The Eastman 2-gallon thermal expansion tank is designed to prevent dangerous build-ups of pressure in your water heater tank. The thermal expansion tank stops hot water from backflowing into the cold water line while preventing hot water from escaping through the pressure relief valve and onto the floor. This constant fluctuations in water pressure can be dangerous and they can damage your water heater tank permanently, properly installing a thermal expansion tank can solve all these issues. Complies with NSF-61 and Lead Free law.