PEX Perspective

Throughout history pipes have been made of many different materials. We should expect to see continuing innovation in the plumbing and heating industry as new materials, such as PEX, gain market acceptance.

The earliest pipes in recorded history were made of cross-linked polymers. Today, many of the most advanced pipes, such as PEX, are also made of cross-linked polymers. History’s early pipes were made of cross-linked cellulose and lignin, molecules made of carbon and hydrogen, as are PEX and pe pipes today. These first pipes, also known as hollow logs, conveyed water, and until the twentieth century, low pressure gas. 

Beginning some 20 centuries ago, sheets of lead were pounded over round forms and seam soldered to form pipes. The use of lead was so common that plumbers received their name the Latin word plumbum; meaning lead. Later in the 18th century pipes began to be made of cast iron, then steel; and in the early twentieth century, copper.

Today we see a growing variety of pipes in the plumbing and heating marketplace. Around the world monowall pipes are now being made of cross-linked polyethylene (PEX), high density polyethylene (hdpe), raised temperature resistant polyethylene (pe-rt), polypropylene, polybutylene (pb), pvc and cpvc and other more exotic polymers.

Composite pipes are made of multiple layers of materials, bonded together for strength and chemical resistance. They’re made of various combinations of PEX, epdm, pe and polypropylene, with reinforcing centers of aluminum, aramid fiber, stainless steel and even copper.

Why has the market rapidly shifted toward these newer forms of pipe? And, what does the future hold for plumbing and heating professionals?

As contractors begin to re-examine their piping options, everyone wants to select materials that will be safe, serviceable and a credit to the industry.

And somewhat surprisingly, the pipes of the future will be from the same materials as our original log pipes. These basic materials used in PEX, epdm, pe and pb are carbon and hydrogen-derived from natural gas. Unlike photosynthetic cross-linking, manmade cross-linking takes common materials and binds the molecules together to make tough, flexible and long lasting PEX pipes.

Many processes are used to activate cross-linking of these molecules, but all PEX cross-linking processes depend upon the use of various chemicals combined with heat, pressure or electron beams. Regardless of the method used, cross-linking effectively unites the molecules to achieve a tougher, more damage-resistant product.

To maintain these benefits, however, the cross-linked molecules must still be protected against chemical attack from sunshine, oxygen, heat and especially chlorine. To achieve this protection, each manufacturer adds protective chemicals.

PEX pipes offer an attractive combination of benefits to contractors and end users.

Lower labor and material costs lead the list of benefits, but there are other important reasons to switch to PEX pipe, such as longevity and resistance to corrosion.

As our nation’s water quality continues to deteriorate, and as many areas add more aggressive water treatment products to the drinking water, metal, and especially copper pipes, are increasingly more susceptible to chemical attack. Chemical issues include a steady drop in many system’s pH (acidity), increased seepage of road salt into aquifers, the use of chloramines to replace free chlorine and the generally higher rate of chlorine usage to compensate for the possibility of terrorist attacks. Many parts of Florida, Arizona, California and other states with water quality issues have seen large scale replacement of copper pipes with PEX and other plastic pipes.

In Germany, PEX pipes have surpassed copper piping in recent years, and this trend continues across Europe.

In North America, copper retains a market edge, but is quickly losing ground to PEX, especially in residential construction. Copper still dominates over PEX, especially in larger diameters used for commercial construction.

Two factors have limited PEX pipes growth into the commercial marketplace, and these two factors will have to be addressed for PEX to gain widespread commercial acceptance.

The first limiting factor is chlorine resistance. Unlike most residential construction, commercial projects will almost always have many lines running domestic hot water on a 24/7 basis. All PEX pipes listed for plumbing in the United States today are rated to the astm 2023 standard, but many believe this standard does not provide sufficient basis for assuming a minimum 40 year life where the pipe is used in continuous domestic hot water recirculation.

For continuous duty at 140F, where the water may be acidic, and/or heavily chlorinated, NSF and UL have developed competing standards for PEX. Confusingly, both of these performance standards are known as Cl-R, meaning that they are rated for continuous duty in domestic hot water service with a life expectancy in excess of 50 years. By contrast, Cl-Td means that a pipe is rated for only a 25% duty cycle of exposure to domestic hot water, appropriate for most residential duty.

The second limiting factor is market acceptance of the necessary fittings and hardware. Many large diameter fitting systems are being introduced, but some time will be required before they are generally accepted by professional specifiers.

For 1-1/4" and smaller sizes, there is a wide variety of fittings and accessories for all applications, both new construction and remodeling. Transitional fittings are available for plumbing PEX to copper, iron, pe, pb and all other types of piping. The use of manifolded “home run” systems is growing in popularity, especially with homeowners who enjoy the convenience of being able to shut off individual fixtures for easy service. Home run systems also enable the use of smaller lines, which means that individual fixtures enjoy faster hot water delivery.

The National Association of Home Builders (NAHB) and other professional associations have accepted the use of PEX in homes, and have published booklets outlining the cost savings of PEX versus copper. Depending on the size of the home, savings can run from hundreds to thousands of dollars. Most importantly, PEX pipe offers price stability to contractors, especially compared to copper.

A very helpful publication, design guide, Residential PEX Water Supply Plumbing Systems, is available from NAHB at

Mike Chiles is president of Watts Radiant.

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