A water heater expansion tank is a small tank used to protect your water heater from excessive pressure. The tank is partially filled with air and cushions the shock from water hammer and absorbs excess water pressure caused by thermal expansion. The modern expansion tank is small and divided in two by a rubber diaphragm. One side is connected to the pipework of the system and contains water. The other side-the dry side contains air under pressure and a Schrader valve for checking pressures and adding air. When the heating system is at the low end of the normal range of working pressure, the diaphragm is pushed against the water inlet. As water pressure increases, the diaphragm moves, compressing the air on the other side. The rubber diaphragm in expansion tanks prevents the undesired transfer of air and helps maintain low levels of oxygen in the pipes and reduces corrosion in the system. De Hart Plumbing Heating and Cooling serving Manhattan Kansas, Junction City Kansas, Wamego Kansas and some surrounding areas.