<Table> <Tr> <Td> </Td> <Td> This article includes a list of references, but its sources remain unclear because it has insufficient inline citations . Please help to improve this article by introducing more precise citations . (June 2012) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article includes a list of references, but its sources remain unclear because it has insufficient inline citations . Please help to improve this article by introducing more precise citations . (June 2012) (Learn how and when to remove this template message) </Td> </Tr> <P> An expansion tank or expansion vessel is a small tank used to protect closed (not open to atmospheric pressure) water heating systems and domestic hot water systems from excessive pressure . The tank is partially filled with air, whose compressibility cushions shock caused by water hammer and absorbs excess water pressure caused by thermal expansion . </P> <P> The modern vessel is a small container or tank 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 Schrader valve (car - tire type valve stem) 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 is pushed against the water inlet; as the water pressure increases, the diaphragm moves, compressing the air on its other side . </P>

Purpose of expansion tank on a hot water heater