Which PEX to use?
The three types of PEX, PEXa, PEXb, and PEXc offer different levels of flexibility for different applications.
I’m interested in changing my plumbing to PEX. The previous homeowner made repairs to the home’s original galvanized plumbing with copper, CPVC, and PEX. I would really prefer the entire system be the same and intend on doing the work myself. PEX seems like the best option for me, but I don’t know which of the three types—a, b, or c—will work best. I’m concerned about chemical leaching, stiffness, and UV light breaking down the pipe.
—Nate Forsyth via email
Max Rohr, academy manager at REHAU, replies: It makes sense that you want to simplify the collage of different plumbing materials in your home. People switch to PEX because it is durable and flexible enough to navigate through tight spaces, reduces the number of required fittings, and, importantly in an occupied house, reduces downtime. PEX is made by crosslinking polyethylene (PE), creating a spiderweb-like structure that is flexible and tough. PE’s molecules normally align in parallel strands, like a picket fence. An example is a red Solo cup, which you can rip like a piece of paper from the lip to the bottom once it’s cracked. Crosslinking PE changes the molecular structure to look more like a chain-link fence, making it resistant to stress cracking and tearing in all directions.
The difference between PEXa, PEXb, and PEXc is in the way the crosslinking is facilitated during manufacturing. PEX is an extruded product—picture a noodle coming out of a pasta maker. PEXa is crosslinked right as it comes out of the extruder with a high-pressure peroxide reaction, known as the Engel method. PEXb and PEXc are extruded into coils and then crosslinked in secondary steps. PEXb uses either a silane reaction or
steam (moisture cure), while PEXc is crosslinked with electron beams. PEXa has the most consistent crosslinking, is most flexible, and has the longest track record in the market.
What should consumers look for in a PEX? The sun breaks down plastics, and PEX is no exception, so look for manufacturers that offer a long warranty against UV breakdown. Typically, an additional layer is manufactured on the pipe to better protect it from UV.
Leaching is a question that comes up regarding PEX a few times a year. Reputable PEX manufacturers obtain NSF/ANSI 61 certification, which is a standard that helps ensure that water-supply piping doesn’t add harmful levels of contaminants to drinking water. For more information about this topic and specific examples, I’d encourage you to look at the NSF’s “Fact Sheet on the Safety of PEX Plumbing Products,” available on NSF International’s website.
Photo: Charles Bickford
From Fine Homebuilding #294