Joining copper and galvanised steel pipe requires the use of a dielectric union. The dielectric union is a special union that prohibits the pipes from touching while they are being joined. When two different metals touch, the running water inside will produce a small electric current that over time will cause the metals to corrode and break down the joint, causing a leak.
Turn the water off to the pipes to be connected. Clean the threads of the end of the galvanised pipe with the rag and remove all moisture.
Wrap the threads on the end of the galvanised pipe with the pipe tape, starting at the end of pipe and working your back up the pipe until the threads are covered. Wrap the tape in a clockwise direction. Connect the threaded end of the dielectric union to the galvanised pipe and tighten it down, using one of the pipe wrenches.
Unscrew the copper slip end of the dielectric union from the bottom galvanised end and remove the rubber gaskets and set these aside for final assembly.
Sand the end of the copper pipe with the emery cloth to remove the outer coating and until the shiny copper shows through. Sand the inside of the copper slip end of the dielectric union.
Brush a coat of flux on the sanded end of the copper pipe and the inside of the slip end of the union. Insert the slip end of the union onto the end of the copper pipe.
Open the gas valve on the torch and light the gas to produce the flame. Heat the joint with the flame until you see the flux bubble and begin to melt. Touch the solder to the joint, opposite the flame, and work your way around the joint as the heat draws the solder into the joint and seals it. This process is called sweating a joint.
Replace the rubber gaskets into the union and thread the copper end of the union onto the galvanised end and tighten them down, using the pipe wrenches.
Turn the water on slowly to the pipes and check for leaks. Tighten the joint as needed to seal any potential leaks.
Remove the rubber gaskets from inside the union before you apply heat from the torch as this could damage the gaskets, causing a weak joint or a leak. Avoid using the open-flame torch around flammable materials as this could result in a fire.