Thermal expansion occurs as domestic water is heated. Left unchecked, the resulting pressure increase can cause relief valve discharge and other potentially unsafe conditions. Therm-X-Trol expansion tanks absorb this expanded water, keeping water pressure at a safe level.Most effective way to control Thermal Expansion. Stainless steel system connection Molded, polypropylene liner for maximum corrosion resistance Broadest line of models to fit any application. NSF 61 approved.