2.1 Gal. Potable Hot Water Heater Thermal Expansion Pressure TankThermal Expansion Tanks are designed for pressure control in potable water systems. The tank uses compressed air to maintain system pressures by accepting and expelling the changing volume of water as it heats and cools. It prevents the water heater pressure relief valve from opening, saves energy and eliminates a potential safety hazard. It is commonly installed in a domestic hot water system that contains a backflow preventer, a check valve or a