How does an expansion vessel work?

You might not have heard of a boiler expansion vessel before, it’s an essential part of your heating system that helps it to maintain a constant pressure. If you have a closed-water heating system, like a combi boiler, in your home, you will have an expansion vessel. It essentially acts as a sort of shock absorber, dealing with the expansion and contraction of water in your central heating system as it warms up and cools down. 

If this part stops working, the pressure in your heating system will change too much, meaning your boiler won’t work as it should. This might leave you without heating and hot water until the issue is resolved. In this guide, we will be looking at how expansion vessels work and how you can repressurise them to ensure they’re in full working order. 

How does an expansion vessel work? 

The first thing to note about expansion vessels is that they contain both air and water. One side is filled with water from the central heating system and the other with gas (normally nitrogen). This is a requirement in a lot of closed-water systems as water expands as it heats up. 

If the additional volume of water had nowhere to go, the pressure could increase to high levels, which would cause the pressure relief valve to open. The expansion vessel provides the extra hot water with pretty much an overflow tank, that can help bring the pressure back down to a normal level. Functioning as a spring, the air within the vessel compresses and absorbs the extra pressure, working to keep the system stable. 

There are several different designs for expansion tanks, but most new boiler systems have bladder style vessels. So, in these systems, the water from the central heating and the compressed air are separated by a flexible membrane (or diaphragm). The vessel includes a Schrader valve on the air side that enables more air to be pumped in when needed in order to maintain appropriate resistance to the expansion of the water. 

Where is the expansion vessel on a combi boiler? 

Where your expansion vessel is located will depend on the type and model of boiler you have. In combi boilers, these tanks are often installed within the casing of the appliance itself, so a lot of people aren’t even aware that these components exist. If you’re unsure where the tank is in your heating system, you’ll probably be able to find out in your boiler’s user manual. 

Remember, only a Gas Safe registered engineer is allowed to take off the casing on a gas appliance. This is the law, so never try to do it yourself. 

What size expansion vessel do you need? 

When water is heated up, it can expand to between 4% and 9% of its initial volume. This means that if an expansion vessel is too small it might not be able to do its job properly and won’t take enough water out of the system to reduce the pressure. 

When you purchase a new boiler, you won’t need to worry about this because it will normally come with an expansion vessel that is suitably sized. If you need to buy a replacement because your boiler expansion vessel has a fault, National Boiler Spares can advise you on the right model for your specific boiler. 

How to fit an expansion vessel 

Installing a new expansion vessel can be a challenging and time-consuming task, especially if it isn’t easy to access because of how your boiler is set up. This is definitely not a job you should try to do yourself, only heating engineers should remove and replace these parts. 

Depending on where it is located, more complicated elements of your boiler might need to be taken out to allow access to the vessel. In certain cases, the boiler will need to be taken off the wall so a new expansion tank can be fitted, which involves disconnecting the flue. 

What pressure should an expansion vessel be set to? 

To properly do its job, the compressed air in an expansion vessel needs to be at a suitable pressure. Your boiler manual should inform you how to work out this pressure, but in residential systems this is typically around one bar. 

You should hear air escaping when you push down on the Schrader valve to check the pressure in the vessel. If you don’t, then there isn’t any pressure. Another indication that something is wrong with the central heating part is if water escapes when you push on the valve. 

Draining and recharging the vessel should always be tried before contacting the manufacturer for a suspected failed vessel. If this doesn’t work though, it might be that the diaphragm inside has failed, which means you may need to get a new vessel. 

Contact National Boiler Spares today or explore our product range to see the boiler expansion vessel replacement cost UK.