What Do You Need to Know About PEX?

Uponor plumbing expert offers an explanation of the product and its use.
What Do You Need to Know About PEX?
Kinked piping can be reformed using a heat gun.

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Primarily driven by increasing material and labor costs, value engineering pressures and a desire for greater system reliability, trends in piping indicate a growing preference for PEX (crosslinked polyethylene) over copper and other traditional metallic materials for plumbing or hydronic piping applications.

The following Q&A with Daniel Worm, senior product specialist for plumbing at Uponor, offers an explanation of the product and its properties relevant to codes, standards and listings for specifying engineers who may have little or no prior experience with PEX and its use.

Q: What are the differences in PEX piping?

A: There are three commonly used methods of producing PEX pipe, and each results in a different material. PEX-a, which uses the Engel method, has the highest degree of consistent crosslinking over the other two methods (PEX-b and PEX-c), and produces a pipe that is more flexible, more durable, and has a thermal and elastic memory.

Q: What is thermal memory as it applies to PEX piping?

A: Thermal memory refers to the pipe’s ability to be repaired from accidental kinks by using a controlled heating source, such as a heat gun. Applying controlled heat quickly eliminates the kink in a matter of minutes, and once the repaired area cools to the ambient temperature, it is no weaker than any other section of pipe.

Q: What about elastic memory?

A: Elastic memory refers specifically to PEX-a piping’s ability to be expanded and have the pipe return to its original dimensions. This allows the use of the ASTM F1960 fitting connection. An expansion tool expands the pipe and an expansion ring before the installer inserts a larger internal diameter fitting. Once the fitting is inserted, the piping naturally compresses around the fitting as the piping returns to its original diameter. This is the only connection currently available that actually gets stronger over time.

Q: Can the pipe and fittings handle surge pressures?

A: Yes. PEX-a pipe and ASTM F1960 ProPEX fittings are extremely resistant to surge pressures and fatigue. In fact, due to its flexibility, PEX can actually reduce surge pressures by up to 40 percent when compared to copper pipe. This is important when discussing water hammer and how it affects piping systems because the piping material plays a large role in where that pressure goes.

Q: How do you ensure the connections and fittings are not the weak points in the system?

A: ASTM F1960 ProPEX fittings hold strong with 1,000 pounds of radial force. PEX-a pipe can withstand a burst pressure of around 800 psi (almost two times the requirement for ASTM F876, the standard specification for PEX tubing). So neither the pipe nor the fittings are weak points, making for an extremely high-performing system in domestic water and hydronic piping applications.

Q: Are the fittings freeze-resistant?

A: The fittings do have a slight freeze resistance. Freezing typically is not a concern, however, as the frozen water will work away from the fittings towards the pipe itself. When this happens, the pipe will expand up to three times its outside diameter and can be thawed to its original size with a heat gun (provided the pipe has not been compromised).

Q: Can you address expansion and contraction of a PEX piping system as well as hangers and supports?

A: Most codes specify 32 inches on center for horizontal hangers and supports. To increase hanger spacing, you can incorporate PEX-a Pipe Support; a galvanized, stainless steel channel that comes in 9-foot lengths to enable hanger spacing similar to copper. A secondary benefit of PEX-a Pipe Support is its ability to control expansion and contraction. The support comes with stainless steel strapping, which is important to include in the installation along with fixed anchor points at specified distances – 65 feet for hot-water applications and 150 feet for cold-water applications.

Q: Can PEX pipe be used as a riser in a high-rise building?

A: Yes, as long as the risers are zoned and the pipe is used in the correct temperature and pressure applications. For example, PEX is approved for use in cold-water applications (60 degrees F) up to 168 psi. If the specific design exceeds the temperature and pressure limits of PEX, consider an alternative material. It really comes down to making sure the system is designed properly for its intended purposes.

Q: Are there special considerations for insulating PEX?

A: Unfortunately, even though PEX has a better R-value than metallic pipes, code does not differentiate between piping materials when it comes to insulation. As a result, currently you still have to use the same insulation thicknesses and materials as with other pipe materials. Use a standard copper-tube-size (CTS) insulation.

Q: What is the recommended maximum temperature for hydronic heating using PEX?

A: The maximum recommended temperature is 200 degrees F, though most systems do not typically operate that high.

Q: Is PEX listed to ASTM E84?

A: Certain manufacturers list their PEX piping to ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials. Check with your PEX pipe manufacturer to see if the PEX you’re specifying meets this requirement.

Q: Is PEX listed for fire-stop applications?

A: Fire-stopping falls under ASTM E814. Ensure the PEX you are specifying is the listed through-penetrant when choosing a fire stop. Many fire stop manufacturers will offer system selectors on their websites, where one can specify nonmetallic piping systems and/or PEX specifically.

Q: Can temperature maintenance cables or heat trace be used with PEX?

A: Yes, as long as they are thermostatically controlled.

Q: How would new installers obtain training on the use of PEX tools and materials?

A: Some manufacturers offer commercial pipe training courses for installers. Check with manufacturer representatives and external sales individuals for information about training sessions.

Q: Can PEX piping be used for compressed air?

A: From a listings and warranty standpoint, it cannot. However, many people do use it for compressed air because of its high burst pressure. Unfortunately, manufacturers usually cannot warranty the piping under those circumstances.

Q: Why is the domestic hot-water return velocity so low?

A: It is recommended that designers and engineers follow the ASPE Plumbing Engineering Design Handbook Volume 2, Plumbing Systems. Keeping velocities low in hot-water return piping saves energy, while still recirculating the amount of water needed to maintain system temperatures. This also requires the use of flow control devices, such as balancing valves, to maintain consistent flow throughout the system.

Q: How does the install cost of PEX systems compare with that of a more conventional system?

A: With PEX, you don’t have to worry about ever-changing commodity prices on the open market, as with copper. On average, savings by going with a PEX system can run 25 to 30 percent.

Q: Does PEX come with a warranty?

A: It depends on the manufacturer. Some offer limited warranties of up to 25 years on the PEX pipe and the fitting system.

Q: Where can I go for more information about PEX?

To learn more about PEX pipe and its applications, check out the following sites:

About the author: Daniel Worm, CPD, is the senior product specialist for plumbing at Uponor. He can be reached at daniel.worm@uponor.com.

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