Expansion vessels, also known as expansion tanks, can be found in most domestic pressurised heating systems. If you have acombi boiler, you will have an expansion vessel. This vessel contains air and water from the central heating system, and its purpose is to maintain the right level of pressure in the system.
There are various expansion tank designs, but the most common among modern boiler designs is the bladder type system. This design consists of a cylinder or disc which is split into two parts by a thin rubber sheet known as a ‘diaphragm’. One side is filled with water from the central heating system, while the other is filled with nitrogen. An expansion vessel also has an air valve that allows for the system to be depressurised and repressurised over time.