Sweat Fitting

A sweat fitting is any coupling, elbow, tee, or other connector designed to accept copper tubing that gets permanently sealed by melting solder into the gap between the fitting's interior and the tube's exterior. You clean both surfaces with flux and emery cloth, apply heat with a torch until the joint reaches solder temperature (around 700°F for lead-free), then feed solder into the joint where capillary action pulls it through the gap. The solder cools and hardens, creating a watertight seal.

Sweat fittings are the standard in residential and light commercial work because they're faster and cheaper than threaded connections, create no stress points, and require no wrench damage. The catch is that you need propane or acetylene and a competent hand—bad technique produces leaks that show up later in the wall. Lead-free solder is now code-required everywhere, and it's touchier than the old 50-50 tin-lead stuff: it needs hotter heat and sometimes a few seconds longer to flow smoothly.

Always use solder-joint fittings in runs where you know you'll need to disassemble and re-solder later. Once you sweat a joint, it's permanent without cutting the line or applying heat again. For isolation valves or anywhere you might want a shutoff without tools, thread adapters into sweat fittings instead.