While PEX has been used for more than 40 years in radiant heating applications, it’s now getting more traction in commercial hydronic heating and cooling applications as well. Contractors and engineers are beginning to spec and install PEX for distribution piping to various terminal units, such as fan coils, reheat coils, heat pumps, baseboard radiators and chilled beams.
However, while PEX is thoroughly tested and proven in the field, it can sometimes be misunderstood. Many wonder if a plastic pipe can really compete with, let alone outperform, metallic piping in these types of applications.
So, to make it easy, I’ve outlined a few important facts you should know before installing PEX in a hydronic distribution piping application.
All PEX is not the same
Professional PEX products fall into three categories: PEX-a, PEX-b and PEX-c. Of those three, PEX-a, manufactured using the Engel Method, has the highest degree of consistent crosslinking — at 80 percent. This produces a pipe that is more flexible and more durable with thermal and elastic memory properties.
Understanding thermal and elastic memory
Thermal memory is the ability to repair accidental kinks in the PEX-a piping with a controlled heating source, such as a heat gun. The controlled heat quickly eliminates the kink in minutes, while the pipe is restored to its original durability.
Elastic memory refers specifically to how PEX-a piping will quickly return to its original dimensions after expansion. This allows the use of the exceptionally strong, ASTM F1960 fitting connection.
In creating a PEX-a connection, the installer uses a specially made expansion tool to enlarge the diameter of the pipe, so that a fitting with a slightly larger internal diameter can be inserted. The pipe quickly compresses over the fitting, creating a durable connection that actually strengthens over time.
Just as important: Because the fitting diameter is a bit larger than the pipe diameter, “dry-fit” pipe connections are physically impossible. This eliminates the problem when an installer neglects to solder or glue a fitting in a copper or CPVC system, resulting in costly leaks inside the walls of a completed project. An ASTM F1960 fitting requires the connection be fully made, permitting quick visual confirmation.
The difference with expansion fittings
Unlike with metallic pipe, an expansion fitting is actually the strongest part of a PEX-a system. It will not leak. In fact, pull testing shows that even 3,000 pounds of force with over 13 inches of displacement does not compromise the integrity of the ASTM F1960 PEX-a connection. In all likelihood, there would be a catastrophic failure of the building structure before enough force is applied to break that connection.
Using hangers and supports
PEX-a pipe can be hung at similar intervals as metallic pipe. It’s true that other plastic piping systems typically require supports every 32 inches. Some manufacturers, however, offer PEX-a pipe supports — a galvanized steel channel that provides continuous support in suspended-piping applications. This support allows PEX-a to behave just like a metallic piping system in many ways, including hanger spacing: ½-inch and ¾-inch PEX-a pipe can be hung at 6-foot intervals; 1-inch through 3-inch pipe, at 8-foot intervals.
Addressing expansion and contraction
The PEX-a pipe support controls the natural expansion and contraction that occurs as the piping heats and cools. Used in conjunction with manufacturer-recommended installation practices, the pipe support allows PEX-a to undergo an expansion rate equal to or, in some cases, less than that of copper.
Temperature and pressure considerations
PEX-a is rated for continuous operation at 200°F at 80 psi. Any hydronic application with water temperatures at or below this value is perfectly applicable for the product.
Fire-resistant construction ratings
PEX-a is ASTM E84-rated for flame- and smoke-resistance inside a non-ducted, return-air plenum. This also applies to fire stopping. For more information regarding these listings, please contact the manufacturer.
Codes and standards compliance
PEX-a is compliant with all codes and standards that pertain to it, be they in the International family (IBC, IRC, IMC, IPC) or the Uniform family (UMC, UPC, CPC) of U.S. Codes.
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