How To Replace A Viessmann Vitodens 200 Expansion Vessel

how to replace a viessmann vitodens 200 expansion vessel

Caution – This article is for information purposes only. Please do be aware that by law only a Gas Safe registered heating engineer is allowed to remove the casing of a gas appliance. Attempting to replace the expansion vessel yourself may also mean that you’ll invalidate your warranty not to mention the risk to your safety.

The Viessmann Vitoden 200 is well known for its reliability, but just like even the best combi boilers & other types of boilers, there will be times when things break down, and repairs are needed. The expansion vessel is a key component within your boiler, and when it’s not functioning correctly, then it is going to mean no hot water or heating in your home.

So, this guide is all about helping you to spot when there’s a problem and what can be done to get it fixed rather than face a new boiler cost.

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What Is An Expansion Vessel?

The role of the expansion tank is to maintain the pressure within your boiler system.

That’s because as water is heated, it expands and that’s caused by the molecules in the water speeding up as it gets hotter and so needing more space. This is what’s called thermal expansion.  

Expansion tanks provide the room for the ‘expanded water’ as it heats. This is set-up so that a flexible diaphragm separates the central heating water and the compressed air. As the water expands, the diaphragm flexes and makes room for the extra water. Once the water has cooled back down or been used, then the diaphragm shifts back again.  All this then means that the pressure in the system stays the same all the time the water is heating.   

There’s also a Schrader valve, the same as you have on your car tyres which is installed on the airside. This then means that more air can be pumped in as needed to maintain the pressure in the expansion vessel and provide that needed resistance to the expansion of the water.

Where Is The Expansion Vessel Located On Viessmann Vitodens 200 Boilers?

The location of the expansion vessel does vary from one model to the next, and it can often be behind the casing. That means that as with boiler installation, only a gas certified engineer is allowed, by law, to gain access to it.

For further information on where all the components are within the Vitodens 200 boiler, take a look at the installation manual.   If you have a Viessmann Vitodens 100-w, then you’ll find the installation manual for this model here.

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How Do I Know If The Expansion Vessel On My Vitodens 200 Is Faulty?

The usual symptom of a faulty expansion vessel is when the pressure goes much higher than expected and then drops down much lower than it should be. This is then followed by the boiler shutting down as it’s unable to operate with such fluctuating pressure levels.

Another sign of an Viessmann expansion vessel problem within your system can come from the pressure relief valve opening and leaking water in an attempt to keep the pressure level down.

How To Check Expansion Vessel Pressure?

The pressure will usually be checked during your boilers annual service. If there are any problems, then the engineer will be able to check out the system there and then.

Do remember that annual servicing is required to keep the warranty valid but that our boiler cover plans are a great way to avoid big unexpected bills or boiler replacement cost should there be any problems.

How To Replace A Viessmann Vitodens 200 Expansion Vessel – Step By Step

Fitting a new expansion vessel can be difficult, and this can be because of how your boiler is set-up. It’s likely that you will need to remove other parts of the boiler to reach the vessel and in some situations, you may also have to remove the boiler from the wall and disconnect the flue to get to it.

So, if the expansion vessel needs to be replaced, we strongly recommend getting in touch with an accredited heating engineer who will have the training and expertise to complete this for you.

What Pressure Should An Expansion Vessel Be Set To?

If you’ve been having problems with your boiler and you’re now wondering what pressure should an expansion vessel be set to, we have the answers for you!

Generally, the pressure of the expansion vessel should be set to 1 bar. Do remember though that this will go up when your new boiler is on and heating cold water, so check the pressure when the boiler has been off for a while to ensure you get an accurate reading.


Having to replace an expansion tank isn’t unusual in a boiler which is coming to the end of its working life. But without it working effectively then you’re just not going to get the supplies of hot water and heating that you need.

Sometimes the cost of the replacement in an older boiler combined with the risk of other problems such as faulty heat exchangers can mean that replacement rather than repairs makes more sense. If things aren’t going well with your boiler, and the time has come to go your separate ways we can have a new system fitted in your home within a matter of days. Check our great boiler deals right now.

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