I believe that any plumber would tell you that the key to successfully soldering or “sweating” a copper fitting is properly preparing the pipe and fitting for the soldering process. Cleaning the pipe and fitting is essential to get the solder to flow properly and insure that the pipe and fitting have been soldered together. I just refinished my bathroom and used the existing lines that brought the water up from the main to the bathroom. I replaced all the branches or “feeds” for the shower, toilet, and sink. When using existing pipe and new pipe together, cleaning and preparation are critical to insure good soldered fittings and joints. I’ll use my bathroom as an example.
The feed for my bathroom is 1/2? copper pipe but because my shower and toilet position would be slightly different, I decided to route my hot and cold pipes differently to accommodate the changes. I needed to put a 1/2′ copper tee on the cold feed so I could run the cold water for the shower and toilet. I turned the main off in my basement to stop the flow of water. Once I had the water off, I could sweat the fitting properly. If you are a novice at sweating pipe, you need to know that you will never get your fittings soldered properly as long as there is water in the pipe. Even if there is the sligtest amount, it will be enough to keep the copper from heating up to the right temperature. Using a tube cutter, I cut the existing copper pipe back to its desired length. One thing about tube cutters for beginners, gradually increase the pressure on the blade as you rotate it. Putting too much pressure all at once can cause your pipe to deform during the cut. Once the pipe is cut and clear of all water, it is time to prepare the pipe and fitting for the sweat. Existing pipe will usually have a darkish brown tarnished look where new pipe or fittings will usually be shiny.
Look at the existing pipe and make sure that there aren’t any burs on the end where the fitting will go. Use a small file to debur the pipe if necessary. Next take a piece of emery paper and work it around the pipe from the end to about an inch in until it is shinyand free from tarnish. I like to take a piece of clean cloth and just wipe the pipe off when I am through with the emery paper. This cleans any dirt or dust off of it. Next, take a 1/2 ” pipe brush and run it through the fitting. If you don’t have a pipe brush, you can use a piece of emery cloth. Apply flux around the end of the pipe and inside the fitting. Place the fitting on the end of the pipe until it slides all the way on. When sweating a tee, I like to prepare all three pipes and sweat them all so that I don’t have to reheat the pipe and disturb the parts of the fitting already finished.
Once you have your fitting in place, it is time to light your propane torch. Adjust the flame until it is a nice, steady blue flame. Remember that the solder will always flow in the direction of the heat so you neeed to place your torch in a position where the solder will flow evenly around the pipe and fitting and be drawn inward toward the heat. On a tee, you would want to place the torch in the center of the cross bar and heat from the bottom. That way you will heat all three pipes at once and the soldler will be drawn inward toward the heat bonding the pipes to the fitting. In the case of a 90 or 45 degree fitting, place the flame from the bottom on the curved part of the fitting. Once you have the flame on the fitting, you will notice the flux start to bubble. Keep watcching until the flux seems to dry up and the color of the pipe and fitting turn dull. Take your solder and start at the top of the pipe where the end of the fitting is. Touch the solder at this point and when it melts, keep feeding solder as you go around the pipe in one direction and stop at the bottom. Then come back to the top and do the same in the other direction. By placing the heat at the bottom and starting your solder at the top, you will fill in the area where the pipe goes into the fitting because the solder will flow toward the heat. Be sure to do this around all the pipes in the fitting. Once you are done, remove the heat. If you used to much solder, you will have a few seconds where the solder will still be wet. You can take a rag and gently wipe around the pipe and fitting to remove the excess solder and smooth it out. Now let the fitting and pipe cool down without disturbing them. Many solder joints become cracked because they are not allowed to cool properly. I recommend doing some practice in the garage before you attempt the real thing. If you know anyone who has done it before, why not ask them to come over for a beer and share their experience.