18 l Lead-Free Potable Water Thermal Expansion TankThe 4. 8 gallon XT 18 Thermal Expansion Tank is utilized on potable water lines between the backflow preventer or pressure reducing valve and the water heater for protection against water thermal expansion in residential applications. As the system pressure increases, water enters the polypropylene lined water chamber of the tank, expanding a diaphragm into a pre charged air chamber to keep the system pressure below the relief valve setting. Materials