What is the difference between Poly B and PEX?
The straight answer
They are different plastics. Poly B (polybutylene, 1978 to 1998) degrades from chlorine in treated water and is no longer an accepted material. PEX (cross-linked polyethylene, late 1990s to today) is engineered against that chemistry, certified under CSA B137.5, and is the standard Poly B replacement in Canada.
The longer version
They look alike and they are not alike. Poly B failed for a specific reason: chlorine in treated water oxidizes polybutylene from the inside until it cracks. PEX is cross-linked polyethylene, a different plastic built and tested against that exact attack, and it goes in with brass or poly fittings rather than the acetal parts behind Poly B’s worst record. The materials chapter explains why PEX is not “another Poly B.”
Telling them apart: Poly B is usually grey and printed PB2110 or CSA-B137.8, with copper-coloured crimp rings. PEX is usually red, blue or white, printed CSA B137.5 with a CL1 to CL5 rating, and joined with black crimp rings or expansion rings. Some PEX is grey too, so read the print, not the colour. The identification chart puts them side by side with real photos.
The two can be joined during a repair, but only with fittings certified for polybutylene. Ordinary PEX push fittings and crimp rings are not interchangeable with Poly B ones. The fittings guide lists the rated options.
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Grounding
This answer summarizes material sourced in full on the chapter pages it links. If the facts change, the chapter changes first and this answer follows.
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