Not all PEX is the same. Here's the technical difference between the three types, why PEX-A is the right replacement material for Poly B, and what the expansion fitting system means for joint integrity.
The Three Types of PEX
PEX stands for cross-linked polyethylene. All three types — PEX-A, PEX-B, and PEX-C — are made from polyethylene with cross-links added to the molecular chain to improve performance over regular PE pipe. The letter designation tells you how those cross-links were created, and that matters for real-world performance.
PEX-A uses the Engel method: cross-linking happens during the extrusion process itself, while the material is above its melting point. This produces the most uniform cross-link distribution in the finished pipe — the ASTM standard requires a minimum 70% cross-link density for PEX-A, and good PEX-A manufacturers achieve 75–80%.
PEX-B uses the silane method: cross-linking happens after extrusion, through moisture exposure. It's cheaper to produce, which is why it's common in big-box retail and lower-cost plumbing jobs. Cross-link density is lower, typically 65–70%.
PEX-C uses electron beam irradiation after extrusion. It's the least flexible of the three and is rarely used in residential repipe applications.
- PEX-A: cross-linked during extrusion (Engel method) — most uniform, most flexible
- PEX-B: cross-linked after extrusion (silane method) — cheaper, less flexible
- PEX-C: cross-linked by electron beam — stiff, rarely used in residential work
- All three are immune to chlorine oxidation — the failure mode that kills Poly B
- PEX-A has the highest cross-link density: typically 75–80%
Why Flexibility Matters for a Repipe
In a repipe, the installer is routing pipe through existing framing — around corners, through wall plates, along joists. Flexibility directly affects how cleanly this can be done. PEX-A can be bent to a much tighter radius without kinking than PEX-B. Where PEX-B would require an elbow fitting at a turn, PEX-A can often be bent by hand — reducing the number of fittings in the system and therefore the number of potential failure points.
PEX-A's flexibility also improves freeze performance. If a PEX-A pipe freezes and the ice expands, the pipe can expand with it and then contract back to its original shape when thawed — a property called elastic memory. PEX-B doesn't have this to the same degree. In BC's climate, freezing is relatively rare but not impossible, particularly in crawl spaces during cold snaps.
The Expansion Fitting Advantage
The other major reason we use PEX-A is the fitting system. PEX-A uses expansion fittings rather than crimp or clamp fittings. Here's how it works: the pipe end is expanded using a special tool (a PEX expansion tool), a ring is slid over the expanded end, and the fitting is inserted. The pipe's elastic memory causes it to contract back around the fitting — and as it does, it grips the fitting more tightly. The joint gets stronger over time as the material tries to return to its original diameter.
Crimp and clamp fittings (used with PEX-B) work differently: a metal ring or clamp is compressed around the outside of the pipe to hold it on the fitting. These joints can loosen over time as the material relaxes. They're perfectly acceptable for many applications, but they're not the strongest method.
For a Poly B replacement — where the whole point is building a plumbing system that will last another 50 years without surprises — we want the strongest possible joint method. PEX-A with expansion fittings is that.
- Expansion fittings: pipe expands over the fitting and contracts back — getting tighter over time
- Crimp/clamp fittings: external compression — adequate, but can relax over decades
- Expansion method creates the strongest joint in residential plumbing
- Fewer fittings needed overall — PEX-A's flexibility reduces elbow count
- All major BC insurers accept PEX-A as the Poly B replacement standard
Colour Coding and Labelling
We run red PEX-A for hot water lines and blue for cold — consistently throughout the job. Every connection point is labelled. This makes the system readable for any plumber who works in the home in the future — they can immediately identify hot from cold without tracing lines. It costs nothing extra and is good practice on every repipe we do.