PEX-a in hydronic piping systems
With copper costs rising, engineers are recognizing PEX-a piping as a technology that retains all the advantages of traditional piping materials, while adding unmatched flexibility and strength.
If you’ve been on a commercial jobsite recently, you may have seen an opaque white pipe being used for the potable plumbing system. This pipe, called PEX — an acronym for crosslinked polyethylene — is quickly gaining popularity in the engineering and trades sectors due to its flexibility, durability, stable material costs and performance. And now that the commercial plumbing industry has gotten on the PEX bandwagon, hydronic piping professionals are starting to look at it as well. They are finding that PEX is also ideal for hydronic piping applications because it eliminates concerns about increasing product and labor costs and provides a solution for projects with value engineering pressures and compressed project cycles.
Still, copper (2 inches in diameter and under) and iron pipe (over 2inches) have dominated the hydronic marketplace for decades. Not only have they become the materials engineers expect to specify, but also the materials that owners and occupants expect to see running through a building. In order for the marketplace to take an alternative material like PEX seriously, the industry must believe in its performance, application, ease-of-installation and adherence to codes and standards.
Not all PEX is the same. Many in the industry don’t realize that there are three different methods for manufacturing PEX — PEX-a which uses the Engel or peroxide method, PEX-b which uses the silane method and PEX-c which uses the electron-beam or radiation method. PEX-a is considered to be the superior PEX in the industry due to the consistent crosslinking throughout the entire pipe wall. This uniformity results from the crosslinking occurring during the manufacturing process when the polyethylene is in its amorphic state, allowing PEX-a to have two fundamental characteristics: thermal memory and elastic memory.
Elastic, or shape, memory allows the use of an ASTM F1960 cold-expansion fitting, which is a watertight pipe-joint connection that actually becomes stronger over time. With an expansion fitting, the installer simply expands the PEX-a pipe and an expansion ring with an expansion tool before inserting a fitting. As the PEX-a pipe and ring shrink back down to their original size, it creates a strong, durable connection that holds tight with up to 1,500 psi (pounds per square inch) of radial force. Because the pipe and expansion ring have to be mechanically expanded before inserting the fitting, there is never an issue with dry-fit concerns. And no torches, glues or solders are necessary, which makes for a cleaner, healthier jobsite. Installers can simply confirm with their own eyes that a proper connection is made.
Thermal memory refers to the ability to repair accidental kinks in PEX-a piping with a controlled heating source, such as a heat gun. As it cools, the pipe will always return to its original size and shape, meeting the same standards and possessing the same qualities as before. This ability to remove kinks quickly and easily is especially valuable during installation, minimizing the time and cost of pipeline repairs.
PEX-a also has a critical performance advantage over copper. Because PEX-a is a static system, meaning its internal surfaces — which are three times smoother than copper — will not pit, scale or corrode, its performance will remain the same during year 10 as it was on day one. This static performance eliminates the need for engineers to compensate for changing conditions during the lifecycle of the system. In contrast, copper performs dynamically, meaning pipe corrosion and erosion will occur and impact friction loss and pipe performance over time.
From an application standpoint, PEX-a can work with any terminal unit in a hydronic heating system — fan-coil units, baseboards, radiators, chilled beams, VAV (variable air volume) reheat terminal units, radiant manifolds, etc.
PEX-a is regulated by ASTM F876, which denotes a maximum temperature rating of 200°F at 80 psi. The latter is well within the range of operation for the vast majority of hydronic systems.
Also, because of its copper tube size (CTS)-controlled outside diameter, switching to PEX-a piping involves minimal process change. All the hangers, pipe supports, insulation, etc. can be the same, off-the-shelf components used in a copper piping system.
Bridging the differences between PEX-a and copper in most applications is the PEX-a Pipe Support, a galvanized steel channel providing continuous support in suspended piping applications. It is available in PEX piping sizes ranging from 1/2 inch to 3-1/2 inches.
The use of PEX-a pipe supports permits hanger spacing that is similar to copper pipe, so it reduces the required amount of hangers by half. And since fewer hangers mean lower material and labor costs, PEX-a pipe support results in a less-expensive installation.
A major criticism of — and misconception regarding — PEX-a in commercial piping applications is its perceived lack of visual appeal. Contractors and building owners typically expect long, evenly spaced, rigid pipelines running through a building. It’s a look of solidity and durability that many associate with copper and believe PEX-a lacks. Fortunately, PEX-a Pipe Supports reinforce and cover the pipe, negating any aesthetic criticisms.
PEX-a Pipe Supports also allow PEX-a to be run inside a plenum without spacing limitations between the horizontal runs. See illustration.
Perhaps the most critical feature of the PEX-a Pipe Support is its ability to control the natural expansion and contraction that occurs as the piping heats and cools. PEX-a has a free-body expansion rate of 1.1 inches per 100 feet per 10F ∆T, or 10 times that of copper. Installing anchors every 65 feet and using PEX-a pipe supports allow PEX-a to function much like a copper system.
Two Installation Methods
With a strut and clamp system, using PEX-a pipe supports and anchoring with fixed points reduces the expansion rate of PEX-a from 1.1 inches per 100 feet per 10F ∆T to 0.08 inches per 100 feet per 10F ∆T, a rate actually less than that of copper.
With a loop and clevis system, the same procedure can reduce the rate to 0.12 inches per 100 feet per 10F ∆T.
Thus, the location of expansion compensation for either a copper system or a PEX-a system would be the same size and in the same location.
This ability to control PEX-a expansion and contraction is critical to allowing for a minimum redesign on a piping system when switching from copper. Thus, PEX-a pipe supports allow designers to leverage similar design schemes for PEX-a as they would for copper.
When it comes to hydronic risers, the goal is to control expansion and contraction forces inside the wall cavity and uphold the integrity of the fire stop. To accomplish this, a riser clamp should be placed at the floor and base of each level, along with a mid-story guide.
Codes and standards
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.
From a specification standpoint, PEX-a is mandated by ASTM F876, meeting all the pressure and temperature ratings. Other relevant codes are ASTM F877 for hot- and cold-water distribution systems; and ASTM F1960 for cold-expansion fittings.
Note: All third-party codes, standards and listings that Uponor PEX-a piping meets for any number of applications can be found on www.uponorengineering.com.
Engineers can be assured that, although PEX-a may still be new to them, it has been reviewed and approved as a reliable means of hydronic piping for more than 40 years. With its elastic and thermal memory characteristics and — in conjunction with PEX-a Pipe Supports — its ability to mimic copper’s rigid properties, PEX-a retains copper’s most desirable qualities, while offering several revolutionary improvements. The common hassles and uncertainties of switching from copper are completely alleviated, allowing the benefits of an improved piping system to stand out.
All of which is why the use of PEX-a continues to grow among commercial system designers, specifiers and installers.
Aaron Stotko is the associate product manager for Heating and Cooling at Uponor. He can be reached at Aaron.Stotko@uponor.com.