12-A102 - Backstop Thermal Expansion Tank - 4.5 GallonElbi 12 A102 Backstop Thermal Expansion Tank 4. 5 Gallon The Elbi Backstop A Series thermal expansion tank protects water heaters and closed loop plumbing systems from excess pressure caused by thermal expansion. Designed for use in residential and light commercial potable water systems, it absorbs expanded water volume to prevent stress on pipes, valves, and appliances. With durable construction and a compact, pre charged design, it ensures long