If you've ever shopped for PEX tube you probably know there are three main types. These are PEX A, PEX B and PEX C. In hindsight, that's an unfortunate naming scheme, because it makes it sound like these are grades and implies that one is better or worse than the other. They are actually types of manufacturing processes to achieve a finished product. Anyone buying or working with PEX should know the differences so as to know how to select the proper tubing.
PEX stands for cross-linked polyethylene, (and is sometimes referred to as XLPE.) Polyethylene is composed of chains of ethylene molecules referred to as polymers, (hence “poly”-ethylene.) These polymers don’t connect, which is what gives polyethylene its flexibility. It's produced in two main forms, low density (LDPE) and high density (HDPE). In HDPE, the polymers are tightly packed, which makes it stiffer than LDPE. HDPE is used in common items such as milk bottles and LDPE is used in film applications, such as food wraps or trash bags.
HDPE, in general, has good impact resistance. When crosslinked by any of the methods, A, B or C the HDPE overcomes some of its natural material properties, making the finished product more resilient for potable and radiant applications. Always check the markings on the pipe, called the print-line, for corresponding fastening methods and ratings for temperature and pressure.
Cross-linking creates connections between the polymer chains in the plastic. There are three ways of cross-linking HDPE, which is where the names A, B and C come from. All tubing is formed via extrusion, a process which subjects the HDPE to heat and pressure through a die.
- PEX A. This is cross-linked using the “Engel” process. It involves introducing peroxides, (a form of oxygen,) to the HDPE prior to forming it. This addition enables more bonding at the atomic level, resulting in the highest degree of cross-linking.
- PEX B. Technically referred to as a silane process, but is more often called steam or moisture cross-linking. Unlike the PEX-A process, it's done after the HDPE is extruded into tube. Passing the finished product through steam in the presence of a silane catalyst encourages the polymers to form cross-links.
- PEX C. Cross-linking here is initiated by subjecting HDPE tube to an electron beam or gamma radiation. The PEX C method is not as common as the first two methods and you may not run into it, but know it’s another type of PEX tube that’s available.
Similarities and Differences
The first thing to note is that PEX A, B and C are actually quite similar. All meet ASTM standards and have enough flexibility for plumbing use. When NSF-certified, all three can be used for potable water.
Differences can be summarized as follows:
- Flexibility - PEX-A is the most flexible, allowing for it to be fastened by cold expansion, which allows the end to be fluted prior to inserting the fitting. PEX B is not expandable and you should never expand the end of a PEX-B pipe.
- Kink-Resistance - PEX-A has the greatest resistance, PEX-B the lowest. You should never install kinked pipe.
- Bursting Pressure – Always check the tubing for maximum temperature and pressure ratings. Remember, the pressure ratings decrease as the temperature of the water increases.
- Resistance to Oxidation - All polyethylene tubing is non-corrosive. ASTM also has standards that measure the performance of tubing when exposed to chlorine over time. Those are on the tubing, too.
Make an Informed Decision
PEX tubing has a lot of advantages in plumbing applications, but buyers should know the differences between the three types. While all three meet applicable standards, cost, and flexibility, the installation methods do vary. Buyers should remember this when deciding which type of PEX to use.