Flexcon Industries HTX15 FLEX2PRO Hydronic Expansion Tank - 2.1 Gallon Oversized 100% butyl diaphragm virtually eliminates diaphragm stretch, further promoting longevity HTX tanks feature a brass air valve with rubber O-ring seal Tanks are finished with appliance quality paint which helps prevent external corrosion HTX tanks are comprehensively tested and backed by Flexcons 5 year limited warranty Volume: 2.1 Gallons