A.O. Smith TW-5-1 Expansion Tank prevents wasting water and provides a smooth operating system for plumbing systems where backflow or check valves are present. When a backflow protection seals off your water system, it requires the use of an expansion tank, otherwise the relief valve can be discharged on every heating cycle. The expansion tank will be internally lined with a polypropylene liner for corrosion protection and its air valve will adjust the air pre-pressure to the actual water pressure. Basically, the expansion tank can add years to life of your water heater.