This Design Guide provides the information and resources necessary to design and install cross-linked polyethylene (PEX) water supply systems in residential buildings. It includes comprehensive design concepts and installation guidelines to increase the acceptance and proper use of PEX. This document is targeted to meet the needs of home builders, designers,and trade contractors. Its purpose is to introduce potential users to PEX and to enable current users to optimize their PEX plumbing and minimize system costs. In addition, it will allow code inspectors and homeowners to become familiar with the applications, performance characteristics, and benefits of PEX water supply systems.

Cross-linked polyethylene (PEX) is a high-temperature, flexible, polymer pipe. Cross-linking technology was first developed in Europe and has since come into use around the world for a variety of applications. PEX has a 30-year history of successful use in the European market with extensive testing for durability and material performance. It was first introduced in North America in 1984 where it has been primarily used for radiant floor heating, and more recently,for domestic water distribution systems. It is approved for potable hot and cold water supply systems as well as hydronic heating systems in all model plumbing and mechanical codes across the United States and Canada.

The comparison of PEX to polybutylene piping (PB) appears to be a major obstacle to mainstream acceptance by some code officials, trade contractors, and homeowners. But not all plastics are the same, just as not all metals are the same. Polymer fittings for PEX pipe are far more robust and reliable than those used for PB. A result of modern polymer technology, PEX piping performs in ways that provide superior reliability, durability, and safety. on

The PEX piping industry is highly regulated. Standards, specifications, and code requirements define tight material and production quality controls. Continuous-use temperature ratings as high as 200ºF (93ºC) are required as well as standardized chlorine resistance testing to ensure that the piping will withstand the most aggressive drinking water conditions. Nationally accredited, third-party certification agencies require strenuous quality control testing, including random plant inspections and annual monitoring testing.There are numerous opportunities for more widespread use of PEX pipe in the U.S. residential market. The development of manifolds and parallel plumbing systems for flexible piping has helped to advance its use. All major residential building codes permit the use of PEX piping,but obstacles to its acceptance still remain. There is anecdotal and research information that shows:

• There is a perception among some that PEX piping systems are inferior as a building product, generally based on knowledge of past failures of PB piping systems.Although these hurdles exist, the following are among the many benefits of PEX piping systems.

• Ease of Installation – PEX pipe uses mechanical connections eliminating the need for solders, flames, and chemicals. Its flexible nature allows it to bend around obstructions. Use of manifolds can speed installation and improve performance.

• Corrosion Resistance – PEX piping will not pit or stress corrode.

• Scaling Resistance – PEX pipe’s smooth interior walls and chemical properties make it resistant to mineral build-up.

• Cost Effectiveness – PEX plumbing systems are less labor intensive and can optimize system performance.

• Availability of Pipe Sizes – PEX piping is available in a wide range of diameters.

• Energy Efficiency – PEX piping minimizes heat transmission through the pipe wall.

• Resistance to Freeze Damage – Under most circumstances, water in the pipe can be frozen and thawed without damaging the pipe.

• Water Conservation – Well designed PEX plumbing systems can reduce the wait time for hot water to reach the fixture.

• Environmentally Sound – PEX is an inert material and does not contain volatile organic compounds (VOCs).

• Certification – PEX pipes and fittings must meet strict performance requirements.

Although general research on hot water systems has been performed on various aspects of plumbing systems, a recent literature search by the NAHB Research Center indicated that specific system design information for flexible water supply plumbing is sparse. Documents relied more on “standard practice” than on engineered or designed systems. Using these approaches often leads to system designs that either supply more water than is needed at the fixture, or do not take advantage of the characteristics of a flexible plumbing system to reduce cost and improve performance.

This Design Guide provides the information and resources necessary to design and install efficient and cost-effective PEX water supply systems in residential buildings. It illustrates various plumbing configurations for a variety of house types as well as installation guidelines for each method. Properly designed and installed PEX piping systems are beneficial for plumbing designers, installers, and homeowners.

Available in sizes from 1/4 to 2 inches, PEX piping can generally be installed in place of rigid piping on a size-for-size basis. Home-run installations with central manifolds can be used to balance pressures at the outlets and minimize hot water delivery wait time, reducing wasted water and energy. Manifolds can be installed that reduce the amount of piping and fittings,speed-up installation, and balance pressures throughout the system.

