Not All PEX Is Created Equal

PEX is PEX, right? Wrong. If you’re installing a PEX piping system, you need to be aware of the differences of the three PEX manufacturing methods.

Not All PEX Is Created Equal

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If you specialize in residential applications, you’re most likely familiar with crosslinked polyethylene, or PEX — the flexible, durable, plastic piping system that’s become increasingly popular in new-home construction. But if you’re in the commercial space where PEX is still relatively new, you might not be aware of the product’s variations that can make or break your installation.

A little background first: PEX was invented in 1968, and by the 1970s, it was being used throughout Europe for water distribution systems. PEX was brought to North America in the mid-1980s for use in residential radiant floor heating applications. By the mid-1990s, it was being used for residential potable plumbing as well. During the early 2000s, its use evolved to combine a home’s cold-water plumbing with fire sprinklers for a “multipurpose” system.

Commercial projects began using PEX in the 1990s for radiant heating (and eventually radiant cooling) applications. However, the past 10 years has seen an increase in PEX use for domestic hot water, as well as hydronic heating and chilled-water applications.

Both residential and commercial industries see the same benefits of PEX, namely:

  • Its flexibility and long coil lengths, dramatically reducing the need for fittings.
  • Its high durability, corrosion resistance and freeze resistance for greater longevity.
  • Its cost-effectiveness compared to materials like copper.

What are the most important facts you need to know about PEX? Currently, there are three manufacturing methods, known as PEX-a, PEX-b and PEX-c. These methods generate pipe that possess varying degrees of crosslinking. Crosslinking is important because the higher the degree of crosslinking, the greater the flexibility, durability and other benefits of the pipe.

PEX-a pipe

PEX-a pipe is manufactured using the “Engel” method (named after German inventor Dr. Thomas Engel). This method is commonly referred to as “hot” crosslinking because the polyethylene molecules are linked during the extrusion process when polyethylene is above the melting point.

This method results in the highest degree of crosslinking of all PEX types — around 85 percent. This level of crosslinking offers the greatest flexibility, allowing the tightest bend radius to eliminate fittings with changes in direction. PEX-a also offers a unique thermal and elastic memory. This means kinks in the pipe can be repaired with the use of a heat gun. It is important to know that only PEX-a can be kink-repaired. You cannot do this with other PEX types.

The higher degree of crosslinking of PEX-a also offers greater durability in freeze/thaw cycles, keeping the integrity of the pipe intact.

Finally, PEX-a is the only PEX pipe manufactured specifically to allow for larger ASTM F1960 expansion fittings. While you can use ASTM F1960 expansion fittings with PEX-b or PEX-c pipe, strength tests have shown the strongest connection is between PEX-a pipe and expansion fittings, due to the thermal and elastic memory of the PEX-a pipe.

PEX-b pipe

PEX-b is crosslinked after the extrusion process by placing the pipe in a hot-water bath or steam sauna. The degree of crosslinking for PEX-b is typically around 65 to 70 percent. Because this method is not as evenly crosslinked as PEX-a, it is a stiffer product.

It also does not have the same degree of thermal memory as PEX-a, so you cannot repair kinks with a heat gun. If you get a kink in the pipe, you have to cut it out and install a coupling.

Lab tests have shown that while you can create an expansion connection with PEX-b, it will not hold as securely as a PEX-a connection. The stiffer quality of PEX-b pipe can create tiny cracks in the pipe wall when expanded, compromising the fitting connection.

This pipe type typically works best with insert fittings using a copper crimp or stainless steel clamp ring around the outside of the pipe to hold the fitting in place. While push-to-connect fittings are approved for use with all PEX types, testing has shown these fittings can blow off with higher pressures.

The main drawback with insert fittings is that the crimp or clamp ring is working against the shape memory of the pipe. Remember, PEX always wants to revert back to its original shape. If you have force pressing against that shape (such as a ring crimping or clamping down on it), you’re not working with the pipe; you’re working against it.

Also, insert fittings have a smaller internal diameter (so they can go inside the unexpanded pipe). The smaller internal diameter can restrict flow and reduce system performance.

PEX-c pipe

PEX-c uses an electron beam to change the molecular structure of the pipe after the extrusion process. The PEX-c method requires multiple passes under the beam to reach a degree of crosslinking of 70 to 75 percent.

Side effects of this process are discoloration due to oxidation (from natural white to yellow, unless other pigment is added), and a slightly stiffer product. PEX-c can also accept expansion-style connections, but it will also not hold as securely as a PEX-a connection.

And, just as with PEX-b pipe, you cannot repair kinks.

PEX standards, codes and listings

There are numerous standards, codes and listings for PEX to ensure it is being manufactured and installed properly to meet the needs of an application. The most common standards are the ASTM F876 Standard Specification for Crosslinked Polyethylene (PEX) Tubing that covers the materials, dimensions and performance for the pipe, as well as the ASTM F877 Standard Specification for Crosslinked Polyethylene (PEX) Hot- and Cold-Water Distribution Systems that covers the requirements, test methods, and marking methods for the PEX components.

Various manufacturers have other standards, codes and listings that their pipe meets. Be sure to check the pipe submittal or the manufacturer’s website to obtain a thorough understanding of all the standards, codes and listings for the PEX product you plan to install.

Also know that while PEX is approved in all national and local plumbing and mechanical codes, some jurisdictions still do not allow PEX. Be sure to check with your local jurisdiction before installing the product.

One last piece of advice:

When installing PEX pipe and fittings, it is important to stick with the same manufacturer for all components. Mixing different pipe and fittings from different manufacturers can limit the warranty and add confusion/frustration if there is a need for service. Using one brand of PEX and fittings will ensure a full warranty and greater confidence with service and support.

For more information about PEX, visit the Plastics Pipe Institute website at plasticpipe.org or the Plastic Pipe and Fittings Association website at ppfahome.org.

About the Author

Kim Bliss is the content development manager at Uponor. She can be reached at kim.bliss@uponor.com.



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