Everything you need to know about plumbing with PEX
Crosslinking is a chemical reaction that occurs between polyethylene polymer chains. Crosslinking causes the HDPE to become stronger and resistant to cold temperature cracking or brittleness on impact while retaining its flexibility. The three methods of crosslinking HDPE are the Engels method (PEX-a), the Silane Method (PEX-b), and the Radiation method (PEX-c). Several industry participants claim that the PEX-a method yield more flexible tubing than the other methods. All three types of PEX tubing meet the ASTM, NSF and CSA standards.
PEX (or crosslinked polyethylene) is part of a water supply piping system that has several advantages over metal pipe (copper, iron, lead) or rigid plastic pipe (PVC, CPVC, ABS) systems. It is flexible, resistant to scale and chlorine, doesn’t corrode or develop pinholes, is faster to install than metal or rigid plastic, and has fewer connections and fittings.
Contents
PEX tubing is made from crosslinked HDPE (high density polyethylene) polymer. The HDPE is melted and continuously extruded into tube. The crosslinking of the HDPE is accomplished in one of three different methods.
PEX plumbing has been in use in Europe since about 1970, and was introduced in the U.S. around 1980. The use of PEX has been increasing ever since, replacing copper pipe in many applications, especially radiant heating systems installed in the slab under floors or walkways. Interest in PEX for hot and cold water plumbing has increased recently in the United States.
- Flexible PEX tube is manufactured by extrusion, and shipped and stored on spools, where rigid plastic or metal piping must be cut to some practical length for shipping and storage. This leads to several advantages, including lower shipping and handling costs due to decreased weight and improved storage options.
- PEX plumbing installations require fewer fittings than rigid piping. The flexible tubing can turn 90 degree corners without the need for elbow fittings, and PEX tubing unrolled from spools can be installed in long runs without the need for coupling fittings.
- Attaching PEX tube to fittings does not require soldering, and so eliminates the health hazards involved with lead-based solder and acid fluxes; PEX is also safer to install since a torch is not needed to make connections..
- PEX resists the scale build-up common with copper pipe, and does not pit or corrode when exposed to acidic water.
- PEX is much more resistant to freeze-breakage than copper or rigid plastic pipe.
- PEX tubing does not transfer heat as readily as copper, and so conserves energy.
- Water flows more quietly through PEX tube, and the characteristic “water hammer” noise of copper pipe systems is virtually eliminated.
- PEX plumbing installations cost less because:
- PEX is less expensive than copper pipe.
- Less time is spent running pipe and installing fittings than with rigid pipe systems.
- Installing fewer fittings reduces the chances for expensive callbacks.
Oxygen Barriers
Some applications require PEX with added oxygen barrier properties. Radiant floor heating (or hydronic heating systems) may include some ferrous (iron-containing) components which will corrode over time if exposed to oxygen. Since standard PEX tubing allows some oxygen to penetrate through the tube walls, various “Oxygen Barrier PEX” tubing has been designed to prevent diffusion of oxygen into these systems. Two types of specialty PEX pipe are offered:
1. Oxygen barrier PEX has a layer of polymer laminated to the outside surface (or sandwiched internally between PEX layers) that prevents oxygen from penetrating. The polymer film is usually EVOH (ethyl vinyl alcohol copolymer), used in the food industry as an oxygen barrier.
2. PEX-Al-PEX (or PEX-Aluminum-PEX or “PAP”) is a specialty PEX tubing manufactured by several suppliers. This tubing has a layer of aluminum embedded between layers of PEX to provide an oxygen barrier. PEX-Al-PEX may also be called multilayer pipe or composite plastic aluminum pipe. PEX-Al-PEX will also retain shape when bent, and may also exhibit less expansion and shrinkage during temperature fluctuations, but may be less flexible than PEX tubing. PEX-AL-PEX costs about 30% more than standard PEX.
The terms PEX pipe and PEX tube have been used interchangeably, however some manufacturers distinguish beween the two by manufacturing to different inside/outside diameters. For example, PEXpipe may be manufactured to IPS-ID (iron pipe size, inside diameter controlled) sizes with varying thickness to meet pressure requirements, while PEX tube may be manufactured to CTS-OD (copper tubing size, outside diameter controlled) sizes, commonly with a standard thickness of SDR-9 (standard dimension ratio).
The PEX tube manufactured to CTS-OD sizes is the most common, with available sizes including 3/8″, 1/2″, 5/8″, 3/4″ and 1″. On this website, “PEX tube” refers to this common CTS-OD product.
Before extrusion, the HDPE can be pigmented to yield color-coded pipe. Common PEX tubing colors are “natural” (hazy clear, unpigmented), white, black, red and blue. The red and blue colors are used to help plumbers and homeowners distinguish between hot and cold water supply lines. The tube will bemarked on the outside to show which standards it meets.
As it is produced, PEX is wound onto spools for storage and shipping. A typical spool of 1/2 inch PEX will hold 1200 feet of tubing.
Standard Connection Method
The standard method for connecting PEX pipe to brass PEX fittings uses a copper crimp ring and a PEX crimping tool. The copper crimp ring is inserted over the pipe, the fitting is inserted inside the pipe, and the copper ring is crimped over the pipe and fitting using the PEX crimping tool. Tools, fittings and crimp rings are available from several suppliers. Information about testing standards for this method can be found on the ASTM standards page.
Expansion Fitting Method
The expansion method involves using an expansion tool to increase the diameter of the PEX tube. Special expansion fittings are inserted into the expanded tube, which shrinks back to shape around the fitting. A plastic ring is then pressed over the fitting to insure a tight connection.
This method was developed as a proprietary solution, and is currently available from one company. Information about testing standards for this method can be found on the ASTM standards page.
SSC Method
The SSC (stainless steel clamp) method uses special clamps designed for PEX connection. The fittings used here are the same used in the “Standard Connection Method” above, but in this method the SSC fastens the PEX tube to the fitting. A special “SSC crimping tool” is used to tighten the clamp around the tube and fitting. Information about testing standards for this method can be found on the ASTM standards page.
Compression Method
Standard compression fittings can be used to make connections between PEX tubing. For moderate to large size jobs this method is more expensive than using the Standard Connection Method, since compression fittings cost more than PEX fittings.
“Push-fit” and other proprietary methods
Several companies offer specialized fittings that will connect PEX to PEX or to copper, PVC and other materials as well. These fittings use one or more of several technologies such as EDPM O-ring seals, stainless steel gripping teeth, and threaded compression nuts. These fittings are faster and easier than most competing methods, but cost more per fitting than standard PEX fittings.
PEX fittings are generally made of brass, although some vendors are offering bronze, copper and engineered plastic fittings for PEX. The characteristic ridges on the “insert” part of the fitting distinguish a PEX fitting from other fittings (see pictures below). The ridges, the PEX tube and the crimped copper ring all work together to form a high-pressure seal.
To work with PEX tubing using the standard crimping method, three basic tools are needed: the main crimping tool(s), a pipe cutter, and a de-crimping tool.
The pipe cutter is used to make a clean, square cut before inserting the tubing into the fitting.
The main crimping tool can be purchased in several configurations from various vendors. One popular model has the capability to crimp either 1/2″ or 3/4″ PEX tube, while another uses interchangeable crimp heads to work with any of the PEX tube sizes.
A de-crimping tool is designed to remove the copper crimp ring from the tube and fitting. Various designs all work by cutting the copper ring. Fittings can be easily re-used.
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