Rheem RRT-12 Therm-X-Guard Expansion Tank, 5-Gallon For best results, use only Rheem replacement parts for repairing your Rheem Water Heater A THERM-X-GUARD thermal expansion tank with a sealed in compressible air cushion provides a space to hold expanded water volume When hot water is used in the system, the expansion tank returns the hot water back into the system for use