How to Make a PEX-A Cold Expansion Joint

Why this matters

A cold expansion joint is the most forgiving joint in the PEX family when it is made right and one of the hardest to inspect when it is not. There is nothing to gauge: the proof is that the fitting went in to its shoulder and the pipe shrank back down onto it. A joint where the fitting stopped an eighth of an inch short looks finished, holds a quick test, and leaves less grip and less seal than the maker designed. Cold weather makes all of it slower, and crews that test on their warm-weather schedule in a 30 F garage find their leaks the next morning.

How the joint holds

PEX-a pipe has shape memory. Stretch it while cold and it slowly returns toward its original diameter. The joint uses that: a ring of the same material goes over the pipe end, the expander stretches pipe and ring together, the fitting goes in while they are open, and both shrink back down over the fitting's barbs. The ring adds the clamping force, and the pipe's own recovery does the rest.

Three things follow. The fitting has to go in while the pipe is still open, within a few seconds of the last stroke. The ring has to sit at the pipe end, so the barbs under it are covered. And the joint is not finished when the fitting is in; it is finished when the pipe has recovered, which takes longer the colder the pipe is.

Only pipe, rings and fittings the pipe maker lists for this method belong in it. The tubing is made to ASTM F876 and fittings for this joint to ASTM F1960, and whether a particular pipe is listed for them is read off its print, per Reading PEX Pipe and Fitting Markings Before You Join. Crimp and clamp joints are a different system with their own card.

 1. RING on the pipe end, flush
          +------+
    ======| RING |   <- pipe end and ring
    PEX   +------+      edge line up
    ======|      |

 2. EXPANDER HEAD in; stroke, release,
    head turns a little, stroke again,
    until the maker's count is reached

 3. FITTING pushed in to its shoulder
          shoulder
              |+------+
    FITTING ==|| RING |======  PEX
    barbs     |+------+
              no gap at the shoulder

Hazards

The expander. Its head opens with real force and closes on anything in it. Fingers stay off the head and out of the pipe end. A battery tool runs on a trigger; set it down with the trigger guarded.

The cutter. PEX cutters have an open blade. Cut away from your other hand and close the cutter between cuts.

Warming in cold weather. Where the pipe maker allows warming, it is with a heat gun at the distance and duration they state, never an open flame, which scorches PEX and weakens it.

The steps

1. Cut square, and cut clean. A PEX cutter with a sharp blade, cut perpendicular. Why: the ring has to sit flush with the end all the way round. A skewed cut leaves one side of the pipe short, so on that side the ring overhangs nothing and the barbs under it are under less pipe. A dull blade crushes the pipe oval and the ring sits unevenly.

2. Slide the ring on, flush with the end. The ring's edge lines up with the pipe end, or overhangs it by no more than the maker allows. Some rings have a stop that sets this for you. Why: a ring set back from the end leaves the first barb clamped by pipe alone. Do not use a ring from a different system, even one that looks identical; ring thickness and material are part of the listing.

3. Expand in rotations. Put the expander head fully into the pipe, stroke, release, and let the tool rotate the head before the next stroke, or turn the tool by hand a little if it does not auto-rotate. Repeat to the tool maker's count for that size. Why rotate: expanding in one spot makes ridges in the pipe and an uneven opening. Why the count: too few strokes and the fitting will not go in; too many and the pipe recovers slowly and loosely. Keep the head lubricated as the tool maker says, and keep grease out of the pipe bore.

4. Insert the fitting fully, right away. Pull the head out and push the fitting into the pipe until the ring meets the fitting's shoulder. A small gap between ring and shoulder is acceptable only within the maker's stated limit. Why right away: the pipe starts shrinking the moment the head comes out. A fitting started after a pause meets a pipe that is already gripping.

5. Hold it still. Hold the fitting in place for a few seconds while the pipe grips, so it does not push back out. Why: the barbs need pipe around them; a fitting nudged out of position during recovery sets short.

6. Wait for the recovery, then test. The maker gives a contraction time before the system can be pressurized, and it lengthens as temperature drops. The test is the one your adopted code requires (IPC 2021 Section 312.5 or the UPC 2021 test provisions, as adopted), before the walls close.

If the fitting will not go in fully: pull it straight back out before the pipe grips, give the pipe one or two more strokes, and push it home. Once the pipe has started to grip and the fitting is still short, cut the pipe off past the ring and start over with a new ring. A short joint does not get pushed home with a hammer.

