Soldering copper pipe is easy and fun. It used to be even easier, back when we used 50/50 tin/lead solder. Lead-based solder, however, is not at all kosher for drinking water supply systems any more, so instead you'll have to use the newer lead-free solders.

Before soldering, of course, you have to cut the pipe to fit, which is worth a small writeup of its own.

Like any soldering process, you will first have to heat the metal -- both the pipe and the fitting -- up to a temperature sufficient to melt the solder. Copper is both a good conductor and container of heat, and there's a lot of metal in copper pipe -- a lot more than an ordinary copper wire, anyway. So a soldering iron won't do; you're going to need a torch. For small pipe -- 1/2 or 3/4 inch -- a handheld propane torch will do, but for anything larger, you're going to want a plumber's acetylene torch. (And for really large pipe, say 2" or larger, you're going to need an extra-large tip for that plumber's acetylene torch.)

If you're repairing a leak, or adding new pipe to an existing line, step one is to make sure that the old pipe is empty of water and that there's no water coming in. Solder melts at 240 degrees (460 degrees F), but of course water boils at 100 degrees. If there's any water in the pipe, as long as it's boiling the temperature will stay at 100 degrees and the solder will never melt. Your torch can boil a small amount of water away in a reasonable time, but if there's any water trickling in your torch will almost never win the battle. So you're going to have to make sure the water is shut off completely. (There are numerous tricks and techniques for doing this, and for overcoming a lack of a shutoff valve where you need one, or a shutoff valve that doesn't completely shut off.)

A good solder joint (which of course for our purposes must be good enough that it will eventually hold the water in, under pressure) demands that the metal surfaces being soldered be scrupulously clean. So you need to sand both the pipe end and the inside of the fitting until they're shiny bright. (If the pipe and fittings are new this will be easy, but if they've been lying around for a while and have oxidized, the sanding process may take some elbow grease.) For sanding the outside of the pipe, it's hard to beat plumber's emery cloth. For the inside of the fittings, you can either use a scrap of the same emery cloth wrapped around your finger, or if you'll be doing more than a bit of this and don't want to chew up your fingers, spend a few bucks and get a couple of the wire brushes which are specifically made for this task. (They come in sizes to match the fittings. I keep a 1/2" and 3/4" brush in my plumbing kit.)

The next step is to coat both the outside of the pipe and the inside of the fitting with a thin layer of soldering flux. The flux serves several purposes: most importantly it reduces any remaining oxides or impurities which were too small for you to sand away (we're talking about the molecular level here). Also it will react slightly with the molten solder, removing impurities from it and helping it flow more smoothly.

Use a flux that's designed for the solder you're using: it'll be "lead-free" flux, and the kind I use also contains a bit of finely powdered metal, so that it "tins" the pipe as it heats up, and this seems to make the soldering easier, so I recommend it. Paint the flux on with a flux brush. (Don't overdo it, though; if you spread on huge gobs of the stuff, it'll either run out and make a mess or pool inside the pipe and maybe make the water taste bad for a while.) You might want to give the assembled joint a quarter turn to make sure that the flux is evenly distributed.

When you've got your joint (or perhaps several at once) sanded, fluxed, and assembled, it's time to fire up the torch and actually start soldering.

Copper, as I mentioned, is an excellent conductor of heat, so you can basically get by with applying the torch to a single point on the fitting or on the pipe just next to it, and leaving it there for a while. But it will take a bit of time (10-20 seconds, depending on the size of the pipe and the size of your torch, etc.) for it to heat up to soldering temperature, and you've got nothing else to do while you're waiting, so unless you're working in a confined space, you might as well play the torch all over the joint. (This won't actually get the joint heated any faster, but it will at least feel like you're doing something.)

The flux will melt almost immediately (and if you used too much it will begin dribbling out of the joint and making a mess). As it approaches soldering temperature, it will begin to smoke slightly. If it's tinning flux, you'll begin to see streaks of gray on the exposed bits of shiny copper near the joint. With experience you'll get a feel for how long it takes to heat a joint up, but whenever you're not sure, you can keep testing with the tip of your solder.

