What is an expansion tank and what is the purpose of it?
A water heater expansion tank or expansion vessel is a small tank used in closed water heating systems and domestic hot water systems to absorb excess water pressure, which can be caused by thermal expansion as water is heated, or by water hammer. The vessel itself is a small 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 normally a car-tire 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 pressure increases, so the diaphragm moves compressing the air on its other side. The compressibility of the air cushions the pressure shock, and relieves pressure in the system that could otherwise damage the plumbing system. When water heater expansion tank is used in domestic hot water systems
