Watts ET Expansion Tanks for Radiant and Hydronic Heating Closed loop systems. Series ET Non-Potable Water Expansion Tanks are for use in commercial and residential closed loop hydronic heating system piping to control the thermal expansion of hot water and maintain system pressure below the relief setting of the relief valve. The pre-pressurized steel tank features a durable expansion membrane that prevents contact of the water with the air in the tank. This rugged diaphragm minimizes loss of the air change and ensures long and trouble-free life for the system. Available in a variety of models and sizes. Maximum Pressure: 60psi (4.1 bar).