If the job calls for PEX piping, following these guidelines can help ensure the material works as it should for the customer
PEX is increasingly being chosen over copper, CPVC and other metal and plastic piping materials in plumbing applications in new commercial construction. Here are the key things to keep in mind about PEX’s properties and the practices that will allow it to operate as designed.
Properties of PEX
There are three types of PEX — PEX-a, PEX-b and PEX-c — distinguished by their manufacturing methods and the resulting characteristics.
PEX-a is considered the superior of the three because its manufacturing method creates uniform crosslinking that provides greater flexibility as well as increased thermal and elastic memory in the pipe. Thermal memory is the pipe’s ability to be repaired after an accidental kink. Elastic memory has to do with the pipe’s ability to expand. For PEX-a, it can expand up to three times the pipe’s diameter and then shrink back down to its original size. This makes the piping highly resistant to freeze damage. It also allows for the use of ASTM F1960 expansion fittings for piping connections, which are easy to do yet very reliable. The fitting holds tight with 1,500 pounds of radial force.
Currently, F1960 expansion fittings are available in sizes up to 3 inches in engineered polymer or brass and in various configurations: elbows, tees, reducing tees, couplings, plugs and adapters. There are also products on the market called “multiport” tees that combine multiple tees into one long tee with numerous outlets. These multiport tees reduce the number of required fittings in an application, saving on installation time and reducing the number of potential leak points.
Hanger spacing and supports
Different installation practices apply for horizontal and vertical runs of PEX piping.
For horizontal installations, use copper-tube-size (CTS) hangers and support the bare PEX according to the applicable code. Depending on the pipe size and code, space the hangers between 32 inches and 48 inches apart.
Vertical installations are classified as either in-wall or riser applications. Again, use CTS hangers and support the bare PEX according to the applicable code. For in-wall installations, space hangers 5 feet apart. For risers, which run between building stories, install a riser clamp at the base of each floor and include a mid-story guide to maintain piping alignment in the stud cavity.
To increase spacing between hangers (thus cutting installation costs), use a PEX-a Pipe Support product. This galvanized steel channel provides continuous support in suspended-piping applications. The supports are available in PEX piping sizes from 1/2 inch to 3 1/2 inches.
Used appropriately, PEX-a Pipe Supports will allow hanger spacing similar to that of copper, between 6 and 8 feet. PEX-a Pipe Supports cannot be placed in the pipe span containing fittings. On those runs, hanger spacing must revert to that of bare PEX, so it’s advisable to do a hanger layout that will allow for the full benefits of the PEX-a Pipe Support product.
Thermal expansion and contraction
PEX-a pipe expands and contracts at a rate of 1.1 inches per 100 feet per 10 degrees F ∆T. Using PEX-a Pipe Supports, along with anchor points, can reduce this expansion and contraction rate to acceptable levels.
Anchor points are required every 65 feet for domestic hot water and 150 feet for domestic cold water applications.
With a loop and clevis system, using PEX-a Pipe Supports and anchoring reduces the rate to 0.22 inches per 100 feet per 10 degrees F ∆T, or by 75 percent.
In a strut system, the same procedure reduces the rate to 0.04 inches per 100 feet per 10 degrees F ∆T, a 93 percent reduction, and less than that of copper, which is 0.11 inches per 100 feet per 10 degrees F ∆T.
Use a CTS riser clamp on the base of each floor to control for expansion and contraction in the risers. For hot water, add an extra clamp at the top of every other floor. For cold water, add a clamp at the top of every fourth floor. Also, use an iron-pipe-size (IPS) support for mid-story guides to guide the pipe and maintain direction.
PEX-a is ideal for use with public fixtures, such as urinals and water closets with flush valves. It can reduce surge pressures by 18 to 40 percent compared with metallic piping. PEX-a also has superior acoustical properties, absorbing 10 to 100 times more sound than metal piping. In these installations, it is best to keep the PEX-a header higher, allowing for longer, sweeping bends down to the fixture connections. Most codes currently require hammer arrestors for fast-acting valves, regardless of pipe material.
PEX-a piping is rated for up to three-hour fire assemblies, per ASTM E119. Listings include assemblies for combustible wood-frame construction as well as noncombustible steel-frame and concrete construction. E119 listings are maintained by each respective manufacturer and are product specific. For specific E119 listings, refer to the Underwriters Laboratory (UL) Online Certifications Directory.
Some PEX-a piping is listed for use in return-air plenums, per ASTM E84. A return-air plenum is an un-ducted ceiling space used for air circulation. It is important to have PEX-a Pipe Support segments no less than 48 inches long in a return-air plenum, and the space should not have more than 18 inches of bare PEX or fittings. This also applies to PEX-a Pipe Supports installed in the vertical position. The pipe can also be installed in return-air plenums with an E84-listed insulation when the use of PEX-a Pipe Supports is not practical.
PEX-a pipe carries numerous listings through ASTM E814 for through-penetrations. Listings range from one-hour penetrations for combustible wood-frame construction to three-hour, noncombustible listings for steel-frame and concrete construction. These listings are maintained by the various firestop manufacturers, such as Hilti, 3M, Specified Technologies, Inc. (STI), and RectorSeal.
PEX-a is listed for underground distribution and water service. It meets American Water Works Association C904 requirements for water service and is rated for direct burial. When using PEX-a for below-grade applications, proper trench preparation is necessary. In good soil conditions (flat, no rocks, etc.), the PEX can be installed directly on the trench bottom. For poor soil conditions (rocky, muddy, etc.), prepare the trench bottom with a granular material to provide a stable base. The material should not exceed 3/4 inch in particle size (sand is a recommended option). In below-grade applications, PEX-a can be run continuously to the desired termination points (use 1-inch insulation to overcome soil compression).
Horizontal directional drilling
This application is used when trenching or excavating is not practical. The Plastics Piping Institute technical report, Guidelines for Use of Mini-Horizontal Directional Drilling for Placement of High Density Polyethylene Pipe, allows for runs of up to 600 feet in depths up to 15 feet, using pipe with a maximum diameter of 12 inches. Ensure the pipe does not come into contact with any sharp objects or exceed the minimum bend radius of six times the pipe’s outside diameter, and always pressure-test the pipe after installation to ensure system integrity.
PEX-a pipe is approved for air and hydrostatic testing. Before testing, allow the system pressure to stabilize and always test in accordance with local codes. If conducting a hydrostatic test, do not allow the water-filled piping to freeze. Expect slight fluctuations of pressure due to ambient temperature changes. For commercial pressure testing, condition the PEX to 1.5 times the test pressure. Then, allow the pressure to drop to 10 pounds per square inch before re-pressurizing to 1.5 times the test pressure. Repeat this for a duration of 30 minutes. After that, quickly relieve the pressure to actual test levels, close the valve and start the pressure test timer after observing a slight rise in pressure.
Flush the system with clean, potable water to remove any construction debris. Where required by code, disinfect, per AWWA C651-86, with either a chlorine solution of 50 ppm for 24 hours or a chlorine solution of 200 ppm for three hours. Be sure to flush the system with potable water after chlorine disinfection.
About the Author
Daniel Worm is an associate product manager for plumbing at Uponor. He can be reached at firstname.lastname@example.org.