How to Solder (Sweat) Copper Pipe
Soldering copper pipe is the process of joining two or more pieces of copper pipe together with a copper fitting. It is also known as sweating copper.
If you’ve ever soldered electronics before, the process is not too different. The biggest difference is the use of a propane torch (or the hotter burning MAP gas torch) instead of a soldering iron. Another difference is the preparation of the copper, including applying flux paste. A different type of solder wire is used as well.
First, you need the obvious: copper pipe or tubing, as well as the fittings to assemble them. Fittings are fairly cheap - often barely a dollar.
You will also need:
Sometimes the soldering wire and paste can be bought as a kit.
- Copper pipe cutter
- Emery cloth / sandpaper
- Propane torch and propane
- Measuring tape
- Marker (such as a Sharpie)
- Measure the length needed for the copper pipe.
- Measure the copper pipe.
- Mark where the cut will be with the Sharpie.
- Cut the pipe with the pipe cutter:
- Clean, roughen, and deburr the pipe with the emery cloth. Deburring is the process of removing burs, which are the little sharp bits that are often left on the edges when cutting metal.
- Clean and roughen the inside of the pipe fitting where the pipe will be inserted.
- Apply flux to the outside of the pipe. There are two schools of thought on how much flux to apply - essentially, one thought is that there is no such thing as too much flux, the other thinks there is a limit. In either case, the flux should cover all areas of the pipe that will be inside the fitting. Putting flux inside the fitting as well is not a bad idea.
- Insert the pipe into the fitting. The pipe should “bottom out” inside the fitting. Rotate the pipe or fitting while inserting to help spread the flux and make insertion easier.
- Prepare the solder wire by unwinding about 10 inches.
- Turn on the propane torch.
- Heat the pipe and fitting; do not use the solder yet.
- Once the soldering paste begins to sizzle (about 5 seconds) touch the solder to the joint on the pipe.
- The solder should melt and be “sucked” into the joint.
- Make sure to get the solder completely around the joint.
- Remove and turn off the flame.
- Gently clean the joint.
The below video depicts me completing steps 10 through 15:
One of the most common problems you’ll encounter when soldering / sweating copper is that the solder won’t take. What happens is the solder will melt, but as soon as it touches the pipe, it beads and solidifies. The reason for this is that there is water in the pipes. You CANNOT solder copper pipe if there is any water in the pipe. Sometimes a little moisture will burn off, but if there is even a trickle of water in the pipe - for instance, if an earlier valve doesn’t completely close - the solder will not take.
The solution is to prevent water from getting to where you are attempting to solder. An “old school” solution is to insert a piece of white bread into the pipe. The bread should dissolve over time. A more modern solution is to use dissolvable pipe plugs.
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