Soft soldering of copper pipes

For domestic water and central heating, copper pipes are normally joined by soft soldering, where the working temperature is below 450 °C. Soft solder for drinking water is typically a tin-based solder wire (often with approx. 2 % silver). Brazing (above 450 °C, up to approx. 1000 °C) is only used for more demanding joints and is a different process. The copper pipe must be intended for the installation (DS/EN 1057).

Cut the pipe with a copper pipe cutter so that the cut is clean and square. A pipe cutter gives a better cut than a hacksaw and does not leave swarf inside the pipe that can lodge in the system.

Remove the burr inside with a deburring brush/tool and outside, so the pipe end is clean. Burrs disturb the flow and can cause turbulence and noise. An uneven edge also prevents the pipe from sliding fully home in the fitting.

Clean the pipe end and the fitting's socket bright with steel wool or abrasive cloth, and wipe with a degreasing sponge. Solder only bonds to clean, bright and grease-free copper — oxide and grease prevent the solder from flowing.

Apply an even layer of soldering paste (flux) all the way round the pipe end. The flux cleans the surface during heating, prevents new oxidation and makes the solder flow into the gap. Do not apply more than necessary — excess flux must be wiped off afterwards.

Push the pipe fully home in the fitting and turn it slightly so the flux is spread evenly throughout the joint. Check that the parts are positioned correctly and in the desired direction before you heat — it is difficult to adjust afterwards.

Heat the joint with a gas torch until it reaches at least approx. 250 °C, so the solder melts on contact. Heat evenly all round the fitting, not just in one spot. Avoid overheating: if the joint gets too hot, the flux burns off and the solder will not bond.

Do not overheat — once the flux has burnt off, the solder will not hold.

Hold the solder wire against the edge of the hot joint — not into the flame — so that it is the heat from the copper that melts the solder. Capillary action draws the solder into the gap all the way round. A continuous shiny ring of solder along the edge shows that the joint is filled.

Let the joint cool without moving or loading it while the solder solidifies. Then wipe off flux residues — flux residues are mildly corrosive and must not be left on the pipe. Check the joint for a full, even soldering.

Soldering is hot work. Place a non-combustible backing/board behind the joint, keep an extinguishing agent ready and follow the rules for hot work with a subsequent fire watch. Take particular care when soldering close to timber, insulation and concealed cavities.