Poly Alloy Vs. Brass PEX Fittings: 6 Key Differences


There is an ongoing debate among plumbing professionals over the quality of poly-alloy (plastic) versus brass PEX fittings. 

Some plumbers swear by plastic fittings; others, by the brass version. As in most products with the same or similar purpose, the superior option largely depends on the specific application. 

Plastic (poly alloy) PEX fittings are cheaper, more lightweight, and the best choice for applications involving acidic water or other corrosive substances. Brass PEX fittings are stronger, easier to install, and the best choice for high-pressure and high-temperature applications.

Poly Alloy vs. Brass PEX Fittings

The table below shows the key differences between brass and poly alloy PEX fittings:

ComparisonPlastic (Poly Alloy)Brass
CostOne-fifth the cost of brass5x the cost of plastic
WeightLightweight polymerHeavyweight metal alloy
InstallationHarder to installEasier to install
Presence of acidic water or corrosive liquidsWell suitedCorrodes easily
High-temperature environment (over 200oF)Can soften and failWell suited
High-pressure environment (over 200 PSI)Can crack or failWell suited

What Is A PEX Fitting?

PEX stands for cross-linked polyethylene. Polyethylene (PE) is among the most widely produced types of plastic used in various applications worldwide. It is a thermoplastic polymer available in both low-density (LDPE) and high density (HDPE) formulations. 

LDPE is used primarily for film applications such as frozen food packaging and disposable trash bags. PEX pipes and tubing are made from HDPE, which is stiffer and denser than LDPE.

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Fittings are the small parts that connect pipes or tubing with other pieces of plumbing hardware and fixtures. Standard fittings include valves, corner sections, connectors, crimp rings, and clamps. 

They are manufactured to American Society for Testing and Materials (ASTM) standards. PEX fittings are generally rated by third-party reviewers such as the National Science Foundation (NSF) and Underwriters Laboratories (UL).

PEX fittings are primarily made of either poly-alloy (plastic) or brass. They may be made from stainless steel when the intended application involves highly corrosive liquids. 

PEX fittings create a mechanical union, as opposed to one that has been solvent-glued. Manufactured through an extrusion process, PEX tubing and pipes have been widely used since the 1980s. 

Poly-Alloy PEX Fittings

Professional plumbers prefer plastic PEX fittings by a factor of about three to one, as evidenced by the rate at which they purchase them.

What Are The Pros And Cons Of Poly-Alloy PEX Fittings?

Advantages of Poly-Alloy PEX Fittings 

  • Lightweight and lower cost (about 20% as much as brass fittings)
  • Easily reusable; can be removed, cleaned, and reused
  • Rarely leak, even years after installation
  • Smooth internal surfaces reduce friction during fluids conveyance
  • Do not corrode; safe for use with acidic liquids
  • Manufacturer warranties are similar to those for brass fittings

Disadvantages of Poly-Alloy PEX Fittings 

  • Installation is more complex than brass fittings
  • Thicker walls (ASTM2159 Standard) can result in significant flow restriction
  • Can soften when handling temperatures above 200o
  • Known to fail when handling pressures above 200 PSI
  • Deteriorate when exposed to high levels of UV light; unsuitable for exposed outdoor use
  • Less crack- and shock-resistant than their brass counterparts

When Are Poly-Alloy PEX Fittings The Better Choice?

Although not exclusive, there are three situations in which poly-alloy PEX fittings are nearly always the better choice:

  • When cost is important:  Plastic fittings cost much less and do not experience the price fluctuations common with copper-containing alloys such as brass
  • When conditions are harsh:  Plastic fittings are well suited for exposure to chemicals, chlorination, caustics, select alcohols, inorganic acids, detergents, and many others.
  • When location makes leak repair impractical:  Plastic fittings rarely leak even after years of use
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Brass PEX Fittings

Brass fittings are essential to plumbing and are found in nearly every installation. Brass fittings are used as valves, water supply lines, drain pipes, water pump connections, relief valves for pressure reduction, and more.

What Are The Pros And Cons Of Brass PEX Fittings?

Advantages of Brass PEX Fittings 

  • Strong and reliable performance
  • Easier to install 
  • Can withstand higher temperatures
  • Resistant to bacteria and other contaminants
  • Larger flow passage than their plastic counterparts

Disadvantages of Brass PEX Fittings 

  • Corrodes pretty easily, particularly when exposed to acidic liquids
  • Prone to leakage, especially during periods of hot weather
  • Cost about five times as much as poly-alloy fittings
  • Can result in conduction heat loss along the conveyance line

When Are Brass PEX Fittings The Better Choice?

Although not exclusive, there are three situations in which Brass PEX Fittings are nearly always the better choice:

  • When installed outdoors with no UV protection: Brass fittings are unaffected by prolonged exposure to sunlight.
  • When used with liquid temperatures over 200oF or pressure above 200 PSI:  Brass fittings offer far superior resistance to these conditions.
  • When connecting disparate plumbing fixtures: Brass fittings provide the strength needed to deal with fluctuations in force and pressure that can exist when connecting, for example, a metal pipe with a plastic tube.

How And Where Are PEX Fittings Used?

There are several types of PEX fittings, including push-to-connect, expansion, compression, crimp, and press-fit. They are used in applications such as connecting segments of PEX tubing or connecting PEX tubing with copper tubing, manifolds, faucets, or plumbing fixtures.

PEX fittings are standard in any situation involving: 

  • Water conveyance
  • Baseboard and fan coil delivery systems
  • Hydronic radiant heating
  • Snow melting systems
  • Permafrost protection
  • Residential fire sprinkling systems
  • Radiant cooling systems
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They are also widely used in food and beverage and miscellaneous industrial applications.

What Styles Of Plastic And Brass PEX Fittings Are Used?

PEX tubing should be installed in such a way as to reduce the number of fittings needed to minimize leaking and disruptions. Beyond that, the style of fitting used depends on the application.

Crimp Fittings

Crimp fittings are the standard since they are the most affordable and widely available. A large variety of fittings can be chosen to make either PEX connections or transitions to other materials. Most crimp fittings are brass, but plastic fittings are also used.

Compression Fittings

PEX compression fittings operate much the same as copper ones. They have three parts: a nut, ring, and insert. The insert is placed inside the end of the PEX tubing to make the connection, then the ring slides over the tubing and is secured with the nut. 

Compression fittings are popular with plumbers, DIYers, and contractors because they are easy to install and take apart.

Push-Fit Fittings

Push-fit fittings (also known as push-to-connect fittings) are a recent plumbing innovation that doesn’t need special installation tools. A locking mechanism allows pushing the PEX tubing into the fitting where it is gripped and sealed. Push-fit fittings are usually made of brass.

Expansion Fittings

Expansion fittings insert the PEX tubing into a plastic sleeve. The sleeve and tubing are stretched to allow inserting a fitting, which is gripped once the tool is released. These fittings can only be used with certain types of PEX tubing.

Press Fittings

Press fittings are similar to crimp fittings without the crimp ring. Once the PEX pipe is placed over the sleeve, a press squeezes the pipe until it locks.

Final Thoughts

Brass has been the fitting material of choice for many years and is not likely to ever disappear from plumbing parts bins. However,  plastic is now a widely used alternative due to its many benefits. Whichever you choose, make sure to install using strict adherence to industry specifications to ensure an outcome that meets expectations.