Despite the fact that you may not have heard of an expansion vessel before, it’s a crucial part of your heating system that allows it to maintain a constant pressure. If you have a closed-water heating system in your home, such as a combi boiler, you will have an expansion vessel. Put simply, it acts as a kind of shock absorber, handling the expansion and contraction of water in your central heating system as it heats and cools.

If this element stops working, the pressure in your heating system will fluctuate too much - meaning your boiler won’t function as it should. This could leave you without heating and hot water until the problem is fixed.

Want to know more about this essential part of your central heating system? Here, we take a look at everything from how expansion vessels work to where you can find them and how you can re-pressurise them to keep them in fully working order.

 How does an expansion vessel work?

The vessel, which will look a little something like the one pictured above, contains both air and water. Water from the central heating system fills one side and gas, usually nitrogen, fills the other. It’s a requirement in many closed-water heating systems because water expands as it heats up. If the extra volume of water didn’t have anywhere to go, the pressure in the system could rise to high levels, at which point the pressure relief valve would open to compensate. The expansion vessel gives the additional hot water a sort of overflow tank, that can help to bring the pressure in the system back to a normal level. Acting like a spring, the air that is present in the vessel compresses. In this way, it absorbs the extra pressure, helping to keep the system stable.

There are different expansion tank designs, but the majority of newer boiler systems feature bladder type vessels. In these systems, the central heating water and compressed air are separated by a flexible membrane - or diaphragm. The vessel has a Schrader valve on the air side that allows for more air to be pumped in when necessary to maintain a suitable resistance to the expansion of the water.

Where is the expansion vessel on a combi boiler?

The location of your expansion vessel will depend on the type and model of boiler you have. In combi boilers, these tanks are usually fitted within the casing of the appliance itself - meaning many people don’t even realise these components exist.

If you’re not sure where this tank is in your heating system, you should be able to get this information from your boiler’s user manual.

Caution - by law only a Gas Safe registered engineer is allowed to remove the casing of a gas appliance - never attempt this yourself.

What size of expansion vessel do I need?

When water is heated, it can expand to between four and nine per cent of its original volume. This means that an expansion vessel that is too small may not be able to perform its job sufficiently and won’t take enough water out of the system to bring the pressure back down.

When you get a new boiler, you don’t need to worry about this as it will generally come with an expansion vessel that’s appropriately sized. If you need to get a replacement because this tank develops a fault, your heating engineer will be able to advise you on the right model for your particular boiler.

How to fit an expansion vessel

Fitting a new expansion vessel can be a difficult and time consuming job, particularly if it is not easy to access because of your boiler setup. This isn’t a job you should attempt yourself. Only heating engineers should remove and replace these tanks.

Depending on where it is situated, complex parts of your boiler may have to be taken out in order to get to the vessel. In some cases, the boiler needs to be taken off the wall in order to fit a new expansion tank, and this involves disconnecting the flue.

What pressure should an expansion vessel be set to?

To do its job properly, the compressed air in an expansion vessel must be at a suitable pressure. Your boiler manual should tell you how to calculate this pressure, but in residential systems this is usually around one bar.

When you push down on the Schrader valve to check the pressure in this vessel, you should hear air escaping. If you don’t, there is no pressure. Another warning sign that something is wrong with this central heating component is if water escapes when you press on the valve. This may just be condensation, which can be drained from the Schrader valve. Draining and recharging the vessel should always be attempted prior to calling the manufacturer for a suspected failed vessel. However, if this doesn’t work, it may be that the diaphragm inside has failed, meaning you might need to get a new vessel.

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How to recharge an expansion vessel on a combi boiler

If you think your expansion vessel may need to be re-pressurised, also referred to as ‘recharged’, the safest and easiest thing to do is arrange for your heating engineer to take a look at it.

If you plan to do this yourself however, you’ll firstly need to switch off your boiler and remove the fuse as an additional safety precaution. This will ensure it doesn’t switch back on by accident. You should also make sure that the central heating side of the vessel isn’t pressurised, meaning it’s important to release any excess pressure first.

Once you’ve taken these steps, you can check the pressure of the compressed air in the vessel against the recommended value provided in your boiler manual. You will need appropriate measuring equipment to test this. For example, you could use a car tyre pressure gauge (pictured below). If you discover the pressure is too low, you can recharge it using any type of air pump and attachment.

After you’ve re-pressurised the vessel, it’s a good idea to make sure the Schrader valve isn’t leaking air. You can do this by simply putting some washing up liquid over it. If you see bubbles forming in this liquid, you may need a replacement valve.

If you have a Viessmann boiler - whether it’s a combi model, storage combi design or anything else - and you have any questions about expansion vessels or another topic, don’t hesitate to contact our customer support team for further information and advice.

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