This PEX Design Guide can be used by anyone considering the installation of PEX piping for a residential plumbing system. It can be used by the novice as an introduction to PEX piping or by the experienced plumber to optimize his/her approach. Building code officials can use this Guide as a consolidated source of information on the application of PEX piping in residential buildings. Builders can use this guide to learn about the advantages, installation issues, and expected performance of PEX plumbing systems for discussions.

Ease of Installation

The installation of PEX pipe is generally easier than rigid pipe. It is available in long coils which eliminates the need for coupling joints. Its flexible nature allows it to be bent gently around obstructions, minimizing the use of fittings. No solvent, chemical, or solder joining is required.The mechanical fittings are secure and reliable when installed properly. The pipe is lightweight,making it safe to transport and easy to handle. For a comparison of the installation of rigid metal pipe to PEX pipe, refer to the PATH Field Evaluation in Lincoln, Neb.1

Based on extensive testing and material performance over the span of more than 30 years,PEX piping has proven to be a durable material that does not suffer from some of the historical problems associated with metallic piping, such as reduced interior dimension, corrosion,electrolysis, filming, mineral build-up, and water velocity wear. PEX piping will typically expand if the system is allowed to freeze, and return to its original size when the water thaws.

PEX plumbing systems have lower installation costs than rigid metallic plumbing systems.Installation time and labor required is greatly reduced. In service, the use of PEX systems can reduce energy and water use by delivering water to the fixtures faster and by reducing losses in the piping.

PEX piping offers reduced heat loss and improved thermal characteristics when compared to metallic pipe. In addition, less energy is used by the water heater because of shorter delivery time for hot water with PEX parallel plumbing systems.2

Properly designed PEX plumbing systems have the potential to conserve water. The flexibility of PEX allows it to bend around corners and run continuously, reducing the need for fittings; this allows downsizing the pipe diameter to 3/8-inch for certain fixtures.Home-run systems and 3/8-inch pipes minimize the time it takes hot water to reach the fixture. Lengthy delivery time for hot water represents a significant waste of water as well as energy; a problem exacerbated in larger homes.

In 2002, the NAHB Research Center conducted software simulations and laboratory tests on a “typical” hot water system using a trunk and branch rigid pipe design and one that included a 3/8-inch diameter PEX home-run system. Results indicated that systems using shorter 3/8-inch runs with a home-run manifold reduced the wait time for hot water and wasted less water than longer runs of rigid pipe with many elbows and connections.

PEX is a modification or enhancement of high-density polyethylene, an economical and highly cost-effective construction piping material. Generally, manufacturing equivalent lengths of plastic pipe consumes far less energy than manufacturing metallic pipe. The lighter weight of PEX compared to metallic piping helps to lower transportation costs and energy consumption,offering even greater benefit.PEX pipes can be recycled as an inert filler material that can be incorporated into other polymers for specific applications. There is also reduced water use through faster delivery time.

In addition, PEX pipe does not contain harmful VOCs.PEX is a material made up of molecules of high-density polyethylene (HDPE) that are permanently linked to each other by a process called crosslinking. Crosslinking makes PEX a“thermoset” polymer, which gives it long-term stability.Polyethylene can be crosslinked using several technologies. All methods induce links between the single strands of PE to form a dense network through radical reactions. The number of links between the strands determines the crosslink density and is an important factor in determining the physical properties of the material. The minimum percent crosslinking for each method is specified in the ASTM F 876 standard. The three most common methods of crosslinking polyethylene are as follows:

Peroxide – Peroxides are heat-activated chemicals that generate free radicals for crosslinking. This is called the Engel Process.

Moisture-cured Vinylsilane – This method involves grafting a reactive silane molecule to the backbone of the polyethylene. This is called the Silane Process.

Beta Irradiation – This method involves subjecting a dose of high-energy electrons to the PE. This is called the Radiation Process.In European standards these three methods are referred to as PEX-A, PEX-B, and PEX-C,respectively, and are not related to any type of rating system.

PEX pipe produced by any of the three methods must meet the same qualification requirements as specified in the PEX standards. Although methods of crosslinking produce different characteristics, all three methods have been utilized to manufacture approved PEX products.As required in any manufacturing process, procedures for each technology must be established and followed with good quality control checks in place to produce quality products.

