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Plumbing glossary

PEX (Cross-Linked Polyethylene)

Reference photograph: PEX (Cross-Linked Polyethylene) (Flexible plastic plumbing pipe that has largely replaced copper and galvanized s).

PEX (cross-linked polyethylene) is a flexible plastic pipe used for residential water supply lines. It has largely replaced copper and galvanized steel in new construction and repipes since 2000 due to lower cost, faster installation, freeze resistance, and corrosion immunity.

PEX comes in three types:

  • PEX-A — most flexible, allows expansion fittings; freezes-and-thaws best without bursting
  • PEX-B — slightly less flexible; uses crimp or clamp fittings; most common in residential
  • PEX-C — least flexible; less common

Advantages over copper: 30–50% lower-cost materials, 50% faster installation (fewer fittings), expands ~3% before bursting (vs copper which bursts immediately when frozen), no corrosion in hard water or acidic water.

Disadvantages: degrades in UV light (must be inside walls), can taste plastic for first few weeks of use, some early PEX-A fittings (Kitec) had high failure rates and triggered class-action lawsuits — modern fittings are reliable.

Repipe cost in 2026: $4,500–$12,000 for a 2-bath PEX repipe, vs $7,500–$18,000 for the same scope in copper.

Sources

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