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Code Classroom Continued from page 18

many problems associated with poor water quality, which can damage metal pipes. The tubing is typically rated at 180 F at 100 psi pressure. PEX tubing also had the high- est polymer oxygen diffusion barrier of any other tubing available at the time. Engel-method PEX is cross-linked while the plastic is still in its melted form. This is done by adding peroxide to the mix and then applying a tremen- dous amount of pressure and temperature to the liquid. The PEX that comes out of the machine is clear. As it cools, it takes on a darker color. The Engel method gives the pipe an essentially uniform distribution of the cross- linking sites throughout the material. Research shows that this is the best way to make PEX tubing. It takes a while longer to make it this way and usually costs a little bit more than other methods. • Irradiation-method PEX (also known as PEX-C). Irradiated PEX starts out as straight polyethylene tubing. As with other methods of making straight polyethylene, the tubing takes on a definite form during the extrusion process. The cross-linking takes place in a second process

Europeans have been using PEX

tubing for plumbing and hydronic piping systems for about 30 years. PEX tubing has a great track record, and there are

now many new manufacturers and many different types to specify.

when the tubing is run through an electron beam cannon. The beam gets the polyethylene molecules so excited that they cross-link. This method can sometimes result in a less uniform cross-linking of the material, however, especially if the pipe is larger than 1" in diameter. Thicker pipe requires a higher dose of radiation. If it’s not done properly, the outer layer of irradiated PEX can become brittle. Quality con- trol is a concern for this method. Major manufacturers are refining the quality control process to ensure better prod- ucts. The safest bet is to use large reputable manufacturers that will stand behind their products. *Silane-method PEX (also known as PEX-B). The

big difference between this method and the other two methods is that, with the Engel and Irradiation methods, the cross-linking consists of a bond between carbon mol- ecules. With the Silane method, the cross-linking takes place across silicon and oxygen molecules. These links are weaker than the carbon-carbon links that result from the other methods; this may have an effect on the long- term chemical stability of the material. The Silane method cross-links the material in the crys-

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talline state. There have been so many advancements made in the process that it’s difficult to generalize about it. A lot of money and research has been directed at it, because it has the best potential, among the various processes, for cost effective production. The extrusion is less aggressive and easy to control and the dimensional tolerances are good, which is very important with regard to fittings technology. Production is fast, raw materials are less expensive, and there is generally less “scrap.” More importantly, the future looks bright, in that the Silane methods will continue to improve over time. These are all good manufacturing methods that meet

the standard requirements for the American Society for Testing and Materials (ASTM). It's good for you to know the differences between these materials, because they all go under the name PEX. You will be the one liable for the installation. There are two other materials approved for water distri-

bution systems in the model codes: • PEX-Aluminum-PEX (PEX-AL-PEX) and PEX-

Aluminum-High Density Polyethylene (PEX-AL- HDPE. I first saw this type of tubing in the mid-’90s at an ASPE engineered plumbing exposition (a great place to visit and learn about new technologies). What I like is that it is made in rigid, straight lengths that can be installed straight in pipe racks. One of my pet peeves with PEX tubing is that it is supplied in coils and that, when installed in a building, it is hardly ever installed in straight runs. From a distance, it is hard to distinguish it from electrical wiring. When there is a large quantity of piping, it can be difficult to trace, and it can look terrible if the piping is all coiled up and not aligned. The installer has a lot of extra work to do in order to have a nice looking finished prod- uct. The aluminum serves as an oxygen barrier. When bent, it stays in that shape. When PEX tubing is stapled to the bottom of a floor

joist or floor it tends to bind at the staple, and you some- times hear the ticking sound associated with thermal growth. This will happen with any piping system if it is fastened tight to the structure. PEX piping also tends to move about in the walls and ceilings when there is water hammer in the plumbing system. If it is in contact with drywall, it can amplify sounds like a drum does. PEX piping must be installed in accordance with the

manufacturer’s installation instructions. I recommend using pipe supports and insulating bushings to address any movement. You should also use fittings supplied by the manufacturer for their tubing. Different manufacturing methods and processes can leave slight variations in wall thicknesses and pipe diameters that will require the prop- er fitting.

Following these tips can provide a safe and quiet piping system that can last the lifetime of the building. n

Ron George is president, Plumb-Tech Design & Consulting Services, LLC. He has served as Chairman of the International Residential Plumbing & Mechanical Code Committee. Visit for more information.

April 2011

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