XT-35 - Thermal Expansion Tank - 9 GallonZurn Wilkins XT 35 9 Gallon XT Thermal Expansion Tank The Zurn Wilkins XT 35 is a 9 gallon diaphragm style expansion tank designed to protect residential potable water systems from thermal expansion pressure. Installed between a backflow preventer or pressure reducing valve and a water heater, it safely absorbs excess pressure as heated water expands. A polypropylene lined chamber and Butyl rubber diaphragm maintain system stability by allowing water