Hydronics – All PEX pipe is made from high-density polyethylene. In the manufacturing process, the molecules are cross-linked, which is what gave rise to the PEX name – polyethylene, cross(X)-linked.
The most useful feature of this type of PEX is its ability to hold a “memory” of its original shape, and thus allows kinks in the pipe to be repaired by merely heating the pipe with a heat gun. PEX-A is more elastic than other types, which has led manufacturers to develop full-flow, insert fittings, where the pipe is expanded around the fitting rather than the fitting being fit inside the pipe. This may make fittings harder to install if you’re working in cold conditions
PEX-B is widely available and compatible with many of the generic fitting types on the market, including compression, pushfit and copper crimp rings. It is also performs better in chlorine testing than other PEX types, although all PEX types are vulnerable to high chlorine counts. The pipe can degrade if exposed to high levels of the chemical.
Unlike PEX-A, if PEX-B kinks it must be cut and repaired with a fitting. While it is often rated for higher pressure than PEX-A, this property does not really produce any advantage in the field as it would be very rare that PEX of any kind would be used in a high-pressure application that would put this limit to the test.
PEX-C does have some of the memory properties of PEX-A, although not enough to be used with full-flow fittings. The pipe is more flexible than PEX-B, although it may feel slightly more “plastic-like” than other PEX types. It brings to the table a lot of “middle ground” attributes, however, since it is fabricated using a more environmentally-friendly manufacturing process it may in fact become a more popular choice in the future.
This tubing isn’t pure PEX, but rather a thin layer of aluminum sandwiched by layers of PEX. It deserves mention because it is commonly used for infloor heating applications. The main advantage of PEX-AL-PEX, aside from its built-in oxygen barrier, is that it retains its shape when bent. If you’ve ever faced a quickly-unraveling coil of PEX, you might appreciate the much-less-ornery coil of PEX-AL-PEX. Although it does use aluminum, and costs more to manufacture, the process to make PEX-AL-PEX requires few chemicals, making it one of the most environmentally-friendly ways to make pipe.
How PEX is made
PEX comes in three types, each of which is defined by its manufacturing process. Although certain types of PEX can be slightly better in different applications, many contractors use them interchangeably.
PEX-A pipe is produced using peroxide using what is known as the “Engel method.” During the manufacturing process, cross-linking is done above the crystal melting point, resulting in more cross-linking and a stronger chemical bond within the pipe.
PEX-B is made using a “silane” or “moisture cure” method of cross-linking, where links form after the extrusion process using a chemical catalyst and exposing PEX tubing to water. PEX-B has been manufactured by a large number of companies.
PEX-C is manufactured using electronic irradiation, also called “cold” cross-linking. Here, cross-linking of the molecules is done after the process of extrusion by exposing the pipe to an electron radiation beam. This cross-linking is done below the crystal melting point, and results in less uniform cross-linking and weaker bonds. However, it is a more environmentally-friendly process, since it uses fewer chemicals.
PEX is vulnerable to:
• Becoming brittle from UV (sunlight) exposure.
• Degradation from heavily-chlorinated water. PEX is sold with a coating of anti-oxidant, which must pass 50-year testing equivalents, but the test is only done at room temperature.
• Increases in kinks, once heated.
What to avoid when using PEX
• Fittings with zinc levels above 20 per cent in a brass alloy (zinc corrodes more quickly).
• Additional fittings. Fittings are a potential failure point. Use straight runs of pipe whenever possible, without couplings.
Eric Riml is a certified hydronic designer working in Calgary, Alta.
He can be reached at [email protected]