PEX piping systems

By Ron George,CPD,
President, Ron George Design & Consulting Svcs.

There are many options when it comes to choosing PEX tubing for plumbing or heating system projects. 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 usually 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 tubing 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 standards for materials and performance. Many jobs experienced 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 manufacturer’s installation and maintenance instructions are followed. 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 methods. 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 following standards:

• Cross-linked polyethylene tubing (PEX) plastic tubing 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 requirements of ASTM F 1281, ASTM F 2262 or the Canadian Standards Association standard CAN/CSA B137.10M.

• Cross-linked polyethylene/aluminum/high-density 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 layers of molecules. Most of us are familiar with polyethylene 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 molecules 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-dimensional 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 aluminum 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 material 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-linking comes into play also. Some ways seem better than others. 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 chemically 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 heating 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 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 highest 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 tremendous 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 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 control is a concern for this method. Major manufacturers are refining the quality control process to ensure better products. 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 molecules. 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 crystalline 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 distribution 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 product. 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 sometimes 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 proper fitting.

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

Ron George is president of Plumb-Tech Design & Consulting Services. He has served as Chairman of the International Residential Plumbing & Mechanical Code Committee. To contact Ron, write him at rgdc@rongeorgedesign.com.