PEX-a & PE-RT tubing.

PEX-a · why not PEX-b or PEX-c

Cross-linked polyethylene tubing is made by three different processes — PEX-a (peroxide method, Engel process), PEX-b (silane method, Sioplas process), and PEX-c (electron-beam method). PEX-a is the highest cross-link density of the three (about 85%, vs 65–70% for PEX-b and 70–75% for PEX-c), which translates to better elasticity, better kink recovery (a PEX-a kink can be recovered with a heat gun; a PEX-b kink requires a cut and a fitting), better burst-pressure margin, and a longer published service life at elevated temperatures.

For radiant hydronic service in southwest Montana — where the tubing is buried in concrete, will operate for fifty years or more, and will see supply temperatures from 95°F (slab idle) to 150°F (snowmelt active-melt) — the PEX-a advantage compounds. We do not install PEX-b in slab radiant; we will install PEX-b on a domestic-water fixture pull-through if a homeowner's plumber has stocked it on a remodel job, but not as our spec.

Default · Uponor AquaPEX

Uponor is our default. AquaPEX is the brand name for Uponor's PEX-a tubing for plumbing applications; Wirsbo hePEX is their oxygen-barrier-equipped PEX-a for closed-loop hydronic service. SKUs we keep on the rack:

Oxygen-barrier is mandatory for closed hydronic loops in our service because dissolved oxygen at the boiler heat exchanger corrodes the stainless and shortens service life. The EVOH layer in Wirsbo hePEX meets DIN 4726 and reduces oxygen permeation to under 0.10 g/m³/day at operating temperature.

Premium · REHAU RAUPEX

REHAU RAUPEX is our premium pick when a homeowner specifies it or when the project's published warranty needs the extended 30-year coverage. Same PEX-a material family, slightly different cross-link profile, very slightly stiffer hand at the same diameter. Compatible with Uponor manifolds via threaded compression adapters (we keep a small inventory of these for the rare hybrid job).

PE-RT · where it belongs

Raised-temperature polyethylene (PE-RT) is a non-cross-linked polyethylene engineered for hydronic service at elevated temperature. It has slightly lower temperature/pressure rating than PEX-a (typically 180°F at 80 psi vs PEX-a's 180°F at 100 psi) and a different connection style (insert fittings or press, not expansion). PE-RT can be a useful supplement in two situations: short towel-warmer drops where the run length and the small diameter make expansion connections awkward; baseboard hookup tails where the tubing terminates at a fixed-position emitter and the connection style is determined by the emitter manufacturer.

We do not install PE-RT in slab radiant. PE-RT in continuous slab service is not, as of this writing, the industry standard, and the warranty position from the major manufacturers is less favorable.

Installation craft notes

PEX-a is forgiving but not infinitely forgiving. The five rules we drill on: (1) Never bend tighter than the published cold bend radius — kinks at the bend ruin the loop. (2) If a kink does happen, recover with a heat gun at 280–320°F until the kink restores to round, then cool slowly — do not cut and fit unless the kink will not recover. (3) UV exposure degrades PEX-a; tubing on the spool is fine, but a finished pull that is left exposed to sun for more than two weeks should be covered. (4) Connection style must be expansion (Uponor ProPEX) for all critical joints — no crimp rings in slab service. (5) Pressure test at 60 psi for 24 hours before the slab pour, period.

What we do with off-spec tubing on a retrofit

About 20% of our retrofit work involves systems where the previous installer used PEX-b, polybutylene (which fails — see the polybutylene class action settlement), copper, or galvanized iron. PEX-b in a working slab can be left in place if the homeowner is satisfied with its performance; we connect PEX-a expansion fittings to PEX-b crimp fittings via brass adapters at the manifold and proceed. Polybutylene we replace, full stop. Copper we keep where it's intact and silver-solder our connections.

Read across

Read the radiant floor scope for which tubing goes where; the snowmelt scope for the ⅝″ trunk runs; the manifold sheet for the connection style at the header.