Designing and Sizing with PEX Pipe

PEX pipe has a number of characteristics that make it a good fit for commercial plumbing projects. Yet, many engineers remain hesitant to specify PEX simply because metallic systems have been the norm for so long and are often the default option.

Here’s a look at how to best design and size a PEX installation in order to overcome some of that hesitation.

PEX Design

The design benefits of PEX begin at the overhead stage. When compared with metallic piping, PEX pipe’s greater flexibility requires fewer fittings, and the coils themselves have a significantly lighter weight, allowing for easier transport. For example, a 300-foot coil of 1/2-inch PEX weighs 18 pounds; the same amount of copper pipe weighs 85.5 pounds.

Applying the PEX Logic design style, you can combine the best attributes of trunk-and-branch and home-run piping systems, while improving on both. Logic utilizes multiport tees, which reduce the overall number of fittings when compared to a trunk-and-branch system and use less overall pipe than a home-run system.

Figure A

Logic’s benefits extend to operation. When comparing the “critical path” of pipe from a water heater to a bath tub between trunk-and-branch, home-run and PEX Logic systems, delivery times and pressure loss are the least for a Logic system. Commercial applications can benefit significantly from the Logic design methodology. Figure A displays Logic in a hospitality application. Multiport tees allow for the joining of back-to-back bathroom groups, reducing the overall number of required risers, penetrations and access boxes for valves.

Figure B

Public fixtures, or those utilizing fast-acting valves or flush valve fixtures, are another ideal application. PEX’s elasticity allows it to absorb 18 to 40 percent greater surge pressures than metallic piping. Polymers also absorb significantly more sound decibels than metallic pipes, which can be eight times louder. Figure B is an example of Logic being used to supply public fixtures.

PEX also offers substantial benefits over metallic piping for in-slab applications. It requires significantly fewer fittings, can avoid obstructions with its flexibility and reduces necessary labor for hangers. Bare PEX is listed for use in concrete as well as below-grade and water service applications.

Thermal Expansion and Contraction

PEX pipe expands and contracts at a rate of 1.1 inches per 100 feet per 10 degrees F ∆T. Utilizing PEX-a Pipe Support in suspended-piping applications in conjunction with fixed anchor points can reduce this expansion and contraction rate to an acceptable level.

When using PEX-a Pipe Support, be sure to use a minimum 300-pound, tensile-rated, stainless steel strap to secure the support to the pipe. Place fixed anchor points at 65 feet for domestic hot water and 150 feet for domestic cold water.

With a loop or clevis system, using PEX-a Pipe Support and fixed anchor points reduces the expansion-contraction rate to 0.12 inches per 100 feet per 10 degrees F ∆T, or by 89 percent.

In a strut system, it reduces the expansion-contraction rate to 0.08 inches per 100 feet per 10 degrees F ∆T, a 93 percent reduction. This rate is actually less than that for copper, which is 0.11 inches per 100 feet per 10 degrees F ∆T.

For risers, use a copper tube size (CTS) riser clamp on the base of each floor to control for expansion and contraction. In hot-water applications, add an extra clamp at the top of every other floor; for cold water, add one at the top of every fourth floor. Mid-story guides are also required by most code bodies and should consist of an iron pipe size support and are meant to guide the pipe and maintain direction.

Pipe Sizing

PEX pipe is manufactured with a CTS outside diameter and a standard dimension ratio (SDR) of nine. The SDR is a correlation between the pipe’s outside diameter and wall thickness. This allows PEX to use the same hangers and supports used with copper, as well as any CTS insulations. PEX-a pipe is burst tested at an elevated pressure, and all pipe sizes will burst in the 800 psi range, nearly double the minimum requirement of ASTM F876 Standard Specification for Cross-linked Polyethylene PEX Tubing. Figure C shows the temperature and pressure ratings:

For design purposes, it is important to understand the various types of fitting connections available for PEX pipe:

• ASTM F1807 Standard Specification for Metal Insert Fittings Utilizing a Copper Crimp Ring for SDR9 Cross-linked Polyethylene PEX Tubing and SDR9 Polyethylene of Raised Temperature (PE-RT) Tubing
• ASTM F2159 Standard Specification for Plastic Insert Fittings Utilizing a Copper Crimp Ring for SDR9 Cross-linked Polyethylene (PEX) Tubing and SDR9 Polyethylene of Raised Temperature (PE-RT) Tubing
• ASTM F2098 Standard Specification for Stainless Steel Clamps for Securing SDR9 Cross-linked Polyethylene (PEX) Tubing to Metal Insert and Plastic Insert Fittings
• ASTM F877 Standard Specification for Cross-linked Polyethylene (PEX) Plastic Hot- and Cold-Water Distribution Systems
• ASTM F1960 (ProPEX) Standard Specification for Cold Expansion Fittings with PEX Reinforcing Rings for Use with Cross-linked Polyethylene (PEX) Tubing

When specifying fittings, one must design with the same system intended to be used in the application. If an engineer specifies a PEX system with F1960 fittings, but the contractor installs any of the alternatives, system velocity — and therefore performance — will be significantly impacted.

ASTM F1960 fittings are, in many cases, preferred over alternate fitting connections. In an F1960 connection, the installer simply uses an expansion tool to expand the PEX-a pipe and PEX-reinforcing ring before inserting a fitting. As the ring and pipe shrink back down to their original size, it creates a strong, durable connection that holds tight with up to 1,500 psi of radial force. A 1-inch F1960 engineered polymer (EP) fitting has a 67 percent greater flow rate, at 8 feet per second, than a F2159 plastic fitting and 22 percent greater flow rate than an F1807 brass fitting. 

When it comes to compiling friction loss data, using the Darcy-Weisbach formula is much more accurate and does not employ correction factors for different water temperatures like Hazen Williams. Friction loss data compiled by Uponor utilizes this method and has been backed by empirical NSF laboratory test data, making it an excellent free resource for maximizing system sizing and efficiency.

There are three ways to size a PEX piping system with ASTM F1960 (ProPEX) fittings. The first is to use model code fixture unit tables (Figure D).

Figure D

This method applies only to systems whose scopes fall within the table. IAPMO and ICC have confirmed compliance with using fixture unit tables to determine pipe size for systems utilizing the ProPEX fitting system, which complies with ASTM F1960.

A second method involves residual pressure, which determines the critical path by the most remote or demanding fixture. Utilize friction-loss data and calculate system loss from the fixture to the source, ensuring that a PEX replacement would provide sufficient pressure. The result may require upsizing high-loss cold-water pipe segments. However, it may be possible to downsize some hot-water pipe segments due to the difference between the design velocities of copper and PEX.