Most of water systems nowadays require backflow prevention. However, when the backflow protection seals off the system, the relief valve can be discharged on every heating cycle. With A.O. Smith TW-12-1 Expansion Tank, there will be no wasted water as it helps protecting your plumbing system to have a smooth operating system. The expansion tank can extend the life of your water heater by internally lining with a polypropylene liner for corrosion protection and it adjust the air pre-pressure to the actual water pressure with an air valve.