Unions allow for more connection type and pipe size options when mounting a circulating pump with union connections between two sections of pipe. Since a pump can work with a range of pipe sizes, the union can act as an adapter between the pump's intake and discharge connections. The union has a threaded (FNPT) connection to the pump and either a sweat (SWT) or a threaded (FNPT) connection to pipe. Unions must match the material of the pump and the system in which it is used, hydronic or potable. Hydronic pumps are made of cast iron. Potable pumps are made of stainless steel or bronze to prevent corrosion in systems that deliver drinkable water.