Whether you're replacing a portion of your home's ageing plumbing with new copper plumbing, or you're just repairing galvanised piping with copper piping, good practice and the Plumbing Code dictate how you should connect copper piping to other types of metal piping. When connecting two different types of metal piping, you will likely need to install a "dielectric union." This coupling is designed to tightly fit two different pipes together while separating them with a washer. The washer keeps the different pipe materials from touching each other, which prevents corrosion. Installing a dielectric union to connect copper pipe to metal pipe can be accomplished in just a few steps.
Determine the pipe size that you are connecting to and purchase a Dielectric Union fitting to match. Dielectric unions are available at most hardware and hom improvement stores.
Screw the threaded portion of the dielectric union to the threaded end of the galvanised piping. Apply pipe thread sealant to the fitting and the pipe before screwing together.
Pepare the end of the copper pipe for a soldered connection and slip the washer and threaded ring that are part of the dielectric union onto the pipe. Slide them away from the end to keep it from heating up when you solder the end.
Solder the brass or copper flange that is part of the dielectric union onto the end of the copper pipe. Apply solder flux, heat with a torch, and solder the flange to end of the pipe with a common soldered connection.
Wipe the pipe clean and position the flanged end into the threaded coupling on the galvanised pipe. Position the washer that is part of the dielectric union between the flange and the coupling, slide the threaded ring over the flange and screw the two end together to complete the connection.