Thermal Expansion Tank with a backflow preventer or check valve. expansion tanks are built with a stainless steel system connection with patented Leak safe technology. Inside each tank is a heavy-duty, butyl rubber diaphragm encased with a polypropylene liner where the water is separated from the pre-charged air chamber. The tank controls excessive pressure generated inside the water heater using a sealed compressible air cushion space to store and hold the addition expansion water volume.