Expansion Vessel – All You Need To Know

Our central heating system is not something that many people take much notice of; they just expect it to work as and when required. Most people could identify the boiler, the radiators, and depending on the type of system, they have the water tank. Other than that, when it comes to the technical aspect of a boiler, it is best to leave the repair of these items to a professional, particularly as most repairs need to be undertaken by a registered Gas-Safe engineer. 

What Is An Expansion Vessel? 

An expansion vessel is a crucial component of any closed-water heating system. If you have a combi-boiler, then your heating system is a closed-water heating system.

The role of an expansion vessel is to maintain a specific level of pressure within your heating system. In layman’s terms, it is essentially an overflow system, which handles the expansion or contraction of water due to an increase or decrease in temperature. 

Expansion Vessel & Boiler Pressure Gauge

What Happens if Your Expansion Vessel Stops Working? 

If and when the expansion vessel in your system begins to malfunction, then the pressure is likely to fluctuate too much. This will typically mean that the boiler stops functioning correctly, which could mean no hot water or central heating in the worst-case scenario. Hopefully, now you are beginning to understand what a critical component of your heating system, the Expansion Vessel, actually is. 

How Does The Expansion Vessel Work? 

An expansion vessel is normally a circular type of container which contains both air and water. The container in most modern boiler systems features a bladder-type vessel. The water and air are separated by a flexible membrane, ensuring that the pressure fluctuates as required. Normally the vessel is split into two sides; on one side is gas, usually nitrogen, and on the other is water. As you may well remember from secondary school chemistry, as the water heats up, it expands.

As a consequence, a combi boiler needs to have a built-in safety solution to facilitate that expansion. Otherwise, the pressure would continue to rise in this situation, leading eventually to the pressure relief valve opening, meaning that the boiler would rarely operate properly. The role of the expansion vessel is to provide the additional hot water with extra storage, which essentially helps to stabilise the pressure in the system. 

Read: How To Clean An Airlock On A Central Heating System

How Do I Locate The Expansion Vessel on a Combi Boiler? 

The expansion vessel on a combi-boiler can be located in several different locations, but it will always be contained within the boiler’s casing. Under current UK law, only a Gas Safe registered engineer is permitted to remove this casing, which is why we cannot nor would ever advise that you try to problem-solve this issue yourself. 

Is Fitting an Expansion Vessel A Big Job? 

As mentioned above, fitting or replacing an expansion vessel is not a job that you can undertake unless you are a qualified Gas-Safe engineer, in which case it is unlikely that you would be reading this article in the first place. Be aware that this is unlikely to be a quick job, even for a qualified engineer. The length of time the job will take will depend on where the expansion vessel is situated; in some cases, large parts of the boiler may need to be removed to gain access to the old expansion vessel. It is even possible that the boiler may need to be taken off of the wall to complete the job. Just be aware that you may need to set aside a day for this repair to be completed. 

How To Recharge An Expansion Vessel on A Combi Boiler?

In some instances, resolving the issues with the expansion vessel might be as simple as re-pressurising or recharging the system. Again this requires the services of a heating engineer. The good news is that just because you may have a problem with your expansion vessel, it does not necessarily need to be a huge and expensive issue, so don’t panic too much initially.

What should I do if my Boiler Is Losing pressure?

If your boiler is losing pressure (all of the pressure in 24 hours or less) then here at ADI Leak Detection we will be able help. Call us now on 0800 731 3843 for the best leak detection service in the UK!