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PEX-a vs. PEX-b: What’s the Difference?

In the case you’re looking for a quick answer, the key differences between PEX-a and PEX-b piping are:

· PEX-a is approximately 64% more expensive. At the time of publishing this article, PEX-a is $0.59 per foot, and PEX-b is $0.36 per foot (1/2-inch piping).

· PEX-a can deliver higher water volume/pressure to each fixture. This is because PEX-a can be used with expansion fittings, and PEX-b cannot.

· PEX-a is more flexible, and more durable. PEX-a is typically 85% – 89% cross-linked, while PEX-b is usually 65% – 70% cross-linked.

Here at Replumb Specialties Inc., we ONLY install PEX-a in our repipe projects. We’ll explain why below.

Comparison between PEX-a and PEX-b piping showing the visual difference, with PEX-a on the bottom and PEX-b on the top.

The Science Behind PEX-a vs. PEX-b

Diagram illustrating the difference between cross-linked PEX piping and non-cross-linked polyethylene piping, showing the molecular structure and stability.

PEX-a and PEX-b are both made from cross-linked polyethylene, but the cross-linking method used in the manufacturing process is different for PEX-a and PEX-b. Think of cross-linking as a crowd of people standing close together but not touching (like regular non-cross-linked polyethylene molecules). Now imagine everyone links arms with the people around them (like cross-linked polyethylene molecules). This makes the crowd much more stable and resistant to being pushed around or separated. As for the PEX, the structure of the piping is left stronger and more resistant to any damage or corrosion.

· PEX-a Cross-Linking Process: PEX-a is manufactured using the peroxide (or Engel) method, resulting in a highly cross-linked structure – typically 85% – 89%. During this process, the polyethylene is heated and then treated with peroxide while it’s being shaped into tubing.

· PEX-b Cross-Linking Process: PEX-b is produced using the silane (or moisture cure) method. In this process, the pipes are first shaped into tubing, and then exposed a chemical called an organosilane. This triggers a reaction that cross-links the polyethelene molecules together. This post-extrusion cross-linking results in a moderately cross-linked structure – typically 65-70% cross-linked.

Cost Differences: PEX-a vs. PEX-b

PEX-a piping comes with a higher price tag than PEX-b, primarily due to the different manufacturing processes mentioned above. At the time of this publication, 1/2″ PEX-a is $0.59 per foot, and 1/2″ PEX-b is $0.36 per foot, which is a 64% price increase. The price gap tends to increase with larger diameter piping – 3/4″ PEX-a piping is $0.90 per foot, and 3/4″ PEX-b piping is $0.39 per foot, which is a 131% price increase.

It’s important to note that prices can fluctuate based on factors such as brand, supplier, and regional market conditions. PEX-a’s higher cost reflects better flexibility, resilience, and durability – which are all covered in this article. Budget-conscious homeowners might find PEX-b to be a more economical choice for their home plumbing. At Replumb Specialties Inc., we only install PEX-a, and we encourage everyone, whether our client or not, to opt for PEX-a if they can.

Water Volume and Pressure: PEX-a vs. PEX-b

Because of the cross-linking process used when manufacturing PEX-a, the tubing is able to expand and contract without the occurrence of any micro cracking. PEX-b cannot be expanded without developing micro cracking, and because of this it cannot be used reliably with expansion fittings.

The following picture (on desktop to the right, on tablet and mobile below) shows the difference between a PEX expansion fitting and a PEX insert or “cinch” fitting.

Comparison of inner diameters between ¾ inch PEX-a expansion fitting and ¾ inch PEX-b cinch fitting, showing the larger inner diameter of the PEX-a fitting.

Each of these fittings is connected to a piece of 3/4-inch PEX piping. Notice how the expansion fitting has a significantly larger inner diameter, which matches the inner diameter of the tubing. During installation, the PEX-a pipe is expanded over the fitting using a PEX expansion tool – allowing it to contract tightly over a barb on the fitting and form a secure seal. In contrast, insert fittings have an outer diameter that matches the inner diameter of the pipe. This design causes a reduction in flow as water passes through. According to Uponor (our preferred PEX manufacturer) every insert fitting added to a water supply line can reduce the water volume at the fixture (sinks, showers, etc.), equivalent to a loss of around 5 psi in water pressure. Because PEX-a piping can be used with expansion fittings, it offers a higher capacity for water volume, making it especially recommended for larger households where multiple users might need water at the same time. If you’d like a visual demonstration on how expansion fittings are connected to PEX, click here.

Failed PEX-b expansion fitting showing compromised seal and leak points

Some plumbers might tell you that using expansion fittings with PEX-b is just fine. But to show you what happens in the real world, we’ve included a picture of an actual PEX-b expansion connection our team removed. This fitting has PEX-b connected on one side and PEX-a on the other. As labeled in the image, you can see where the PEX-b was leaking, and another area where the seal is compromised. This is caused by micro-cracking in the PEX-b pipe under the expansion ring, causing the pipe to ‘let go’ of the connection and in turn the expansion ring fails to maintain it’s support on the outside of the piping.

