What is PEX?
Everything You Wanted to Know About PEX
By Kim Bliss
First used in water distribution systems in the 1970s, PEX (an acronym for crosslinked polyethylene) has quickly grown to be the most-installed piping product for potable plumbing systems in residential applications nationwide.
Its popularity has increased exponentially due to the flexibility, durability, cost-effectiveness and installation ease of the product. In fact, it’s now installed in more new-home construction for plumbing than copper and CPVC combined.
So, if you haven’t used it (or even heard of it) yet, here are some basics you need to know before you do your first installation.
Not all PEX is created equal. There are currently three different methods for manufacturing PEX, known in the industry as PEX-a, PEX-b and PEX-c, which generate pipe with varying crosslinking degrees. The higher the crosslinking, the greater the flexibility and other benefits of the pipe.
PEX-a, also called the Engel method, is named after German inventor Dr. Thomas Engel. This method, also referred to as a “hot” crosslinking process, crosslinks the polyethylene molecules during the extrusion process when polyethylene is in its amorphic state (above the crystalline melting point). This method results in the highest degree of crosslinking of all PEX types (around 85%). This high level of crosslinking offers the greatest flexibility of all three PEX types, providing the tightest bend radius for fewer required fittings. PEX-a also offers a unique thermal and elastic memory which means kinks in the pipe can be repaired with the use of a heat gun (note that this is only applicable to PEX-a pipe). Additionally, PEX-a pipe can be expanded to accept higher-flow ASTM F1960 expansion fittings.
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%. Because this method is not as evenly crosslinked as the 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. Also, tests have shown that while you can create an expansion fitting with PEX-b, it will not hold as securely as a PEX-a connection.
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 70 to 75% degree of crosslinking. 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.
PEX fitting systems
Depending on the pipe style (PEX-a, PEX-b or PEX-c), make sure you’re installing the fitting system properly. It is important to note that ASTM F1960 expansion fittings are specifically engineered to connect with PEX-a pipe. While you can use 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.
For ASTM F1960 expansion fittings, square-cut the pipe for a straight cut. Then, slide on the expansion ring. When sliding on the ring, ensure it overhangs the pipe 1/8″ so it doesn’t slide down the pipe when you start the expansion process. Some rings have a stop edge on one side, so it can only go on the pipe one way. The stop edge ensures you have the proper 1/8″ overhang over the pipe. After the ring is properly on the pipe, insert an expander tool to expand the pipe and ring. After expanding, insert the fitting. As the pipe and ring shrink back to their original size, it creates a strong, durable connection that can never be dry fit, eliminating the risk of potential leaks from incomplete connections.
For copper crimp rings, square-cut the pipe for a straight cut, then insert the fitting onto the end of the pipe. Slide the ring over the end of the pipe at the proper position where the pipe and fitting overlap (position depends on manufacturer). Use a crimp tool to compress the ring until the tool stops, then use a go/no-go gauge to verify the connection is made properly.
For stainless-steel clamp rings, it’s very similar to copper crimp. First, however, you must calibrate the crimp tool. Then, follow the same instructions as copper crimp rings: square-cut the pipe for a straight cut, insert the fitting into the pipe, slide the ring over the end of the pipe, and compress the ring until the tool stops.
For push-to-connect fittings, some manufacturers require a stiffening sleeve inside the PEX pipe prior to making the connection. After inserting the sleeve, push the pipe into the fitting per the manufacturer’s instructions.
In addition to its extreme durability (PEX will not pit, scale or corrode), it is also very flexible. That flexibility changes the way you can install the piping system.
Because PEX is available in long coil lengths and offers a tight bend radius (for example, ½” PEX-a pipe has a bend radius of a mere 3½”), you can simply bend the pipe to make a change in direction without having to stop and cut in a fitting. This takes some getting used to for plumbers who have traditionally installed rigid systems like copper or CPVC.
Eliminating fittings improves your installation times, minimizes costs and limits your liability by removing additional potential leak points. You can also use bend supports to help make 90-degree rigid bends for ½”, ¾” and 1″ pipe. However, to alleviate stress on fittings, it’s important to allow a minimum of two times the pipe’s outside diameter (OD) as minimum distance before changing direction (e.g., ½” pipe has an OD of 0.625″, so the minimum distance would be 1¼”).
Due to its flexibility and expansion characteristics, PEX pipe is much more freeze resistant when compared to copper and CPVC. Because of its thermal and shape memory, PEX-a, for example, can expand up to three times its extruded diameter. So if water freezes in the pipe, the pipe can flex to accommodate the expansion and then shrink back down to its original shape after the ice thaws.
All PEX manufacturers are required to test their pipe’s resistance to hot, chlorinated water and list the PEX designation on the pipe’s print stream. Some PEX manufacturers meet the highest requirement for chlorine resistance at end-use conditions 100 percent of the time at 140F.
It’s important to consult with the PEX manufacturer to verify their PEX designation meets the intended application of the product.
PEX pipe does not sweat like copper due to its very low coefficient of thermal conductivity of 0.219 Btu/(hr•ft²•°F). Copper has a coefficient of thermal conductivity between 300 and 400 Btu/(hr•ft²•°F) depending on wall thickness (Type K, L or M). The thicker walls of PEX act as an insulator, offering insulation values of approximately R-0.19.
The heat transfer from copper is also much greater. In fact, PEX can offer up to 30 percent better insulating value when comparing un-insulated PEX to un-insulated copper pipe.
PEX pipe is approved for direct burial in soil or concrete. Some PEX fittings and fitting materials, like engineered polymer (EP), are also rated for direct burial. Again, it is important to check with the manufacturer for approved applications and installation guidelines.
Some manufacturers also offer pre-sleeved pipe for direct-burial applications. This corrugated sleeving provides added protection for installations in concrete slabs or soil. Also, having the PEX inside the sleeve allows for quick and easy removal and replacement of the PEX pipe, if necessary, without having to break up the slab or soil.
More PEX resources
If you’re interested in learning more about PEX piping and its applications, check out plasticpipe.org or ppfahome.org. These are two of the industry’s most valuable websites for general information about PEX piping systems.
Or, if you’ve read and heard all you need to know and you’re ready to add PEX to your repertoire, there are manufacturers that provide regional and jobsite training for licensed contractors. It’s fast and easy to learn and offers many benefits for installation efficiencies and cost savings.
Kim Bliss is the Content Development Manager for Uponor North America