Hydronic system with the closed expansion tank is called closed system, and with the open expansion tank is called open system. In closed system the expansion tank should be installed on the return pipe and about 3 feet away of the suction side of the circulating pump or to the suction header of the multiple parallel circulating pumps. In open system the expansion tank shall be installed at least 3 feet above the highest point of the system to prevent the sub atmospheric system pressures caused by the pump operation. The open expansion tank should have an internal overflow drain. As required by the ASME boiler and pressure vessel codes, water must be prevented from freezing in the tank. The minimum volume of the open expansion tank should equal to 6 percent of the total system water volume.
