The PEX Quotient
By Bill Allen, PE
The history of plumbing goes back thousands of years. Moses of Biblical fame struck a rock about 1,500 BC and became the first water utility. The Roman aqueducts delivered water to Rome from 300 BC to 200 AD. Indoor plumbing is only about 100 years old. The Uniform Plumbing Code was first published in 1946.
The first pipes were clay, wood and lead. After World War II, most indoor water piping was galvanized steel. As the economy boomed, piping systems with better corrosion resistance and ease of installation were introduced. In the 1960s and 1970s, plastic piping systems challenged the metallic materials in use. Some like PVC and CPVC were successes; Polybutylene piping for residential use was a failure. Also during the 1970s we saw PEX introduced.
PEX tubing has proven to be a good system for radiant floor heating, ice melting and residential water piping. There are several processing methods to provide the cross-linking of the polyethylene material. Make sure you verify which performs best in your situation.
Caution: When used properly, every piping material or system can provide good service. Placing galvanized material in corrosive soil will cause it the turn into a sprinkler system. Place copper in contact with concrete or directly connected to steel and it will turn green and disappear. Plastic pipes, if not properly treated, will deteriorate when exposed to direct sunlight. Some plastics can handle high temperatures. Some plastics are approved for use in drinking water and domestic water systems. Some should not be exposed to chlorine. Read the instructions!
There have been fitting failures of some PEX systems in the past when the metal alloys reacted with the water chemistry and failed. When invited to analyze a home, which the night before, had 18 inches of water covering the first floor, it was discovered that the incoming water line had failed at a fitting resulting in a ¾" diameter tube, pouring hundreds of gallons of water into the home. I must report that the builder did a great job on the seals for the doors as they contained the water in the home. Further inspections uncovered leaking fittings that were caused by inexperienced operators not crimping the fitting to specs or using the wrong ring material.
Every product goes through growing pains after they are released. As Pogo, the comic character, stated, “There are no problems, only unsurmounted opportunities.”
When I started in this business back in the Dark Ages, water hammer was a problem. We had faucets that were not stingy with the flow and had valves that could be closed rapidly. The fast velocity of the large flow combined with the sudden stop resulted in noisy piping systems. On more than one occasion I have seen pipes peeled open like a banana from those forces. With the improvements in valve design, water-saving design and the application of water hammer arrestors, this has generally become a thing of the past.
Have you compared the inside diameter of the various piping products available? PEX has a smaller inside diameter than the other materials of the same trade size. This affects the velocity and pressure drop.
In some areas of the country plumbers have used 3/8" diameter PEX to reduce costs and to deliver hot water to the faucet faster. Care must be taken to avoid a low-flow condition due to low pressure and water hammer from the increased flow velocity.
During the last 20 years, I have had requests to inspect and correct PEX systems because the newly installed systems were having water hammer problems and other difficulties. The PEX design manual points out that the flexibility of the PEX allows it to absorb the forces of the pressure surge. This is true, but if the installer does not properly secure the tubing to the structure, the tubes will slap the wall surfaces and create a loud pop.
PEX cannot be stretched tight when secured to the structure. PEX can extend as much as 0.01 inch per foot for a 10°F rise (manufacturer’s published specification). A hot water pipe can easily see variations from 60° F to 120° F. In an 8-foot high wall, the pipe can move ¾". This creates an installation problem as to how to provide expansion and contraction while preventing water hammer noise. The design guide shows examples of ways to solve this and prevent the offensive water hammer. To secure the tubing as it passes through the drywall use a prefabricated pipe support bracket that spans the studs to limit movement.
As the PEX is routed through the structure, the tubing must be located in the center of the stud. If located too close to the drywall an errant trim nail or drywall screw can penetrate it. Use metal plates where nails or screws may penetrate the tube — especially at the top and base plates of wood construction. A homeowner purchased some new solid oak flooring to dress up their new home. The installer removed the baseboard to install the wood and replaced it using 2-inch wire brads fired from a pneumatic gun. Need I say more? They had an interesting contoured flooring system. A small piece of metal where the tube came out of the floor for a water closet would have saved $25,000.
My personal residence has PEX and it gives great service — now that a couple extra clamps have been added.
William Allen, P.E., is the mechanical engineering manager, Geo-Marine Inc., Knoxville, Tenn. He has held titles such as facility and environmental engineer with General Motors; corporate energy manager with Litton Industries; and an engineering consultant with several engineering firms. He attended college at General Motors Institute and received a bachelor’s degree in Plant Engineering in 1971. In 1973, he received a master’s degree in Environmental Engineering from USC.