It is important to remember that the prep work is as important as the actual soldering of the pipe joint. More on that after a couple of important housekeeping items. To wit:
We would be remiss if, in this discussion, we didn’t stress the importance work safety and the use of PPE (Personal Protective Equipment) -namely, safety glasses and gloves. In an industrial construction and/or maintenance environment under the jurisdiction of OSHA regulations, the use of safety glasses and gloves would be a requirement. Of course, it’s unlikely anyone would challenge you while working in your own home. It is up to you. Common sense would mandate, in my opinion, that working with molten metal would compel someone to reach for the safety glasses and gloves. I know it’s not always the case. Just remember -your eyes need to last you a long time. Your quality of life and lifestyle would be unalterably changed forever if you suffered the loss of an eye. Eye injuries, along with hand injuries constitute the lions share of all injuries recorded by OSHA. All injuries are preventable.
Protection of Property
Whether you are running a new 1/2″ copper line for a new garden hose connection or a new 3/4″ line to replace your old galvanized piping through the basement, or perhaps repairing a leaking fitting -you will want to plan the routing of the line in advance to locate all your fittings (couplings, elbows, etc.) where you can apply heat without risk of igniting your wood framing, wall -or, what have you. We are working with an open flame here and many a house or other structure has burned as a result of soldering copper piping in too close proximity to the building structure.
Always have a properly sized fire extinguisher on hand. After use of the torch, continue to monitor the area for at least 30 minutes in case something is smoldering and is not immediately apparent. Have some rags on hand and a bucket with water to cool and/or smother any hot components or structural members.
One other minor point -we talk about ‘sweating a joint” or “sweating copper pipe.” It is the same as “soldering copper pipe.” Sweating and soldering -both references mean the same thing. To explain, for the benefit of the uninitiated, copper piping is joined with fittings of different configurations. A ‘coupling’ joins two straight sections, an elbow is used to fashion a 90 degree angle. There are other configurations -a 45 degree, for example, that enables one to route the pipe just about wherever you need to go, as well as fittings which allow the transition to a male or female threaded connection (threaded adapter).
Please note that a good portion of any trade is learning what hardware or parts and pieces are available to accomplish what needs to be accomplished, as well as knowing what specialty tools are available. If you find yourself challenged to figure out how to get your line from point A to point B, ask your local plumbing supply counter man. They are generally very knowledgeable and helpful in that regard. As it is impossible to know what you do not know, ask the question -and remember -there is no stupid question. Also, note that the most knowledgeable help and widest variety of tools and fittings will come from a plumbing supply house -not the big box store. There may be exceptions but generally speaking, it is true.
The fittings are used to join sections of pipe. Solder is an alloy used as a filler metal to unite two pieces of (in this case) copper together. The solder has a relatively low melting point -lower than the copper and so, enables the fusing of the metal parts together without bringing the base metal (copper) to the melting point. The fitting is heated until it is sufficiently hot enough to melt the solder -so, when the solder is introduced to the fitting/ pipe joint, the molten solder will be ‘sucked’ into the space between the fitting and the pipe -by “capillary Action.” If you recall your physics class, capillary action is the same phenomenon that causes water to be absorbed by a sponge.
Plumbing design and installation is governed by national, state and local codes. The work you do must be in compliance with the applicable code having jurisdiction. It is the homeowners responsibility to be informed about such matters. Some areas require plumbers to be licensed.
Back to the Prep Work
The fitting must be cleaned thoroughly with a wire brush made for the purpose (specialty tool -get one, they’re cheap). Accordingly, the end of the pipe to be joined (the portion of the pipe to be inserted into the fitting) must be cleaned as well. Use emery cloth or ‘plumbers roll’, as it is sometimes called. Fine sandpaper will do in a pinch but plumbers roll holds up better, especially if it gets wet. You cannot over-clean the copper. If it is not clean the solder may not bond to the copper and may leak -creating re-work. It then must be re-heated, taken apart, cleaned and sweated all over again. Who wants to do that?
After you clean the parts thoroughly, apply flux paste liberally to both pieces. Fit the pieces together and get the assembly in a position to heat it safely. Ideally, it should be self supported. Clean off the excess flux with a rag.
Unroll a short (4-6″) section of solder and bend about a 2 inch ‘hook’ in the end of it so you’ll be able to reach around to the far side. (Note: If there is no room for you to get around on all sides of the fitting, there is a side you will not be able to see. The hook will enable you to apply solder to the side you cannot see.)
CAUTION: DO NOT ever position your head and face beneath a fitting you are sweating (even if wearing safety glasses). Dripping molten solder causes serious burns on your skin.
After donning your safety glasses, apply heat to the fitting -NOT to the pipe. Test by touching the solder to the fitting to see if it is hot enough to melt the solder. The solder will begin to be ‘drawn up’ (remember capillary action?) into the fitting. Apply solder all around the fitting until it is ‘capped off.’ Do not overheat the fitting. Once the solder is flowing, pull the heat away. Re-apply heat as necessary -only enough to get the solder to flow. Then remove the heat. Wipe the excess solder off of the joint with a rag.
Horizontal and vertical joints are both accomplished using the same approach. Heat the fitting -not the pipe. Bend a hook in the solder so it can be applied to the side of the fitting hidden from view. Apply the solder to the far side and it will be drawn around to the heat.
Some other references from some respected sources: