Within sealed systems the expansion vessel replaces the expansion tank and takes up the extra volume of water that is created as water is heated.
They work due to a diaphragm made from neoprene separating two sides of the vessel, on one side is nitrogen or air pressurised to around 0.8 bar and the other side is connected to the boiler or system pipework and filled with primary system water. Naturally as the water gets hotter it expands and this expansion forces the neoprene diaphragm to compress the pre-charged pocket on the other side of the diaphragm – as the pre-charge pocket compresses the pressure within it increases. If there weren’t any pre-charge pressure the water would fill the expansion vessel and there would be no room for it to expand – therefore pressure would fill up in the boiler and the system pipework and could potentially cause serious pipework and boiler issues. However to prevent any damage the pressure relief valve (PRV) opens and vents any pressure build up. So if you have water exiting the PRV discharge pipe you probably have a problem with your expansion vessel.
Expansion vessels come in various sizes – what is important is to ensure that the expansion provided within the combination boiler is also large enough to accommodate the expansion of the volume of water in the system that it is fitted to too. If it is not an additional expansion vessel may be required to cope with the extra volume.
Common faults with expansion vessels are:
1- Blocked Feed Pipe
2- Split Diaphragm
3- Oversized System
4- Condensation in the Vessel
5- Loss of Pre-Charge Pressure