Since PEX is the newest and fastest-growing material in common use for supply piping today, we are going to devote this segment to a closer look. (This will take us a bit beyond just the supply piping applications we have been examining in the preceding articles, since PEX has significant use in heating and refrigeration systems, as well.) PEX, short for crosslinked polyethylene, has become the most widely used flexible plastic piping material suitable for both hot and cold supply applications. Originally developed and used in Europe (since way back in the 60s), its use in this country was pretty much limited to non-plumbing applications until the mid-90s. Those initial uses included hydronic radiant, snow-melting, ice rink and refrigeration systems. Until that time, polybutylene (PB) had been the flexible plastic piping material of choice, but when massive field failures finally forced that off the market, the door swung wide open for an alternative.
The succeeding heir, PEX, is composed of high-density polyethylene which undergoes a physically or chemically induced molecular change by one of several processes. Manufacturers explain the result as a three-dimensional "bridging" or "networking" of molecules that form a thermoelastic material, stable at high temperatures while retaining flexibility and resistance to chemical attack. Translation for the rest of us: the stuff is stronger and more durable than regular PE under a wide range of temperature extremes (up to 180?F for plumbing supply applications and 200?F for others).
Currently, there are three primary methods for producing the cross-linking structure of PEX tubing: (1) Engel, (2) Radiation and (3) Silane. Suppliers typically identify themselves with, and promote the advantages of, one of these production processes. While there are varying degrees of performance characteristics between them, to be sold in the U.S., all types must meet relevant ASTM and NSF standards and be certified by agencies such as NSF, IAPMO, ICBO-ES or UL (and marked accordingly). PEX is now included in all the major model plumbing codes in the U.S. and Canada, including CSA, IAPMO, SBCCI, BOCA, ICBO, IPC and NSPC. It is also approved by HUD for hot and cold potable water plumbing use.
PEX is produced in copper tube sizes ranging from 1/2 inch to 2 inch, available in coils as long as 1,000 feet, as well as in straight 20-foot lengths.
While the material is sometimes joined with compression fittings, by far the most common method of joining is by means of insert fittings. There are two basic components involved with this approach: the insert body itself, and the crimp rings. The crimp rings, usually copper, are first slid over the ends of the tubes to be joined. Next, male ends of the insert fitting body are pushed into the ends of the tubes to be joined. Finally, the rings are positioned into their correct alignment over each tube end and a special tool is used to crimp the assembly together. The crimping squeezes the compressible wall of the tubing onto the rigid insert inside, providing a secure mechanical connection and a leak-free seal.
Following are the key advantages claimed by suppliers of PEX piping:
Installation Ease: Because of its flexibility, PEX can be installed with fewer directional connections and related fittings. For under-slab radiant heat installations, the material makes possible a single, continuous length without need for joints. Connections are quick, and "non-messy," and labor costs are low.
"Forgiving" Characteristics: The material can stretch to accommodate the expansion of freezing water, and also tends to absorb the shock of water hammer. Note: suppliers make a careful distinction in terms here. Their claim is "freeze-break resistant," -- not "freeze-break proof." In other words, they are saying that PEX will resist freeze rupturing better than most other piping materials, but they make no ultimate "never happen" claims in regard to such breakage.
Stress Resistance: PEX tubing installed in panel heating applications is well-suited to withstand extreme stresses related to installation within a concrete slab or on a structural wood floor. These stresses include expansion and contraction from the constant cycles of operation, mechanical abrasion, scoring and stretching from installation, and structural movement related to seasonal ambient temperature changes.
Resistance to Chemical Attack: The inert nature of the composition makes PEX invulnerable to attack from most chemicals, including those found in concrete.
Quick Testing of the System: Once fitting connections are made, the system can be pressurized immediately.
One limitation cited by PEX manufacturers concerns exposure to sunlight. The properties of PEX can degrade through UV exposure, and the material is therefore not recommended for aboveground, outdoor applications. (No problem below-ground outdoors, however.)
Like copper, PEX lends itself to use in manifold (sometimes called "home run") plumbing systems. Sometimes compared to the layout of electrical systems with breaker boxes, the manifold approach provides a common location from which all the plumbing outlets in the home are supplied. Variations on this concept include "termination manifolds" which feed the plumbing requirements of a specific room or set of rooms. Manifold systems can reduce water and energy consumption by providing relatively short dedicated runs to the fixtures, reducing the amount of cool water that must be purged in order to get hot water flowing from a given outlet. This also results in faster delivery of hot water, reducing water waste and the frequency of water heater heating cycles.