The goal, as I mentioned, is to get the metal hot enough to melt the solder. You're not using the torch to melt the solder; you're using the torch to heat the metal, and the hot metal to melt the solder. You must melt the solder on the metal, otherwise you can't be sure that the metal was hot enough, and if it isn't, you'll end up with a "cold joint", meaning that the joint will leak. (Perhaps not right away, but a couple of weeks later, after you've got the wall replastered and the carpet laid and everything.)

Although it's vital to get the joint hot enough, you also have to be careful not to get it too hot. If you leave the torch on the joint for too long before soldering, all the flux will burn up or melt away, and the too-hot metal will begin to oxidize again, and you'll have a dirty joint, full of burned flux crud, which the solder won't flow into nicely and certainly won't bond well. If the joint starts smoking a lot, or if you've got lots of black streaks where there used to be puddles of molten flux, the joint is too hot. Sometimes you can rescue it by applying some fresh flux, but in extreme cases you'd have to stop, cool the joint down, re-sand everything, and start over.

So until you've got a feel for the heat, as the joint heats up, test it frequently with the tip of your solder. As the joint reaches soldering temperature, you'll move the torch well aside and start melting solder into the joint. When it's truly hot enough, the solder will melt easily: pressing a length of solder against the joint will feel like pressing a crayon against a hot griddle, or something. Capillary action will draw the molten solder into the joint. You can get away with applying the solder to a single point, and it will flow in and completely surround the joint, though again, if you want to feel like you're doing something, you can run it all around the joint (or as much of it as you can reach).

You'll know the joint is "full" when you can see the solder just welling up around the entire circumference of the joint. Again, don't overdo it -- it's possible to keep piling the solder in, but the excess will pool inside the pipe, making a bump which will impede the water flow (or, back in 50/50 solder days, provide a nice timed-release lead source for the building's inhabitants).

The last step is to wipe the joint. With an old, old rag (which you're going to throw away and never use again), perhaps moistened slightly, folded to several thicknesses to protect your hand from this very hot pipe, quickly wipe the excess solder and flux away. Your goal is a bright, clean, shiny joint, with clean copper on both sides and a little band of silver-gray right at the joint, with no big blobs of solder or streaks of flux. You might imagine you can skip this step, because an unwiped joint doesn't look too bad at first, either, but a few weeks later, the residual flux will have corroded the exterior of the pipe and left ugly green streaks. (Also the greasy flux residue will be impossible to paint, if it matters.) So do take the extra few seconds to wipe the joint clean.

If you're using a wet rag, it may cool the joint too quickly, leaving a pebbly or blobby texture to the exposed solder. If this happens, just play your torch briefly over the joint again to remelt and smooth the solder. (And yes, the appearance does matter; our mascot here is "The Great Gonzo -- Plumbing Artist!".)

You'll want to make both joints on the same fitting (or all three joints, if it's a tee) in succession, because the heat you put into one joint will be almost enough for the nearby ones. If you've got several joints with short lengths of pipe between, you'll want to do all of them at once. But when you're done, and don't need any of that heat any more, you can either wipe the whole joint with a very wet rag, or spray water on it with a little spray bottle, or if you're working with an isolated subassembly, drop the whole thing in a bucket of water, or hold it under a faucet. It will make satisfying scorching and boiling sounds in any case, and once cooled, you won't burn yourself on it as you work on your next steps. (Beware: the glassblower's lament applies!)

If you pay attention and follow these instructions, you can get a good joint, every time. With one exception, I have never had a soldered copper pipe joint leak after I turned the water on. (The exception was some two inch pipe in a forced hot water heating system, which I simply couldn't get hot enough with the torch I was using. After three unsuccessful attempts, requiring tediously re-draining the system each time, I walked over to the friendly neighborhood welding supply store, and got an extra-large tip for the torch, which made all the difference.)

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