sweat

Sweating is the process of joining copper pipe using solder and a flame, typically a propane torch. The solder melts and flows into the joint by capillary action, creating a permanent seal. It is the standard method for joining hard copper tube in water supply and small refrigeration lines where threaded or compression fittings are not practical or desired.

The joint is prepared by cleaning both the inside of the fitting and the outside of the pipe with abrasive cloth or a wire brush, then applying a thin coat of flux. The torch heats the fitting and pipe evenly until they reach about 700 degrees Fahrenheit. When the solder (typically a 50/50 or 95/5 lead-free alloy) touches the joint, it melts and is drawn into the gap by capillary action, forming a strong bond as it cools.

Common problems arise from inadequate cleaning, insufficient heat, or moving the joint while it cools. A dry joint forms when solder was applied directly to the flame rather than the pipe, creating a weak crystalline surface that looks shiny but lacks strength. Overheating can weaken the solder or damage nearby components. On vertical runs, solder can drip away from the joint if too much is applied or if heat is applied from below.

The term reflects what happens visually: moisture on the pipe surface turns to steam and rises off the fitting, resembling sweat. The word entered plumbing vocabulary because the phenomenon was noticeable to early plumbers working with torch and solder.

Sweating is faster and cheaper than compression fittings for new installations but requires skill and practice. It is forbidden in many jurisdictions for gas lines due to joint reliability concerns. Modern alternatives like press fittings have reduced sweat work in commercial settings, but it remains the default method for residential copper water lines and is a core competency in plumbing apprenticeships.