Expansion vessel

An expansion vessel is a metal container divided in two by a rubber diaphragm. One side is connected to the pipe work of the heating system and therefore contains water. The other, the dry side, contains air under pressure and you will find a car-tyre type valve for checking pressures and adding air. When the heating system is empty or at the low end of the normal range of working pressure the diaphragm will be pushed against the water inlet. As the water expands so the diaphragm moves compressing the air on its other side and giving rise to a moderated increase in pressure that you can see on the pressure gauge.

An expansion tank or vessel is a small tank used in closed water heating systems and domestic hot water systems to absorb excess water pressure. Excess pressure can be caused by thermal expansion as water is heated, or by water hammer. The expansion tank usually contains a rubber diaphragm which divides the tank in two sections. One side contains a small volume of air, which is compressed so that it is equal and opposite to the water pressure. The other side of the tank contains water, which is openly connected to the plumbing system. Anytime the water pressure rises, it will push against the diaphragm, and gently compress the air. The compressibility of the air cushions the pressure shock, and relieves pressure in the system that could otherwise damage the plumbing system.