ASTM F 876: Standard Specification for Cross-Linked Polyethylene (PEX) Tubing covers

ASTM F 877: Standard Specification for Cross-Linked Polyethylene (PEX) Plastic Hot- and Cold-

Water Distribution Systems covers requirements and test methods for PEX hot- and cold-water distribution system components made in one standard dimension ratio, and intended for 100psi water service, up to and including a maximum working temperature of 180ºF. Components are comprised of piping and fittings. Requirements and test methods are included for hydrostatic sustained pressure strength, thermocycling resistance, fittings, and bend strength.

The flexible nature of PEX allows it to be bent gently around obstructions and installed as one continuous run without fittings. Slight changes in direction are made easily by bending the pipe by hand. There is a predetermined bend radius of a 90-degree change of direction without installing a fitting (reference manufacturer’s installation instructions). Minimizing mechanical connections can result in quicker installations, less potential for leaks at fittings, and less resistance due to pressure drops through fittings.

As water flows through pipes, pressure in the system gives moving water energy, known as kinetic energy. Kinetic energy increases with the speed of water and also with the mass of water that is flowing. When the flow of water is stopped, such as when a valve or faucet is closed, this kinetic energy must be dissipated in the system.The ability of a plumbing pipe to dissipate energy due to surge in water pressure is based on the pipe’s modulus of elasticity, a measure of material stiffness. A higher modulus of elasticity means the material is more rigid. Copper pipe is 180 times more rigid than PEX pipe.Ultimately, this means that with rigid piping systems, pressure surges can produce noticeable banging sounds as energy is dissipated, thus causing what is known as “water hammer.” The pressure surge that causes water hammer can produce instantaneous pressures of 300 to 400 psi (2070 to 2760 kPa), which can cause damage to rigid pipes, fittings, and connections.The flexibility of PEX pipe allows the pipe itself to absorb energy from pressure surges and eliminate or reduce the occurrence of water hammer.

PEX pipes are less susceptible to the effects of cold temperatures retaining their flexibility even below freezing. This flexibility means that if water-filled PEX piping freezes, the elasticity of the material allows it to expand without cracking or splitting, and then to return to its original size upon thawing. This applies when PEX pipes have room to expand evenly along their length, as is typical when installed within walls or ceilings. PEX pipes inside a slab may not be able to expand evenly.

The U.S. Environmental Protection Agency (EPA) recommends that all drinking water be disinfected, typically using free chlorine, chloramines, or other less common methods.Currently, the majority of potable drinking water in the United States and Canada is disinfected using free chlorine. For water treated with free chlorine, the EPA sets a maximum disinfectant level of 4.0 parts per million (ppm) within the water distribution system.The second-most common disinfectant is chloramines. Research conducted by Jana Laboratories, at the request of the Plastics Pipe Institute (PPI), indicates that free chlorine is generally more aggressive to cross-linked polyethylene (PEX) pipes than chloramines.To ensure the reliability of PEX piping systems in hot chlorinated water applications, it is a requirement of the PEX pipe product standard specification ASTM F 876 that all PEX pipes intended for use with potable water have a minimum extrapolated lifetime of 50 years when tested in accordance with test method ASTM F 2023: “Standard Test Method for Evaluating theOxidative Resistance of Cross-linked Polyethylene (PEX) Tubing and Systems to Hot Chlorinated Water.” The minimum requirement applies to traditional domestic applications.

The test conditions of ASTM F 2023 require that the test fluid has a minimum oxidative reduction potential (ORP) of 825 mV. To produce test fluid with this high ORP, third-party test laboratories typically use reverse osmosis-purified water with a free chlorine concentrationof 4.3 +/- 0.3 ppm (4.3 mg/L) and pH of 6.8 +/- 0.2, resulting in an ORP of 825 mV or higher.This represents a very aggressive water quality, which gives conservative results. This test procedure is designed to extrapolate the life expectancy of a hot-water plumbing pipe when used at a water temperature of 140°F and a pressure of 80 psi. Continuous recirculation and traditional domestic4 conditions can both be evaluated by ASTM F 2023.PEX pipe manufacturers must have pipes tested and certified by NSF International, UL and/or other third-party certification agencies to meet the requirements of ASTM F 876, including chlorine resistance. In addition, manufacturers may have pipes certified to NSF International protocol P 171: “Chlorine Resistance of Plastic Piping Materials.” PEX piping systems use fittings that also must comply with ASTM standards, and are made from brass, copper, or hightemperature engineered polymers that are chlorine-resistant.

