PEX pipe

Entry in PEX and plastic supply pipe · Updated October 29, 2026 · Written by the Know Your Pipes editors from the sources listed under References

PEX pipe is flexible water-supply tubing made from polyethylene whose molecular chains have been chemically linked, or cross-linked, so that it keeps its strength at hot-water temperatures. In the trade it is more accurately called PEX tubing, because its outside diameter follows copper tube sizes.^[1][3]

PEX was first used for radiant floor heating in Europe in the early 1970s and was introduced to the United States and Canada in 1984 for heating and plumbing.^[3] The Plastics Pipe Institute, citing Home Innovation Research Labs, reports that PEX use in residential plumbing grew from under 10% in 1997 to more than 60% as of 2016.^[3]

1 Manufacture and types

Cross-linking turns ordinary high-density polyethylene into a "semi-thermoset" material with better high-temperature strength, crack resistance and toughness.^[1] Three methods are common, and each gives its name to a type: the peroxide method (PEX-A), the silane method (PEX-B) and the electron-beam method (PEX-C).^[1] The Plastics Pipe Institute states that these letters are not a performance rating; tubing made by any of the three methods must meet the same product standards.^[1]

2 Sizes and ratings

PEX tubing is made in nominal sizes from 1/4 in to 4 in, with an outside diameter 1/8 in larger than the nominal size, the same as copper tube.^[1] The wall is SDR 9, meaning the wall thickness is about one-ninth of the outside diameter, so every size carries the same pressure rating.^[1][3]

The 73°F and 180°F figures are the standard ratings; the 200°F rating applies only to products listed for it.^[1] One maker's submittal lists all three and notes that they are hydrostatic ratings; for domestic hot water and recirculation lines it limits service to 140°F at 80 psi under ASTM F2023.^[2] Tubing for drinking water also carries NSF/ANSI/CAN 61 health-effects certification, and the print line on such tubing often shows "NSF-pw".^[1][2]

3 Fittings and joining

PEX is joined with mechanical fittings rather than heat or glue. Each fitting system has its own standard: ASTM F1807 metal insert fittings with copper crimp rings, ASTM F2159 plastic insert fittings, ASTM F2098 stainless steel clamp rings, ASTM F1960 cold-expansion fittings with PEX rings, ASTM F3347 and F3348 press fittings, and ASSE 1061 push-fit fittings.^[1][3] Not every tubing maker approves every fitting. Each maker tests its tubing with chosen fitting systems and prints those standard numbers on the tubing, so the print line shows which fittings are allowed.^[1]

For an emergency repair, a push-fit fitting is the one connection a homeowner may reasonably make, provided both the fitting's instructions and local code permit that use. Permanent work is normally done by a licensed plumber. Supply houses sell PEX by size, color and coil length; PlumbingSupplyAndMore's potable PEX pipe category (PlumbingSupplyAndMore has the same owners as this reference) is one example.

4 Limits

Sunlight. Ultraviolet light shortens the life of PEX, so it should not be stored outdoors or installed where it stays exposed to direct, reflected or diffused daylight; buried tubing is the exception.^[4] Each maker sets a maximum exposure time based on ASTM F2657 testing.^[4] Uponor, for example, gives its white tubing a one-month UV rating and its red and blue tubing six months.^[5] See sunlight and UV limits.

Heat sources. The installation guide published by the Plastics Pipe Institute and the Plastic Pipe and Fittings Association says PEX should stay at least 6 in from a gas or oil water heater's flue, and suggests a metal water heater connector at least 18 in long, also for electric tank heaters.^[4] Heat over 180°F, such as from soldering a nearby copper fitting, can damage the tubing.^[4]

Freezing. PEX stays flexible at temperatures as low as -40°F and can expand when water inside it freezes, then return to size after thawing.^[3][4] That does not make it freeze-proof: tubing frozen inside a slab or packed soil may not expand evenly, and the same guide says systems should not be allowed to freeze.^[3][4] More is in frozen pipes by material.

5 Code status

PEX plumbing is recognized by the International Residential Code, International Plumbing Code, Uniform Plumbing Code and National Standard Plumbing Code.^[1] Which code applies, and with what local changes, is set by each state or city, so the local building department has the final word; see plumbing codes.