Using PEX Pipe in Hydronic Systems

Copper has been a mainstay in hydronic applications, but the benefits of PEX also make it a viable solution.

Converting a nearly century-old building into a modern hotel is no easy task. That’s a lesson Texas-based Lowen Hospitality learned while converting an old Dallas railroad company headquarters into a Homewood Suites by Hilton hotel.

One of the challenges was the hotel’s hydronic heating system. Initial cost estimates were high, but then the general contractor on the project considered using PEX. Many mechanical contractors and engineers may have a perception of PEX pipe as a cheap and flimsy alternative to copper that is bought at big-box stores for plumbing applications. But PEX is increasingly proving its mettle in various commercial hydronic applications, such as the Dallas Homewood Suites project. Its flexibility, durability, price stability and overall performance make it superior to copper in more ways than one.

Still, copper (2 inches in diameter and under) has largely dominated the hydronic marketplace for decades. Understanding how PEX pipe can offer superior performance to copper in hydronic applications can reassure contractors and engineers who are contemplating making the switch.

Properties of PEX

Many in the industry do not realize there are three different methods for manufacturing PEX — PEX-a, which uses the Engel method; PEX-b, which uses the silane method; and PEX-c, which uses the electron-beam or radiation method. PEX-a is widely considered to be the superior PEX because the Engel method chemically cross-links the pipe material during the manufacturing process when the polyethylene is in its amorphic state. As a result, PEX-a is more than 80 percent cross-linked, compared with 65 to 70 percent for PEX-b, and 70 to 75 percent for PEX-c.

The higher cross-linking percentage creates the physical property for the pipe to always return to its original shape and size, thereby boosting the entire system’s integrity and performance.

One major benefit of this “shape memory” is it allows for the use of an ASTM F1960 cold-expansion fitting, a watertight pipe-joint connection that actually becomes stronger over time. With an expansion fitting, the installer simply expands the PEX-a pipe with a specialized tool before inserting a fitting. As the pipe naturally shrinks to its original size, it creates a strong, durable connection that holds tight with up to 1,500 psi of radial force. In fact, unlike other types of pipe, the connections actually become the strongest part of the system.

Since the pipe and the expansion ring have to be mechanically expanded before the fitting is inserted, the problem of dry-fitting is never an issue, unlike with copper. Furthermore, no torches, glues or solders are necessary, which makes for a cleaner, healthier job site. Installers can simply confirm with their own eyes that a proper connection is made.

This same property allows accidental kinks in the PEX-a pipe to be repaired with a controlled heat source, such as a heat gun. As it cools, the pipe will always return to its original shape and size, meeting the same standards and possessing the same qualities as before. The 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 long-term advantage over copper when it comes to performance longevity. It is a static system, meaning its internal surfaces — which are three times smoother than copper — will not pit, scale or corrode and its performance will remain the same during year 25 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, which can be up to 100 years. In contrast, copper performs dynamically, meaning pipe corrosion and erosion will occur and impact friction loss and pipe performance over time.

Hydronic Application

From an application standpoint, PEX-a is a very durable, cost-effective solution for transporting water to terminal units such as chilled beams, fan coil units, baseboards, radiators, hydronic VAV (variable air volume) reheat coils, and radiant manifolds.

PEX-a is regulated by ASTM F876, which denotes temperature and pressure ratings of 200 degrees F at 80 psi, 180 degrees F at 100 psi and 73.4 degrees F at 160 psi. These values are well within the range of operation for the vast majority of hydronic systems. This is especially true with the industry’s focus on energy efficiency and the trend to use higher water temperatures in cooling and lower water temperatures in heating.

Although superior to traditional piping in many ways, PEX-a would be significantly less appealing to a contractor if installing it in a hydronic system meant spending a fortune. Without knowing anything about PEX-a, one might reasonably assume that — because of its flexibility — the pipe would require more supports than copper and be difficult to make aesthetically similar to metallic piping. These added costs would most likely outstrip the advantages already discussed.

However, bridging the differences between PEX-a and copper in most hydronic 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 the support permits hanger spacing similar to copper pipe, so it reduces the required number of hangers by half. And since fewer hangers mean lower material and labor costs, use of the PEX-a Pipe Support results in a less-expensive installation.

The PEX-a Pipe Support also addresses aesthetic concerns some may have with PEX. Commercial contractors and building owners typically expect long, evenly spaced, rigid pipelines running through their hydronic systems. It’s a look of solidity and durability that many associate with copper and believe PEX-a lacks. Supporting the PEX resolves this by reinforcing and covering the pipe.

Certain PEX manufacturers offer an oxygen barrier with their product. Since all non-metallic (plastic or rubber) pipe is permeable to the passage of dissolved oxygen molecules through its walls, without a barrier, oxygen can corrode ferrous components in the system, such as valves, strainers and pump volutes, which impacts the overall life of the system. The built-in oxygen barrier limits oxygen diffusion to levels below DIN 4726, ensuring the integrity of any metallic components within the hydronic system.

Similarly, PEX-a offers resistance to chemical-dissolving agents contained within the hydronic system to prevent the otherwise damaging effects of water. The unique molecular structure is stable, inert and unaffected by chemicals commonly found in most commercial HVAC systems.

Hydronic Installation

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 10 times that of copper. Installing anchors every 65 feet and using PEX-a Pipe Supports allows PEX-a to function much like a copper system.

Essentially, PEX-a retains all the benefits of a traditional piping system while providing superior lifetime performance. The pipe’s copper-tube-size-controlled outside diameter further minimizes process change. All the hangers, pipe supports, insulation, etc. can be the same off-the-shelf components used in a copper hydronic piping system.

For these reasons, PEX-a works perfectly with any terminal unit in a hydronic heating and cooling system. Mechanical contractors and engineers can rest assured that installing PEX-a in their hydronic applications won’t jeopardize performance or sacrifice the benefits of rigid pipe.

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