THERMAL-PLUS thermal expansion tank to absorb expanded water in closed, hot water systems. The expansion occurs as domestic water is heated. THERMAL-PLUS expansion tanks are designed to safely maintain water pressure levels in water heating systems to avoid the relief valve discharge and other potentially hazardous conditions. Thermal expansion tanks work in conjunction with a backflow preventer or check valve. THERMAL-PLUS expansion tanks are built with a stainless steel system connection, a polypropylene liner separated the water with the tank shell, heavy-duty, butyl rubber diaphragm, and 40 PSIG nitrogen pre-charge. The tank controls excessive pressure generated inside the water heater and provide the safest system of your home.