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Specifying Plastic Plumbing: Beyond Material Selection

By: Jonathan Simon on September 9th, 2020

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Specifying Plastic Plumbing: Beyond Material Selection

Specification  |  PEX  |  CPVC

A successful plumbing system starts with a good specification. This means more than selecting a certain material and its representative ASTM standards – and taking that simplistic approach to specifications can result in significant risk. Each plumbing material has different characteristics and limitations that can impact its specification, installation and performance. Failing to account for these factors in the specification can lead to system underperformance and/or premature failures.

When specifying plastic potable water plumbing systems, there are a number of factors to consider when specifying PEX or CPVC that should be considered.

Chemical Compatibility

The molecular structure of plastic piping materials can be compromised by certain materials, such as insulation, fire-stopping materials, pipe tape, caulk and thread sealants. The only way to be sure that the materials that come into contact with pipes do not cause compatibility issues is through testing and certification.

One example is the FBC Compatible Program developed by Lubrizol Advanced Materials. It enables manufacturers of ancillary products to assist in determining their products’ chemical compatibility with FlowGuard Gold® CPVC. PEX material manufacturers have yet to develop a similar program, creating uncertainty in compatibility with ancillary products.

Chemical compatibility issues can also be exhibited through permeation. In this case, chemicals that come in contact with the outside of the pipe pass through the piping material and enter the water supply. CPVC is not subject to permeation, but PEX is. One leading PEX manufacturer warns in their installation guide against allowing “any organic chemicals, pesticides, strong acids or strong bases to be exposed to PEX tubing.”

Other general guidance on compatibility offered by PEX manufacturers recommends against allowing contact between PEX pipe and/or fittings with:

  • Adhesive tape
  • PVC cements and primers
  • Closed-cell or urethane insulation
  • Solder flux

Expansion and Contraction

Materials expand and contract at different rates and these changes should be accounted for in specification and installation. To help plan for expansion and contraction, refer to the manufacturers and other online tools. For example, the FlowGuard Gold CPVC Thermal Expansion Calculator.

PEX piping will expand nearly 3 times more than CPVC, while the flexibility of PEX can partially accommodate that excess expansion, special care should be taken with PEX systems to ensure that the effects of thermal expansion don’t result in excessive noise or damage to the system.

Flow Rate

CPVC and PEX use different joining methods, which can impact flow rates. CPVC uses a solvent welding process in which the pipe is inserted in the fitting similar to copper.

PEX pipe fits around the outside of the fitting and is secured by a crimp ring. The internal diameter within the PEX fitting is smaller than the internal diameter of the pipe, causing pressure drops that are roughly six times higher than an equivalent FlowGuard Gold CPVC fitting.

PEX insert fittings can cause water velocity to accelerate through the restricted area, creating turbulent zones in the pipe. Eliminating this turbulence may require reducing the velocity of the system by as much as 50%.

Hot Water Recirculating Velocity

How water recirculation systems are often designed with flow rates that are comparable to the rest of the plumbing system however, several PEX manufacturers and Appendix I of the UPC limit the design velocity for PEX hot water recirculation systems to 2 fps. FlowGuard Gold CPVC does not require special velocity restrictions for hot water recirculating systems.

Plenum Ratings

Another thing to consider when specifying plumbing systems is plenum ratings. Codes require that all plumbing piping be listed and labeled to ASTM E84 for flame and smoke performance in a plenum.

All FlowGuard Gold pipe and fittings are acceptable for use in return air plenums without special restrictions. PEX plenum ratings vary by brand and pipe diameter and are contingent on either spacing the pipes a minimum of 18 inches apart or using plenum-rated insulation or pipe supports.

Chlorine Degradation

Plumbing specifications typically don’t mention a material’s ability to handle chlorinated water; however, not all piping material is equal when it comes to this attribute. PEX and CPVC vary significantly in their compatibility with the chemicals used in municipal water treatment. These chemicals, most notably chlorine, chloramine and chlorine dioxide, remain in the water supply as it travels through the distribution system.

The Plastic Pipe Institute’s 2018 Guide to Chlorine Resistance Ratings of PEX Pipes and Tubing for Potable Water Applications states: “Frequent or continuous exposure to water conditions beyond those used in ASTM Test Method F2023 (i.e., aggressive water quality with an ORP above 825 mV and/or pressures above 80 psig and/or temperatures above 140°F) may cause premature oxidation and eventual brittleness of the PEX material, reducing its ability to meet long-term requirements.”

CPVC is a chlorinated compound and so effectively resists chlorine attacks. CPVC will not exhibit premature oxidation even in aggressive water conditions.


In addition, most warranties from PEX manufacturers contain exclusions for damage to the system caused by water conditions due to the chemical incompatibility of the material with chlorine.

FlowGuard Gold CPVC is backed not only by the manufacturer’s warranty but Lubrizol also offers a 30-year warranty on the material to qualifying contractors. These FlowGuard Gold CPVC warranties apply regardless of the water treatment methods.

Quality Assurance and Testing

With CPVC, some plumbers may use a “dry fit” method when installing the pipe. This is not recommended as it can lead to installation error when installers forget to go back and solvent weld the fittings. Processes should be put in place to ensure installers follow manufacturer recommendations.

With PEX, ensure installers check for non-bottomed out fittings in which the fitting is not fully seated within the pipe. This weakens the joint and increases the likelihood of failure. If a gap larger than one-sixteenth of an inch exists, the joint should be replaced.

There are also differences in testing methods. CPVC should be tested hydrostatically. Some PEX manufacturers allow air testing, but it can be dangerous because air is compressible and explosively decompresses, while water is not compressible.

Material selection shouldn’t be treated independently from other decisions in the design and specification process. While both PEX and CPVC have limitations that should be considered in the specification, FlowGuard Gold CPVC provides specifiers more flexibility and control in areas of chemical compatibility, flow rates, hot water recirculation design, plenum design and accommodating local water conditions.

Download the FlowGuard Gold CPVC sample specification and submittal


flowguard gold specification