Pre-Insulated PEX Pipe Benefits Underground Hot-Water Applications

Here’s information to keep in mind when applying PEX to hydronic heating/cooling or hot-water distribution systems
Pre-Insulated PEX Pipe Benefits Underground Hot-Water Applications
Workers do a heating/cooling system install using pre-insulated PEX pipe at the Wolf Ridge Environmental Learning Center in Finland, Minnesota.

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If you’ve worked in the residential plumbing or HVAC industry for the past two decades, you’ve undoubtedly seen an increase in the use of flexible PEX piping systems.

But when it comes to underground piping systems for commercial or multifamily applications, such as hydronic heating/cooling or domestic hot-water distribution, traditional copper or steel still dominates the landscape. Here’s why pre-insulated PEX piping systems can also be a good fit.

PEX defined

What makes flexible pre-insulated PEX piping good for underground applications? The answer lies in its inherent mechanical properties, such as the ability to operate continuously at temperatures up to 200 degrees F and its flexibility for faster, easier installations.

Of the three types of PEX — PEX-a, PEX-b and PEX-c — PEX-a is considered superior because its manufacturing method crosslinks the polyethylene molecules during the pipe-extrusion process when the material is in its amorphic state (above the crystalline melting point). The resulting uniform crosslinking delivers greater flexibility and increased thermal and elastic memory in the pipe.

Thermal memory refers to the pipe’s ability to be repaired after an accidental kink in the piping. This is done with a controlled heat source, such as a heat gun.

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 to its original size. This characteristic makes the piping highly resistant to freeze damage and also allows for the use of ASTM F1960 expansion fittings for piping connections.

An ASTM F1960 expansion connection is a simple, yet highly reliable, way to make a PEX fitting. An installer uses an expansion tool to expand the pipe and an attached expansion ring before inserting a fitting. As the pipe and ring shrink back to their original shapes, it creates a strong connection that holds tight with 1,500 pounds of radial force.

Benefits of pre-insulated PEX systems

Pre-insulated PEX piping systems typically consist of one or two service pipes surrounded by insulation and covered by a jacket or casing. This type of system offers numerous advantages over rigid systems: lighter weight, fewer connections and installation efficiencies.

The significantly lighter weight of flexible PEX pre-insulated piping systems allows installers to maneuver the pipe around a job site much easier, faster and more effectively — and with less impact on the body. The systems are also available in continuous coils (rather than straight, segmented pieces of rigid pipe), which eliminates the need to dig a straight or level trench since the coils can curve around existing structures and obstacles.

Longer coils also mean fewer — or sometimes no — underground connections for faster installations. This benefit eliminates the concern of potential leak points underground. Plus, because the insulation is already added to the pipe, it eliminates the two-step process of installing the pipe and then insulating after installation.

All of these benefits add up to labor-cost savings of around 67 percent, depending on the application.

Bonded vs. slip systems

There are two different types of PEX-based pre-insulated piping systems: bonded and slip systems.

In a bonded system, three components — the pipe, polyurethane insulation and casing — are all bonded together. The rigid polyurethane foam is essentially a “glue” that adheres to the core service pipe as well as to the outer casing of the piping system. The bonding not only makes the piping more rigid, but it is also more difficult to separate the foam from the pipe.

Manufacturers recommend cutting the jacket, peeling it away from the foam, removing the foam with a chisel or saw, and removing the excess foam with sandpaper. Using sandpaper is a delicate process: It is important not to perforate the oxygen-diffusion barrier on the service pipe. The oxygen-diffusion barrier on PEX protects the ferrous components in the system from corrosion.

In a slip system, the service pipe is covered by multi-layer polyethylene or PEX foam insulation and then by a corrugated HDPE jacket. This allows increased flexibility because the different layers in the piping system can “move” independently from one another. Also, it’s much simpler to cut away the foam insulation if an installer needs to access the pipe.

Both bond and slip systems are more flexible and installer-friendly than a rigid metal piping system, but a slip system offers even greater flexibility, ease of access and faster installations. A slip system also delivers greater thermal performance and significantly less off-gassing, compared with bonded polyurethane foam systems.

Off-gassing is a natural occurrence of the manufacturing process for both PEX-foam and polyurethane-foam systems. How and when the foam comes in contact with the carrier pipe can have an adverse effect on the system’s thermal performance.

Testing by Uponor, a manufacturer of PEX, has shown polyurethane foam conductivity stabilizes in just two years, with a significant decrease in insulation value and thermal performance. This is why Uponor allows the PEX foam to sit idle for six days following its manufacture to stabilize before being placed on the PEX carrier pipe. Off-gassing takes place during this idling period and ensures steady thermal performance throughout the system’s lifespan.

With bonded systems, the polyurethane foam is sprayed into the casing and hardens. Once the foam hardens, low-conductivity, high-pressure (compared with atmospheric pressure) gases exist inside the foam’s cells. Over time, the higher-pressure gases diffuse through the cells outward, escaping the foam.

As the pressures inside the cells stabilize, the net conductivity of the remaining gases increase. This diffusion process continues until the pressure inside the cells reaches a balance with the surrounding air. This process yields acceptable performance for the system initially, but thermal performance diminishes as time progresses.

Single vs. twin pipes

When deciding to install a PEX-based underground piping system, it is important to understand the thermal performance differences between a single-pipe system and a twin-pipe system. Some code jurisdictions do not permit twin-pipe configurations due to the reduction in insulation thicknesses, which can result in greater heat loss in heating applications.

But because twin-pipe configurations significantly reduce the amount of surface area exposed to the ground, they can have a positive effect on heat loss. In fact, a study of heat-loss performance for both twin-pipe systems and single-pipe systems showed thermal performance actually increased with twin-pipe configurations.

The study found that, as the difference (Delta T) between the average water temperature and the ground temperature increased, so did the difference in performance between the two configurations. The study demonstrated that the increase in performance by incorporating a supply and return line in the same jacket can be as much as 46 percent, with a Delta T of 110 degrees F between the average water temperature and the ground temperature. That would be similar to designing a system with a 160 degree F supply water temperature, 140 degree F return water temperature and a design ground temperature of 40 degrees F. Again, as the differential between the average water temperature and the ground increases, so would the differential in performance between a twin-pipe system and multiple single pipes.

Note that a single-pipe configuration might be the better option when the Delta T between the supply and return pipes is greater than 20 degrees and there is a large difference between the supply water temperature and ground temperature. In this case, a single-pipe configuration would help prevent heat migration from the supply line to the return line, as well as minimize the heat loss from the supply line to the ground.

The next time you have the opportunity to design or install an underground piping system for a hydronic heating/cooling or domestic hot-water distribution application, think about the efficiencies, durability and performance of a pre-insulated PEX piping system. With labor cost savings, thermal performance and overall system longevity, pre-insulated PEX is a good option.

About the Author
Kim Bliss is the content development manager at Uponor. She can be reached at


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