PEX: Crimp and Expansion

What PEX is good at

PEX is crosslinked polyethylene: polyethylene whose molecules are chemically linked into a network, which makes it tough and heat-resistant enough for hot water. The three ways of crosslinking give PEX-A (peroxide), PEX-B (silane), and PEX-C (radiation) ¹.

What it does better than copper ¹:

• Bends around obstructions, so long runs have few fittings and fewer places to leak.
• Quiet. Copper is about 180 times stiffer than PEX; the flexible tube absorbs the pressure surge when a valve slams shut, so water hammer is rare.
• Freeze tolerance. Water-filled PEX can freeze and swell without splitting when it can expand along its length, as in walls and ceilings; in a slab it may not. That’s tolerance, not permission.
• No flame, no solvent. Joints are mechanical.

Its weaknesses: sunlight degrades it, and it can’t be used right at a water heater’s connections under the model code the guide cites ¹.

Three ways to join it

Crimp (ASTM F1807). Slide a copper crimp ring onto the pipe, push the pipe over a barbed brass or plastic insert fitting, center the ring over the barbs, and squeeze it with a crimp tool. Check every crimp with a go/no-go gauge ¹. A variant uses a stainless clamp (cinch) ring and a ratcheting tool that only releases once the clamp is complete.

Cold expansion (ASTM F1960). Put a PEX reinforcing ring on the pipe end, stretch pipe and ring together with an expander tool, and push in the fitting. The pipe’s “memory” shrinks it back down onto the fitting ¹. Expansion fittings are mostly used with PEX-A; check what your tubing is listed for.

Push-fit fittings seal with an O-ring and hold with a stainless grab ring, the same design used for copper: cut square, deburr, and push the tube in to its full insertion depth. Removable types release with a clip ². They’re the go-to for a quick repair.

Not every fitting system is approved with every PEX pipe, so use the one your tubing is listed for. Wisconsin’s rule is short: PEX joints must be made per the manufacturer’s instructions ³.

Installing it

From the design guide ¹:

• Sunlight. Don’t store PEX outdoors; keep it in its packaging until use. Where it stays exposed to sunlight after installation, even indirect light in an unfinished basement, cover it with a UV-blocking sleeve or insulation.
• Slack. Don’t pull runs tight. Leave 1/8 inch of slack per foot so the tube doesn’t stress fittings when it cools and contracts.
• Support. Strap horizontal runs every 32 inches (3/8 to 1 inch sizes) and vertical runs at every floor with a mid-floor guide. Strap at changes of direction so the bend doesn’t pull on a fitting. Sleeve holes through metal studs.
• Water heaters. The model code quoted in the guide keeps PEX out of the first 18 inches of piping connected to a water heater; run copper or a listed connector there.
• Layouts. A home-run system runs a separate line from a central manifold to each fixture; a remote manifold system feeds small manifolds near fixture groups. Both cut fittings and let each line be shut off at the manifold.

What to take from this

PEX bends, stays quiet, and tolerates freezing better than copper, but hates sunlight. Join it by copper crimp ring checked with a go/no-go gauge (F1807), cold expansion with a PEX ring for PEX-A (F1960), or push-fit, using the system your tubing is listed for, as Wisconsin requires. Sleeve exposed runs, leave 1/8 inch slack per foot, strap every 32 inches, and keep it out of the first 18 inches off a water heater.