Copper pipe used in home plumbing systems is joined by soldering. Soldering, or “sweating”, as plumbers call it, creates a watertight seal at the fitting between two pipe lengths. The basic steps are- flux is applied to the outside of the copper pipe and the inside of the fitting; the fitting and pipe are then assembled and heated with a soldering torch and the solder is applied to the joint. It’s called sweating because tiny sweat-like droplets of hydrochloric acid are formed on the surface of the copper pipe when it is heated.

Safety warning: Soldering not be performed without a fire extinguisher within reach. Safety goggles and gloves are to be worn. Do not put the torch flame directly against combustible material such as wood framing or floor joists; use a flame shield.

Tools and Materials Required for Sweating Copper Pipe

Flux: soldering flux comes in paste form or liquid form. It is used to ensure the pipe is clean and that oxidation of the metal’s surface is prevented so that the solder forms a good bond. Make sure you the flux you use is lead-free flux. “Tinning” flux is also available; it contains powdered metal which makes soldering a little easier.

Solder: Solder is fusible metal alloy that comes in the form of thick, wire-like rod on spools. The use of lead in plumbing systems is under federal regulation, and solder must comply, particularly in regards to water supply systems. Lead-free solder has a higher melting point, and cools off faster, which means that you can work faster than when using the old 50% lead solder.

Torch: You can use either a propane torch or plumber's acetylene torch for your heat source. Handheld propane torches are the simplest, the kind with a disposable propane cylinder attached to a torch combustion tip. These will work for pipe of ½ inch or ¾ inch diameter, but they can be hard to use in confined areas. Propane torches are available with swirl-combustion torch tips (like the TurboTorch) that mount on the end of a hose connected to a propane tank; these types produce a higher temperature flame (around 1700 F) with better maneuverability. For pipe 1 inch diameter or larger, a plumber’s acetylene torch works best, although acetylene is dangerously explosive and needs to used with care.

Torch Lighter: Never use matches or a cigarette lighter to get the flame going on a torch, they produce too big of an ignition. Torch lighters are spring loaded flints mounted on a handle that you squeeze to produce a spark.

Safety Equipment: Have a working fire extinguisher nearby always. You might also need to solder pipes in place, adjacent to wood framing members. Pieces of scrap sheet metal can serve as a flame shield, slipped between the pipe and the wood frame or joist. You can also buy flame-resistant aluminum or woven glass blankets at plumbing supply stores for the same function. Protective goggles are also required.

Brush: For applying flux, use an acid brush.

Gloves: One pair of latex or thin leather gloves for getting a better grip when working with flux paste. One pair thick leather gloves for soldering.

Soldering

Joining rigid copper pipe is easier than soldering coiled pipe, but they are basically the same process, except coiled pipe ends may need to be brought back into round by a swaging tool. In either case, if you’ve never soldered before, practice on a few scrap pieces and fittings held in a vise before trying the real thing. When the solder cools down, cut the joined fitting and pipe in half lengthwise to check how far into the fitting joint the solder has flowed.

It is important to make sure the copper pipe has been cut properly. See the page on cutting copper pipe.

1. A good watertight joint requires that both metal surfaces being soldered be thoroughly clean. This is the secret to soldering. Using plumber’s emery cloth, sand the end of the pipe ¼ inch past the depth of the fitting, as well as the inside of the fitting, so that they are brightly polished. You can use a wire fitting brush for the inside of the fittings; they are specially made in sizes that match different fitting diameters.

2. Next put on some gloves and start applying the flux. Using the flux brush, paint a liberal coating of flux on the outside of the pipe, and a thinner coat on the inside of the fitting. Join the pieces together firmly, giving them a half-twist to ensure even distribution, then remove any excess flux from the outside with the flux brush.

3. Put on the goggles and heavy gloves. Light the torch. The idea is not to use the torch flame itself to melt the solder, but to heat up the copper pipe until it is hot enough to melt the solder. This can be done by applying the tip of the flame to the fitting socket and move it back and forth from the socket to just off the edge of the fitting onto the pipe, for around 20-30 seconds, depending on pipe size and torch type, until you see sweat beads appear on the pipe and fitting.

4. Once the metal is hot enough to melt the solder (if you're not sure, keep testing with the tip of your solder until it melts), pull the flame away to about 4 inches from the pipe. Move the tip of the solder around the edge of the fitting and start melting solder into the joint. It should melt easily and be drawn into the joint via capillary action. When the joint gap is completely filled, solder will well up around the joint and start to drip out the bottom. At that point, you can turn off the torch and let the join cool.

5. Lastly, wipe the join clean with an old dry rag. The goal is to clean away any remaining solder and flux, as it can corrode the pipe exterior and leave green unsightly streaks. Be careful not to disturb the joint by bumping or moving it in any way until the solder turns a hazy color from shiny silver. Once it has turned frosty-hazy, you can cool a joint quickly by quenching it in a bucket of water.

Photo by johncarljohnson, Creative Commons Attribution License