In summary, PEX pipe has shown itself to be resistant to attack from chlorine and chloramines under a wide range of conditions, and has performed reliably in all regions of North America..

 Corrosion Resistance

PEX pipe and fi ttings have been tested extensively with aggressive potable water conditions and did not pit or corrode. PEX pipe and fi ttings are tested with corrosive pH levels between 6.5 and 6.7, much lower and more aggressive than levels found in common water systems.A related aspect of corrosion in pipes is concerned with fl ow erosion. Flow erosion tests of PEX fi ttings were conducted by the PPI High Temperature Division (HTD). See “Erosion Study on Brass Insert Fittings Used in PEX Piping Systems,”

Indirect (diffused) and refl ected sunlight also have UV energy. If PEX will be exposed to sunlight continuously after installation, such as in an unfi nished basement, cover the pipe with a UV-blocking sleeve (black preferred) or approved pipe insulation. Different manufacturers’pipes have different degrees of UV resistance as indicated on their labels; always follow the recommendations provided by the particular manufacturer.

Caution

Inert Material – Safe for Drinking Water

Since PEX piping is used to transport potable water, it must comply with federal regulations for public safety. PEX materials are inert (not chemically reactive) and cannot contaminate the potable water passing through them. The fittings are mechanical and do not require the use of solvents or chemicals that might leach into the water when the system is first used.

Testing and certification must comply with NSF/ANSI Standard 61: Drinking Water System

Components - Health Effects, and Standard 14: Plastic Pipe System Components and Related

Materials. The primary focus of Standard 61 is to establish minimum health effect requirements for chemical contaminants and impurities that are indirectly imparted into drinking water from products, components, and materials used in potable water systems. PEX piping systems are tested at water pH levels from 5.0 to 10.0, both excessive acidity and alkalinity, beyond levels encountered in potable water systems. PEX pipe does not corrode, and it is resistant to mineral build-up. NSF/ANSI Standard 14 covers physical, performance, and health effect requirements for plastic piping system components used in potable hot- and cold-water distribution systems.

PEX Piping Dimensions and Flow CharacteristicsACCEPTAnCE

PEX plumbing systems are recognized in all major building model codes and are commonly used for hot- and cold-water distribution applications, water service lines, and radiant fl oor heating systems. The following is a summary of relevant model code requirements which specifi cally pertain to PEX and plastic pipe and fi ttings used for domestic water supply.

P2605 Support, P2605.1 General. Piping shall be supported at distances not to exceed those indicated in Table 2605.1. For PEX, maximum horizontal support spacing is 32 inches, and maximum vertical support spacing is 10 feet (mid-story guide for sizes 2 inches and smaller).

P2608.3 Plastic Pipe, Fittings, and Components. All plastic pipe, fi ttings, and components shall be third-party certifi ed as conforming to NSF 14.

P2903.8 Parallel Water-Distribution System Manifolds. Hot and cold parallel waterdistribution system manifolds with individual distribution lines to each fi xture or fi xture fi tting shall be sized and installed in accordance with Sections P2903.8.1 through P2903.8.7.The user must determine which codes are applicable to his/her specifi c project, and also must ensure compliance with all local, state,and federal codes, regulations, and standards.Codes are constantly reviewed and updated.PEX water supply piping has been adopted in the model codes since 1993.

16 P2903.8.1 Sizing of Manifolds. Manifolds shall be sized in accordance with Table P2903.8.1.

A maximum gallon per minute (gpm) is specified for different nominal inside diameter sizes for plastic and metallic manifolds.

P2903.8.3 Maximum Length. The maximum length of individual distribution lines shall be 60 feet (18.2 m) nominal.

P2903.8.5 Support and Protection. Plastic piping bundles shall be secured in accordance with manufacturer’s installation instructions and supported in accordance Section P2605.Bundles that have a change in direction equal to or greater than 45 degrees shall be protected from chaffing at the point of contact with framing members by sleeving or wrapping.

P2904.5 Water Distribution Pipe. References Table P2904.5. PEX plastic tubing shall conform to ASTM F 877 and CSA B137.5 standards.

P2904.9.1.4 Cross-linked Polyethylene Plastic (PEX). References Section P2904.9.1.4.1or Section P2904.1.4.2.

