Step 1: Cut and Prepare the Pipe. The best way to cut copper pipe before soldering is to use a pipe cutter. By tightening the jaws of the cutter around the pipe, the sharp blade touches the metal and scratches as you rotate the cutter around the pipe and gradually tighten the clamps. In some cases, you have no choice but to cut the pipe with a hacksaw. When doing this, be sure to cut perpendicular to the length of the pipe. Use a deburring tool or file to remove burrs and debris from the metal, otherwise the pipe will not slip into the fitting.
Step 2: Set up the heat shield. If you are welding fittings to existing pipes. It is important to protect the wall or framing members behind the joint from the heat of the torch. The best way to do this is to use a piece of fire and heat resistant fabric, usually sold as a "heat shield" or "welding flame protector." If you don't have a real heat shield, a fiber cement board will work in a pinch. If you don't want to hold it while you work, attach the shield to the wall or support it in some way.
Step 3: Propagating the flux. Wipe the outside of the pipe with a rag. Clean the inside of the joint with a wire brush. Apply flux to the outside of the tube and the inside of the fitting. Flux is a type of solder paste that you can find next to solder at hardware stores. Its purpose is to remove oxides from the metal and ensure good copper/solder adhesion. Brushes for application usually come in a container.
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Step 4: Connect the pipes to the fittings and heat the connections. Slide the pipe into the fitting, light the propane torch and adjust the flame to extend about 2 inches. Keeping the flame on the connection, move it around the pipe to heat the metal evenly until the flux begins to smoke, pop, and boil.
Step 5: Apply Solder. Unwind a roll of solder to get a straight length of about a foot. Remove the heat and immediately touch the tip of the solder to the connection. If the metal reaches the proper temperature, the solder will melt and be drawn into the joint by capillary action. Solder will usually fill the entire gap without moving the coil, but move the coil around the pipe to make sure it's not harmful. If the solder doesn't melt, apply more heat to the joint, but don't overheat. Burning the flux will make it more difficult for the solder to wick into the joint.
Water Problems: When repairing existing water pipes, it can sometimes be difficult to get all the water out of the pipes before welding. It's important to do this, though, because when you use the torch, the water evaporates, keeping the temperature at 212 degrees Fahrenheit - not enough to melt the solder. Sometimes it helps to turn on the faucet in the water main and let the air help the water flow. Keep the faucet open while soldering to allow residual steam to escape. If the pipe is dripping continuously, try to support the end of the pipe. This will probably control the dripping long enough for you to sweat the soldered joints. If you can't move the pipe, inserting a paper towel or rag into the end to absorb water may help. Leave it there until you're ready to solder, and quickly complete the connection when you remove it. If you can't stop the dripping water, repairing with a compression fitting may be better than trying to heat weld the fitting.
