap is somewhat warm but doesn’t deliver any hot water, it could indicate an issue with the Vaillant diverter valve. If the valve is slightly stuck open on the heating side, some of the heating power will escape without heating the water up.

Consider how your annual energy bills would be affected if this occurs. The more heat lost, the more money is wasted. Therefore, it is important to resolve this as soon as possible to avoid extra costs and bring hot water back to your taps.

2. The water only gets hot when the heating is on

In the summertime, when your heating stays off most of the time, you may notice that your tap water stays cold. Try switching the heating on for a few minutes and see what happens. If you notice that hot water runs when you turn the heating on, this could mean that the diverter valve is broken and not doing its job, as it fails to regulate the amount of heat going to taps and radiators.

3. The water is hot, but the radiators are cold

Your radiators are likely to remain cold when the diverter valve gets stuck on the hot water side. Even though your combi always prioritises the hot water, it should switch to heating when you require it and after you turn off your taps or showers.

4. Your boiler is leaking

If you notice wet patches around your Vaillant boiler or if the boiler is losing pressure, this could signal a problem with the Vaillant diverter valve. Dirty contaminated water in the heating system can, over time, cause leaks in the small O-ring seal in the valve. This fault is most likely to happen in the older Vaillant boilers, installed over 10 years ago.

How to fix a faulty Vaillant diverter valve

We never recommend attempting to repair a boiler yourself; instead, you should contact a Gas Safe engineer. Other boiler problems can have symptoms that are similar to a malfunctioning diverter valve. For example, it could be anything as basic as valves on the pipework not being open if you recently moved into a new property.

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

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If the problem is caused by a malfunctioning diverter valve that has simply become stuck, then it may not require the replacement of the whole diverter unit. The Vaillant diverter valve can become stuck due to a build-up of limescale. In this case, all that is required is the diverter valve to be removed and then cleaned with a chemical solution. It can then be placed back into the boiler system and continue functioning as usual.

However, if the Vaillant diverter valve opens and shuts at the wrong times or is not responsive at all, it might need replacing.

Vaillant diverter valve replacement

Certain diverter valves are compatible with certain boilers. In most cases, a gas safe engineer will be able to select the suitable replacement Vaillant diverter valve for you and get it installed within an hour. Click here to see the different Vaillant diverter valves that are available. This might give you a feel for how much a replacement part will cost (expect to pay around £100-£200, although it could be more depending on the model of your Vaillant boiler).

If your Vaillant boiler is still under warranty, you may be able to replace the Vaillant diverter valve free of charge. If you’re not sure if your boiler is still under warranty, click below to contact Vaillant directly.

Contact Vaillant

In some cases, replacing the boiler itself may be more effective in the long run, as is the case with combi boilers over 10 years old. Keep in mind that if the boiler is old, something else could be failing. Vaillant’s latest range of combi boilers comes with longer guarantees and more durable parts. Check out the Vaillant combi boiler review here.

Still need help with your Vaillant boiler?

Call Smart Plan and speak to our expert engineering team free of charge.

0333 772 6247