P2904.9.1.4.2 Mechanical Joints. Mechanical joints shall be installed in accordance with manufacturer’s instructions. Fittings for PEX plastic tubing as described in ASTM F 1807, ASTM F 1960, ASTM F 2080, and ASTM F 2159, shall be installed in accordance with manufacturer’s instructions.

P2904.16.2 Plastic Pipe or Tubing to Other Pipe Materials. Joints between different grades of plastic pipe or between plastic pipe and other piping material shall be made with an approved adapter fitting.

605.3 Water Service Pipe. Water service pipe shall conform to NSF61 and shall conform to one of the standards listed in Table 605.3 (ASTM F 876, ATM F 877, and CSA-B137.5).

605.4 Water Distribution Pipe. Water distribution pipe shall conform to NSF 61 and shall conform to one of the standards listed in Table 605.4 (ASTM F 877, and CSA-B137.5).

605.5 Fittings. Pipe fittings shall be approved for installation with the piping material installed and shall conform to one of the standards listed in Table 605.5 (ASTM F 1807, ASTM F 1960,and ASTM F 2080).

605.17 Cross-linked Polyethylene Plastic. Joints between cross-linked polyethylene plastic tubing or fittings shall comply with Sections 605.17.1 and 605.17.2.

605.17.3 Mechanical Joints. Mechanical joints shall be installed in accordance with manufacturer’s instructions. Fittings for PEX tubing as described in ASTM F 1807, ASTM F 1960, and ASTM F 2080 shall be installed in accordance with manufacturer’s instructions

605.23.2 Plastic Pipe or Tubing to Other Piping Material. Joints between different grades of plastic pipe or between plastic pipe and other piping material shall be made with an approved adapter fitting.

3.4.1. Plastic Piping. Plastic piping materials used for the conveyance of potable water shall comply with NSF 14 and be marked accordingly.

3.4.2. Water Service Piping. Water service piping to the point of entrance into the building shall be of materials listed in Table 3.4, and shall be water pressure rated for not less than 160 psig at 73°F. Table 3.4: PEX Plastic Water Distribution Systems (ASTM F 877 with ASTM F 1807, F 1960, or F 2098 Fittings)

3.4.3. Water Distribution Piping. Water piping for distribution of hot and cold water within buildings shall be of materials listed in Table 3.4, and shall be water pressure rated for not less than 100 psig at 180°F. Plastic piping used for hot water distribution shall be installed in accordance with requirements of Section 10.15.8. NOTE: The working pressure rating for certain approved plastic piping materials varies depending on pipe size, pipe schedule, and methods of joining.

10.15.8 Plastic Piping. Plastic piping used for hot-water distribution shall conform to the requirements of Section 3.4 and Table 3.4. Piping shall be water pressure rated for not less than 100 psi at 180°F. NOTE: The working pressure rating for certain approved plastic piping materials varies depending on pipe size, pipe schedule, and methods of joining. Plastic pipe or tube shall not be used downstream from instantaneous water heaters, immersion water heaters or other heaters not having approved temperature safety devices. Piping within 6 inches of flue or vent connectors shall be approved metallic pipe or tube. Normal operating pressure in water distribution piping systems utilizing approved plastic pipe or tube for hotwater distribution shall not be more than 80 psi.

604.11 PEX. Cross-linked polyethylene (PEX) tubing shall be marked with the appropriate standard designation(s)  for which the tubing has been approved. PEX tubing shall be installed in compliance with the provisions of this section.

604.11.1 PEX Fittings. Metal insert fittings, metal compression fittings, and cold expansion fittings used with PEX tubing shall be manufactured to and marked in accordance with the standards for the fittings.

604.11.2 Water Heater Connections. PEX shall not be installed within the first 18 inches (457 mm) of piping connected to a water heater.

Reports as evidence that their products meet code requirements and warrant regulatory approval. Several PEX manufacturers have ESRs or ERs. Evaluation Reports can be obtained from www.icc-es.org.

The IAPMO Guide Criteria (IGC) procedure provides manufacturers and product developers an opportunity to draft IAPMO standards as a vehicle for introducing new products, when no applicable standard exists for the product. Once an IGC is accepted, IAPMO can list products manufactured in compliance with the new requirements. Some PEX and PEX fitting manufacturers have products listed under IGCs. Lists of IGCs can be obtained from www.iapmo.org.