Why cold changes the joint

Cold PEX is stiffer and recovers more slowly. Two consequences, one on each side of the insertion.

Before insertion, the ring resists. A cold ring takes more effort to expand and springs back faster when the head comes out. Many crews keep rings in an inside pocket or a warm bucket in the cab until they are used. The maker's count may need its upper end, and the insertion has to be faster.

After insertion, the recovery is slower. The pipe takes longer to shrink fully onto the fitting, and the maker's contraction chart reflects that. A test started too soon finds a joint that has not finished closing and may weep, or may hold at low pressure and leak later when pressure rises. Some makers permit a heat gun to speed the recovery within limits they specify; others want the joint left alone. That decision is theirs, and it is in the installation guide.

A worked case

The job. A new 1/2 in PEX-a home-run manifold in an unheated attached garage in January, 30 F on the thermometer, 16 joints: eight branch ends at the manifold ports and eight at stub-outs through the wall plate. Pipe, rings and fittings all from the one listed system.

Ring handling. Rings came out of a warm cooler in the cab in groups of four. The first ring tried straight from the cold truck bed had sprung back enough after expansion that the fitting stopped about 1/8 in short of the shoulder. Pulled straight back out at once, two more strokes, pushed home: ring against the shoulder. That ring stayed, since the pipe had not yet gripped; the rest came from the warm cooler.

The stroke count. The tool's count for 1/2 in pipe seated 15 of the 16 joints on the first insertion; the sixteenth was the cold ring above. Separately, on one stub-out the pipe end had been dragged across the slab and was slightly scored, so the cut was moved back 2 in to clean pipe before the ring went on.

The wait. The maker's chart for this pipe size at 30 F called for a wait several times its warm-weather figure. The crew did the last joint, finished the hanger work, and started the test only after the chart's time had passed from that last joint, not from the first. Air in the system on fill, purged from the highest outlet.

The result. All 16 joints were checked for ring-to-shoulder contact before the test, and all held at the code test pressure through the full test time with no weep. Count: 16 joints made, 15 seated on the first insertion, one seated after two more strokes, none cut out.

What it would have looked like on a summer schedule. Test started five minutes after the last joint at 30 F: the last few joints are still recovering. A slow weep at one of them on test, a joint cut out and remade, and a second wait. The hour saved by not waiting buys back the same hour, plus a wet slab.

Checking the finished joints

• Ring against the fitting shoulder on every joint, or within the maker's gap limit. Look at both sides of the joint; a ring can sit tight on one side and gapped on the other if the cut was skewed.
• Ring flush with the pipe end under it. A visible length of bare pipe between ring and shoulder means the ring was set back.
• No ridges or splits in the pipe just beyond the ring, which point at over-expansion or expansion in one spot.
• The test record with the start time, the temperature, the pressure and the time held.

What changes the method

Larger pipe. Sizes above 1 in usually need a heavier tool or a different head, and longer recovery. The steps are the same; read the tool maker's count for that size.

Repairs near an existing joint. You cannot re-expand an existing joint to take it apart cleanly; cut the pipe beside the old ring and make a new joint.

A threaded adapter on the fitting. Where the fitting carries a male or female thread for a valve or a stub-out ell, make up the thread before the expansion joint where you can, or hold the fitting body with a backup wrench while you tighten. Torque applied through the PEX twists the pipe on the barbs and can leave the ring sitting on a pipe that has rotated under it.

Polymer versus brass fittings. Both are made for this method. Brass fittings for potable water must be lead-free brass. Polymer fittings tolerate less bending load, so support the pipe close to the joint where it changes direction, especially at a stub-out that will carry a valve someone leans on.

No room for the tool. The expander needs straight access to the pipe end. In a tight joist bay, make the joint outside the bay on a loose length and then feed it into place, or leave the pipe long enough to pull the end out toward you. Expanding at an angle stretches the pipe unevenly, which is the same failure as skipping the rotation.

PEX-b or other pipe. Unless the pipe's print lists the cold expansion method, the joint is a crimp or clamp joint instead.

References

• ASTM F1960 (cold expansion fittings with PEX reinforcing rings) and ASTM F876 (PEX tubing), with the pipe and tool makers' installation instructions, which are part of the listing
• IPC 2021 or UPC 2021 water supply test provisions, as adopted by your jurisdiction
• See related: Reading PEX Pipe and Fitting Markings Before You Join
• See related: PEX Installation Procedure Reference and The Joining Method Standard SOP