How to Sweat a Copper Water Joint That Passes First Time

Why this matters

A sweat joint that leaks on test costs the drain-down, the dry-out and the re-heat, which on a finished ceiling is the better part of an hour for one fitting. A joint that holds on test and weeps a month later costs a ceiling. Almost every failed joint traces to one of four things done before the torch was lit: a crooked cut, a burr left in, tube that was not bright, or water in the line. This card owns the torch-soldering steps for the shelf. Which joint goes where, sweat versus press versus PEX, is owned by The Joining Method Standard SOP; alloys, fluxes and brazing are covered in Soldering + Brazing Reference.

How the joint actually works

A sweat joint is a capillary joint. The tube and the fitting socket are sized to leave a thin, even gap all the way around and all the way down. Heat the fitting to the solder's flow temperature, touch the wire to the socket face, and the molten solder is pulled into that gap by capillary action, filling it end to end. The strength and the seal both come from that filled gap, not from the fillet you see at the face; the visible ring is only the evidence that the gap filled.

Two consequences follow, and they explain every step below. Solder flows toward heat, so you heat the fitting and let it draw the solder in. And solder only wets clean, fluxed, hot copper, so anything that keeps any part of the gap dirty, cold or wet leaves a void, and a void that runs from the face to the stop is a leak path.

          flame on the fitting body,
          on this side of the joint
                    |
                    v
      +-------------------------+
      |   FITTING SOCKET        |
======|=========================|  <- tube end
TUBE  |   capillary gap, solder |     bottomed on
======|=========================|     socket stop
      +-------------------------+
      ^
      socket face: wire touched
      here, on the side away from
      the flame; a full ring at
      the face means the gap filled

Hot work and fume, before the torch

Know what is behind the joint. Look behind and above before you light: framing, insulation facing, wiring, plastic pipe, the back of a finished wall. Put a fire-resistant shield cloth between the joint and anything that can burn or melt. Heated PVC or CPVC gives off hydrogen chloride, so shield plastic pipe as well as wood.

Extinguisher at the joint, not in the truck, and on a commercial or occupied building, the hot work permit the owner requires. How to run that permit properly is Running a Hot Work Permit That Would Hold Up. OSHA's general-industry rule in 29 CFR 1910.252 requires a fire watch for at least half an hour after the work ends where a fire watch is called for; 29 CFR 1926.352 covers construction work. NFPA 51B, in the edition your jurisdiction or the building's insurer applies, sets permit and fire watch terms of its own. A shop default of an hour of watch, with a hand on the framing behind the joint before you leave, costs little on a service call.

Fume. Flux smoke is an irritant, and reheating an old joint in a house that predates lead-free solder can release lead fume from the old fillet. Work with air movement, keep your face out of the plume, and in a crawlspace or closet run a fan that pulls the smoke away from you. Wash hands before eating.

Burns and fuel. Gloves for handling fresh joints. Torch cylinders ride in an outside rack, never in the cab, and the torch valve is closed whenever the torch is down.

The steps, and what each one prevents

1. Get the line dry. Shut the water, open the lowest fixture or drain on that run and a high fixture to admit air, and let it drain. If water still creeps toward the joint, a vacuum on the line or a dissolvable plug made for the purpose holds it back. Why: water in the tube holds the joint below the solder's melting point, and the steam it makes blows solder out of the gap. Skip it and you see the classic wet-line joint: solder that balls up and rolls off, or a ring that looks complete over a gap full of steam voids.

2. Cut square. Use a tubing cutter with light pressure, a quarter turn tighter each revolution. Why: a crooked cut lets the tube touch the socket stop on one side only, so the gap is shorter on the other, and a cutter cranked down hard rolls the tube end inward. A hacksaw cut is acceptable only when dressed square with a file.

3. Ream the burr. Take the inside burr off with the cutter's reamer or a deburring tool. Why: the burr narrows the bore, and the turbulence it causes at the joint erodes copper over years, especially on a recirculating hot line. It also lets chips fall into the line.

4. Clean tube and socket to bright metal. Sand cloth on the tube end, out past the depth of the socket; a fitting brush in the socket. Stop when the copper is bright and even all the way around, and do not touch the cleaned surfaces with your fingers. Why: solder will not wet oxide, oil or old flux. A dull band on one side becomes a dry stripe in the gap.

5. Flux, thin and even. On potable lines the solder (ASTM B32) and flux (ASTM B813) must meet the federal Safe Drinking Water Act's lead-free limits, and the steps on this card follow ASTM B828 in substance. Lead-free, potable-listed flux on the tube end and a light coat in the socket, applied with a brush. Why both: the flux protects the cleaned copper from oxidizing while it heats and helps solder wet it. Too much flux runs into the line and can keep corroding the inside of the pipe; brush off the excess.

