The solder/sweat method is a little more advanced than the cold weld method but it can be done really quickly and there is no curing time to slow down the process. Before covering this, a few preliminary notes are required:
1). Even though this method uses only a propane torch (which is not that hot by torch standards) it will produce enough heat as to where the flame or the copper heated with it can cause an extremely serious burn.
2). The heat from the torch will spread through copper amazingly fast – never hold any piece of copper that is being heated with bare hands or fingers or you will have first-hand knowledge of the burn previously mentioned.
3). Copper can retain heat for a very long time. Even after it might appear to be cooled down, it can still be more than hot enough to sear bare skin.
In order to add the fittings this way, the copper tubes and the fittings should be prepared in the same way as with the cold weld method (i.e. roughened and cleaned). Once the fittings and tubes are prepared, the core should be secured so that it will not be able to move or fall during the soldering. If you have one, a bench vise is a good way to keep the heatercore in place during this process but make sure to either pad the jaws of the vise or use only enough pressure to hold the heatercore in place. Over tightening a vise onto a heater will lead to nothing but trouble as it will damage the fins or the seams.
After the heatercore is in a fixed and stable position, the inside of the copper fitting and the outside of the tubing should be lightly coated with flux. If acid core solder is used, the flux layer is not necessary.
Sweating a Copper Joint
Next, the inside of the copper fitting should be pre-tinned. To tin the fitting, it should be held with a pair of pliers and heated with the torch to the point that solder with easily melt when touched to the inside wall of the fitting. Do not use the flame’s heat to melt the solder – get the metal hot enough to melt the solder when the flame is not touching it. A thin layer of solder should be added around the smooth, inner wall of the fitting where it will meet the surface of the copper tubing. Be sure not to get any of the solder in the threads of the fitting as it will likely make properly tightening the brass barb into the fitting very difficult, if not impossible, later on. The outside of the copper tube on the heatercore can also be tinned before attaching the fitting to insure a really solid and leak-free joint but this can also be achieved in another way that I will cover shortly.
To add the tinned fitting onto the smaller diameter copper tube, the fitting should be held with a pair of pliers near the tube and both the copper tube and the fitting should be heated with the torch until the solder inside the fitting and the solder on the tube melts. Once the solder is liquid, the fitting can be slid into place onto the tube and allowed to cool. To make certain that the joint is solid and 100% leak-free, an additional technique known as sweating can be used. While it is most effective when the inside of the fitting and the outer section of the tubes are pre-coated with a layer of flux, it can be done with acid-core solder as well. To sweat the joint, hold the fitting in place on the tube with pliers at a point as far from the joint as possible. Then use the torch to heat the fitting and pipe. Once it is hot enough to melt the solder, touch the end of the solder to the seam of the joint. The solder should flow freely and disappear into the joint. If it does not, heat the joint a little more until it does. Then, wipe the joint with a wet towel to clean and set the joint.
For the larger tube, the fitting normally has to be slipped inside of it. If it is overly loose, the copper tubing can be lightly crimped onto the fitting and then solder can be used to seal the seam and lock the fitting into place. While working on either tube with the torch, keep a close eye on the solder joint that holds the copper tube into the tank. Most of the time, fittings can be added onto the tubes without harming the tank-to-tube solder joint but occasionally it will melt. If it does just touch the point where the tube meets the tank with the solder and it should pull enough solder back into the joint to reseal it.
Although the below image is slightly blurred, the solder can be seen touching the edge of the joint right behind the fitting. Sorry for the picture quality but torching, sweating and attempted digital camera operation produces more of a circus act than a precise operation…Look! It’s Jo-Jo, the amazing torch-wielding, pipe-sweating, picture-taking, unicycle-riding bear! Never mind.
Once the soldering job is completed, give the heatercore enough time to cool down to room temperature before moving it. Moving the heatercore around while the solder is still hot can weaken or crack the solder joint. The worst-case scenario is likely a hairline crack in the solder as it might not leak today, tomorrow or even next week, but at some point in the future it will definitely leak. Once the heatercore has cooled, it can be cleaned up in the sink with an SOS pad or some soapy water and a brass or stainless steel brush to get rid of any remaining flux or other grime. While it is in the sink, give it another leak test to make certain that none of the seams or joints became unsound during the soldering process. Since the heatercore will be nice and clean after the SOS/brush bath, it would be a good time to add a coat or two of primer and paint to make it look as good as it will perform. :)