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Code Classroom PEX piping systems

Ron George, CPD President, Plumb-Tech Design & Consulting Services, LLC


here are many options when it comes to choosing PEX tubing for plumbing or heating system pro- jects. For commercial condo and high rise projects,

I have begun using PEX piping for branch piping. In order to have straight runs of piping in pipe racks, I use copper piping for the pipe mains. For a hotel or condo floor, I usu- ally provide a brass manifold, and I use PEX distribution piping for the hotel room or condo unit. Europeans have been using PEX tubing for plumbing

and hydronic piping systems for about 30 years. PEX tub- ing has a great track record, and there are now many new manufacturers and many different types to specify. The tubing suffered a few setbacks in the last several years, when cheap brass fittings were imported from China and introduced into the wholesaler market as a commodity. Because of poor quality control in Chinese brass foundries, these fittings did not meet the required stan- dards for materials and performance. Many jobs experi- enced fitting failures from dezincification, which made the fittings brittle. Manufacturers have realized the need for fittings that

will withstand the test of time. The piping is great as long as the correct fitting is used and as long as the manufac- turer’s installation and maintenance instructions are fol- lowed. PEX systems have come down in price, and they are almost considered a commodity item by many in the industry. Manufacturers don’t want the tubing to become a commodity item, however, because, when that happens, better materials lose out to lesser materials on price alone. PEX manufacturers began taking the time to point out the differences between the different types of PEX tubing and fitting products and the differences in their joining meth- ods. There are significant differences in the way different companies make PEX tubing and fittings. Also, some manufacturers only make tubing; others make both tubing and fittings.

PEX piping and fitting material standards PEX tubing and fittings used in building service piping

or in hot and cold water distribution systems must meet the material requirements in the model plumbing codes. The model codes require PEX tubing to meet the follow- ing standards: • Cross-linked polyethylene tubing (PEX) plastic tub-

ing must conform to the American Society of Testing and Materials standards ASTM F 876, ASTM F 877 and to CSA B137.5. • Cross-linked polyethylene/aluminum/cross-linked polyethylene (PEX-AL-PEX) pipe must meet the require- ments of ASTM F 1281, ASTM F 2262 or the Canadian Standards Association standard CAN/CSA B137.10M. • Cross-linked polyethylene/aluminum/high-density

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polyethylene (PEX-AL-HDPE) must conform to ASTM F 1986. PEX piping gets its name from Polyethylene (PE). The

X denotes cross-linking, which is the crossing of the lay- ers of molecules. Most of us are familiar with polyethyl- ene products in the form of thin plastic cold beverage cups. The material in those cups is usually polyethylene. The cups tear easily; if a cup is torn or placed upside down on the floor and stepped on, it will tear down the side in a linear fashion. This happens because the mole- cules of the polyethylene material line up in a linear arrangement. This linear arrangement and physical weakness would not work well in a piping system; the pipe would easily split when subject to stresses, physical damage or pressure spikes. Manufacturers have developed a way to take multiple

layers of the polyethylene material and layer them over one another in a pattern that crosses the layers in a basket weaving pattern. The layers are fused together in a process called cross-linking, which gives the pipe strength and durability. PEX differs from HDPE in that there is a three-dimen-

sional link between the molecules or macromolecules. The network of macromolecules gives PEX a memory for its original shape. If PEX pipe is bent or kinked, when heated it should return to the shape in which it was first cross-linked. PEX-AL-PEX or PEX-AL-HDPE piping cannot be heated because these types have a layer of alu- minum and will not return to their original shape. How a manufacturer chooses to link the molecules

affects the properties of the final product. Linking all the molecules together would make the PEX very brittle. On the other hand, if too few molecules are linked, the mate- rial won’t be any better than HDPE, from whence it came. Manufacturers must find just the right combination of linked and non-linked molecules. The angle of cross-link- ing comes into play also. Some ways seem better than oth- ers. The following methods are what manufacturers are using today to make PEX: • Engel-method PEX (also known as PEX-A).

Thomas Engel invented this process for producing chem- ically cross-linked polyethylene (PEX) tubing in 1968. In the early ’70s, a plumbing manufacturer used Engle’s technology to develop a practical manufacturing process for PEX tubing for use in hydronic systems. The cross- linked tubing was introduced to the European floor heat- ing market in 1972 and to the potable water market in 1973.

PEX tubing solved a number of problems that occurred

with metal pipes and some other types of plastic tubing. PEX will not corrode or erode, and it is immune to the

Continued on page 20 April 2011

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