6. Assemble fully and support. Push the tube in until it bottoms on the socket stop, with a slight twist to spread the flux, and wipe the excess off the outside. Support both sides so nothing moves while the solder sets.

7. Heat the fitting, not the tube. Put the inner cone of the flame on the fitting body near the socket, moving it around the fitting so it heats evenly. On a fitting larger than 1 in, work the heat around rather than holding it in one place. Touch the wire to the socket face on the side away from the flame. When it melts on contact, the joint is ready; if it does not, take the wire away and keep heating. Why away from the flame: if the wire melts in the flame instead of on the copper, solder lands on a fitting that may not yet be hot enough to draw it in.

8. Feed until a full ring shows, then stop. Feed the wire along the socket face until a continuous bright ring shows all the way around. A common shop rule for 1/8 in wire is a length of solder about equal to the nominal pipe size, roughly 3/4 in of wire on a 3/4 in joint. Much more than that and the extra drips inside the pipe or onto the floor. Take the heat away as soon as the ring is complete. Overheating burns the flux to black, and solder then refuses to flow or runs out of the gap.

9. Wipe, and let it cool undisturbed. Once the solder has set, wipe the joint with a damp rag to remove flux residue. Do not move or quench the joint while the solder is still freezing; a joint jarred in that window can crack inside the gap and look perfect from outside.

10. Flush and test. Run water through the line to clear flux, then test.

A worked repair

The job. A pinhole in 3/4 in type L copper in a basement ceiling, joists 12 in above, the wood rim joist 4 ft away, and a 1/2 in CPVC line from a previous repair running 5 in beside the copper. The plan is to cut out about 10 in of tube and replace it with a new piece and two couplings: four socket joints. (Cutting out a pinhole and checking the rest of the pipe is its own card, How to Cut Out and Repair a Pinhole Leak in Copper.)

Drain. Main off, laundry sink at the low end opened, upstairs tub open to admit air. After ten minutes the line stops running at the sink, but the copper sags slightly between hangers, and a trickle still runs from the cut end on the downstream side. A dissolvable plug goes into the downstream tube before the coupling, and the trickle stops.

Preparation. Four square cuts, four reamed ends, the replacement tube and both cut ends sanded bright 1 in back, both couplings brushed, flux on all eight surfaces, assembly pushed to the stops. Shield cloth over the joist bottom and a second cloth folded between the copper and the CPVC, because the CPVC is close enough to soften from a torch held on the near coupling.

Soldering. About 3/4 in of 1/8 in lead-free wire per joint, 3 in for all four. On the first joint, the solder balled and did not draw in; the flux had gone brown before the copper reached temperature because the flame was held too far back. That joint was taken apart hot, recleaned, refluxed and redone, which is faster than chasing a doubtful joint after the line is full. The other three drew a full ring each.

Test. Line refilled with the upstairs tub open until it ran without air, the plug flushed out at the laundry sink, then the system held at its 64 psi static with every joint dry under a paper towel after 15 minutes. Fire watch: the joist bottom and the rim joist checked by hand at the end of the hour, cool.

How to read a finished joint

• Pass: a continuous, smooth, concave ring at every socket face, with no gap anywhere around it.
• Starved: a ring that stops partway round. The gap did not fill on that side, usually because it was dirty or cold there. Redo it.
• Balled: solder sitting on the surface in beads. The copper was too cold or wet. Redo it.
• Burned: black or crusted flux, solder that ran away. Overheated. Clean and redo.

A visual pass is not the test. The joint is accepted only when the line holds pressure, which for a repair means system pressure with every joint dry after an observation period, and for new work means the water supply test your adopted code sets, before anything is closed in.

What changes the method

An existing joint has to come apart nearby. Heat travels; wrap a wet rag around a joint you want to keep, a foot or so away, before you heat the new one.

A valve with soft seats. Open it and take out the cartridge or stem where the maker allows, or heat away from it, so the seats are not cooked.

Larger pipe, or a line that cannot be made dry. Above about 2 in, or on a line that will not stop weeping, press joints remove both problems. That choice is The Joining Method Standard SOP's, and the steps are How to Make a Copper Press Joint.

References

• ASTM B828 (making capillary joints by soldering copper tube and fittings), ASTM B813 (fluxes) and ASTM B32 (solder), with the lead-free limits for solder and flux in the federal Safe Drinking Water Act
• 29 CFR 1910.252 (general industry) and 29 CFR 1926.352 (construction) for hot work fire prevention, and NFPA 51B as adopted or required
• See related: Soldering + Brazing Reference and Running a Hot Work Permit That Would Hold Up
• See related: The Joining Method Standard SOP