Flexibility: PEX-a vs. PEX-b

The flexibility of PEX piping impacts both installation and the performance of the water system. PEX-a is extremely flexible, making it easy to install around obstacles and route through tight spaces. This flexibility also allows PEX-a to make 90-degree turns without the need for additional fittings. Why are additional fittings be problem you might ask? According to Uponor (our preferred PEX manufacturer) each 90-degree fitting whether expansion or insert, reduces water flow by the equivalent of 5 psi at the fixture (sinks, showers, etc.). Using less fittings also means fewer potential leak points, as connections are typically the most leak-prone areas in a plumbing system.

On the other hand PEX-b is significantly less flexible, making installation more challenging as it cannot maneuver as easily around obstacles or through tight spaces. Usually this won’t increase labor costs, but PEX-b does require more fittings to make the same turns – which increases the risk of the water system leaking. The additional fittings in a PEX-b system also reduce water volume, which users might notice as decreased water pressure.

To visually demonstrate the flexibility difference between PEX-a and PEX-b, we had one of our team members, Chace, bend a piece of PEX-a pipe and a piece of PEX-b pipe, each held about 18 inches apart. The tape on the wall served as a reference point to ensure both pipes were bent equally. As shown in the image, the PEX-a pipe makes a 180-degree turn without any sign of kinking. On the other hand, while the PEX-b pipe hasn’t fully kinked, you can see a kink starting to form – which would likely worsen when exposed to temperature fluctuations over time.

Side-by-side comparison of PEX-a and PEX-b pipe flexibility, showing PEX-a bending without kinks and PEX-b developing a kink

Kink Resistance and Repairability: PEX-a vs. PEX-b

As shown above, PEX-a piping is more resistant to kinking than PEX-b piping. But what if a pipe does kink by accident? PEX-a pipes can be easily repaired by applying heat to the kinked area, typically with a heat gun or a light flame from a torch. Uponor states that heating the pipe in this way actually strengthens the affected area, so there’s no downside to making repairs this way. Unfortunately, PEX-b can’t be fixed this way. Instead, the kinked section must be cut out and replaced with a new piece of piping. The issue here is that additional fittings are required to connect the new section, which as mentioned earlier, introduces more potential points for leaks.

Freeze Resistance: PEX-a vs. PEX-b

As mentioned earlier in this article, PEX-a can expand without forming micro-cracks while PEX-b cannot. In turn, PEX-a is more resilient to freezing conditions because it can expand around the frozen water inside it and return to its original shape after the ice is melted without being damaged. Although PEX-b might initially expand when frozen, it will develop micro-cracks in the process, and ultimately leak at a later time. Jeff Hughes from Rise Tide Plumbing demonstrates how PEX-A piping can withstand freezing in a video that you can watch here.

Chemical Resistance: PEX-a vs. PEX-b

PEX-a’s higher degree of cross-linking (85% – 89% vs PEX-b’s 65% – 70%) contributes to a greater chemical resistance than PEX-b. For instance, According to NSF/ANSI 61 standards, PEX-a can withstand up to 5.0 ppm (parts per million) of chlorine at 140°F for 50 years compared to PEX-b’s 4.0 ppm under the same conditions. In environmental stress crack resistance (ESCR) tests, PEX-a typically withstands over 5000 hours without failure, while PEX-b often fails between 1000-2000 hours under similar conditions. In accelerated aging tests simulating long-term exposure to aggressive chemicals, PEX-a retains up to 90% of its original tensile strength after 8000 hours, while PEX-b typically retains only 70-75% under the same conditions. PEX-a’s better chemical resistance makes it particularly suitable for applications involving diverse or harsh chemical environments, with some manufacturers reporting up to 20% longer expected service life in such conditions compared to PEX-b.

Temperature Resistance: PEX-a vs. PEX-b

According to ASTM F876, the standard specification for cross-linked polyethylene (PEX) tubing, both PEX-a and PEX-b are rated for a maximum continuous operating temperature of 180°F (82°C) at 100 psi. However in pressure testing as per ASTM F2023, PEX-a shows a higher degree of long-term strength. For example, at 180°F (82°C) and 80 psi, PEX-a is rated for a 50-year lifespan, while PEX-b is rated for 30-40 years under the same conditions (Plastics Pipe Institute, TR-3). In cold impact tests conducted at -4°F (-20°C) as specified in ASTM F2023, PEX-a shows failure rate of less than 5%, while PEX-b exhibits a failure rate of 10-15% (Jana Laboratories, 2007). Thermal cycling tests between 73°F (23°C) and 180°F (82°C), as outlined in ASTM F876, show that PEX-a can withstand over 5000 cycles without failure, whereas PEX-b fails between 3000-4000 cycles (PPI TR-3, 2019). These performance differences make PEX-a particularly suitable for applications with frequent temperature fluctuations or those requiring sustained high-temperature resistance, such as hydronic radiant heating systems.

Additional Questions About The Differences Between PEX-a and PEX-b?

PEX-a is our choice piping here at Replumb Specialties Inc. After more than 30 years in business, we’ve repiped more than 28,000 homes with PEX-a piping – and we’ve even installed it in our own homes. If you have an additional question about the differences between PEX-a and PEX-b, we’re always happy to help. You can contact our office here for anything we might be able to help you with.

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