PEX, Copper & CPVC

Notes

• Nominal size is a name, not a measurement, and PEX is the narrowest of them. ¾″ PEX has a 0.671″ bore against 0.785″ for the same size in Type L copper. Friction rises with the fifth power of diameter, so that 17% difference in bore becomes 1.89× the pressure loss.
• This is why a repipe in the same size can feel like a downgrade. Copper out, PEX in, same nominal sizes, and the shower at the far end is noticeably weaker. Nothing was done wrong — the pipe is simply narrower inside. On any run over about 40 feet, or on a branch feeding two fixtures, go up one size when switching to PEX.
• PEX wins on the things that are not flow. Continuous runs from a manifold mean almost no joints inside walls, and a joint you never made is a joint that never leaks. It survives a freeze that would split copper, it installs in a fraction of the time, and it does not tempt anyone to steal it out of a vacant house.
• Velocity here is 4 ft/s, comfortably inside the 8 ft/s ceiling used for cold water. Hot water is usually held to 5 ft/s, because erosion in copper accelerates sharply with temperature.
• Copper still has two arguments in its favour. It is rated for higher temperatures, which matters on a recirculation loop; and it is unaffected by UV, while PEX degrades in sunlight and must not be stored or run where daylight reaches it. Against that, copper needs a competent solderer and priced-in time — most of a copper repipe is labour.
• CPVC is the cheapest of the three and the one to think hardest about. The bore is close to PEX, so the flow story is the same, and it becomes brittle with age: a twenty-year-old CPVC line can crack when a fitting nearby is disturbed. It also will not tolerate many common thread sealants and lubricants, which is a surprisingly common cause of early failure.