PEX (cross-linked polyethylene) has become a very popular option for residential plumbing over the past couple of decades. PEX is flexible, durable, and easy to install. But there is an ongoing debate about whether plastic PEX fittings are reliable enough for long-term use or if metal fittings are better. This article will examine the pros and cons of using plastic vs. metal fittings with PEX pipe.
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What is PEX?
PEX stands for cross-linked polyethylene. It is a flexible plastic tubing that is used for water supply lines in residential and commercial plumbing systems.
Here are some key facts about PEX:
- PEX tubing has a cross-linked molecular structure, which makes it more flexible and durable than regular polyethylene tubing.
- It can withstand high temperatures up to 200°F and won’t crack under freezing conditions.
- PEX is resistant to scale buildup and chlorine corrosion.
- It comes in red, blue, and white colors for hot, cold, and supply lines.
- PEX tubing is often manufactured with an oxygen barrier to prevent oxygen permeation.
The cross-linking process makes PEX tubing more rigid and able to withstand pressure compared to standard polyethylene tubing. PEX can be used for hot and cold potable water lines, radiant heating, and flexible connections.
Types of PEX
There are a few different manufacturing methods used to produce cross-linked PEX tubing:
PEX-A
PEX-A uses the Engel method to cross-link the polyethylene. This involves exposing the tubing to high heat and pressure along with liquid peroxide. PEX-A is very flexible and durable.
PEX-B
The silane method is used to manufacture PEX-B. The tubing is soaked in liquid silane and then exposed to moisture to cause cross-linking. PEX-B also has good flexibility.
PEX-C
PEX-C is made using electron beam radiation to cross-link the polyethylene molecules. It has similar properties as PEX-A and B.
All three types of PEX tubing have very similar properties when installed. The main difference is in the manufacturing process. PEX-A and PEX-B tend to be more commonly used than PEX-C.
Benefits of PEX
There are several reasons why PEX has become so popular for residential plumbing:
Flexibility
The flexible nature of PEX tubing allows it to bend and fit into tight spaces. PEX can easily navigate around corners and joists in basements or crawl spaces. This flexibility also reduces strain on the tubing from shifts in the building.
Freeze resistance
Unlike copper pipes, PEX tubing can withstand freezing temperatures without bursting. In very cold climates, PEX flexible lines have an advantage over rigid metal pipes.
Easier installation
PEX tubing is much faster to install compared to rigid metal pipes. For new construction or remodels, PEX can save on labor costs because it requires fewer connections and no welding torches.
Corrosion resistance
PEX tubing won’t corrode over time like copper and galvanized steel pipes. The flexible plastic is resistant to chemical breakdown and mineral buildup in water supplies. This leads to longer lifespan plumbing.
Cost savings
PEX is competitively priced with other plumbing materials. It also saves money on labor for installation. The overall costs of using a PEX plumbing system are generally lower than using metal pipes.
Disadvantages of PEX
While PEX tubing has many benefits, there are also a few potential drawbacks:
Rodent damage
Like any flexible plastic tubing, PEX lines can be vulnerable to chewing damage caused by rodents, including mice, rats, and squirrels. If lines are not well protected, rodents can bite through PEX.
UV light degradation
PEX tubing that is exposed to prolonged and excessive UV light can become brittle and cracked over time. Most PEX is made with UV stabilizers but it still should not be used in above ground outdoor applications.
Unsuitable for high heat
Although PEX tubing is heat resistant to 200°F, it should not be used near very high temperature heat sources. PEX has a lower heat rating than metal pipes.
Difficult repairs
Because PEX uses flexible plastic fittings, it can be more challenging to cut out and repair sections of damaged PEX lines compared to straight copper pipes. PEX repairs often require cutting back to the last joint.
Limited size options
PEX tubing is only manufactured in smaller diameters, usually 1⁄2 inch to 1 inch. For larger commercial applications, PEX may not be available in the sizes needed.
PEX Fittings
PEX tubing requires specially designed plastic or metal fittings for connecting pipes and fixtures. There are a few main options for PEX fittings:
PEX Crimp Fittings
These fittings use a crimp ring with teeth that bite into the PEX tubing when compressed with a special crimping tool. This forms a very secure metal-to-plastic connection.
PEX Compression Fittings
Compression fittings use an insert and tightening nut or ring to form a seal between the plastic tubing and the fitting. The compression holds the joint together.
PEX Expansion Fittings
These fittings have a tapered end that gets inserted into the PEX tubing. When the fitting is expanded with a tool, it pushes the tube against the taper to create a tight seal.
PEX Push-Fit Fittings
Push-fit PEX fittings use O-ring seals inside the fitting that get compressed when the PEX tubing is inserted. This creates a tight pressure seal without requiring special tools.
PEX Clamp Fittings
These fittings use metal rings or sliding clamps around a barbed fitting end to hold the PEX tubing in place. Screws or other devices tighten the clamp.
Plastic vs. Metal Fittings for PEX
Both plastic and metal fittings can be used successfully with PEX tubing. There is some debate about which fitting material is better.
Plastic PEX Fittings
The most common type of PEX fittings are made from plastic polymers that are engineered for durability and chemical resistance. Here are some benefits of using plastic fittings with PEX tubing:
- Lightweight, easy to install and transport
- Generally lower cost than metal fittings
- Wide range of options from push-fit to crimp styles
- Resistant to corrosion and mineral deposits
- Quieter water flow without metal-on-metal contact
Plastic fittings designed for PEX are very durable and reliable. Most manufacturers provide 25-50 year warranties on plastic PEX fittings. The flexible nature of all-plastic plumbing joints allows them to withstand shifts and expansions.
Metal PEX Fittings
Brass, copper, and stainless steel fittings can also be used with PEX tubing. Benefits of metal over plastic PEX fittings include:
- Greater structural strength and reinforcement
- Standard threads for adapting to metal pipes and valves
- Better support for vertical lines
- Some find metal fittings more aesthetically pleasing
- May last longer than lifetime of the building
Metal fittings provide robust support and solid threading. All-metal connections are rated for higher pressures and temperatures compared to plastic. They give builders and plumbers more confidence in critical joints.
PEX Fittings Issues
There have been some reports of problems and failures with certain types of PEX fittings:
Leaking Compression Fittings
Some early brass compression fittings with PEX would develop leaks years later as the brass fitting expanded at a different rate than the plastic tubing or under pressure surges. This issue seems to have been resolved with improved fitting manufacturing and technology.
Split Tubing with Crimp Fittings
In the past, crimp fittings with sharp edges had a reputation for splitting or slicing PEX tubing over time. Manufacturers have addressed this by rounding and smoothing the crimp bands.
Corroded Brass Fittings
Brass has natural corrosion resistance. But inferior brass PEX fittings can still corrode, leading to weakened joints and leaks. Using name brand fittings helps avoid corrosion issues.
Failing Push-Fittings
Some push-fit PEX fittings have had sealing problems that allow leaks around the o-ring. Using push-fit fittings only from reputable brands can help reduce failure risks.
Choosing the Best PEX Fittings
To maximize the benefits of PEX plumbing, it’s important to choose the right PEX fittings. Here are some tips for selecting quality PEX fittings:
- Use fitting types designed for PEX – Avoid using adapters for other plumbing materials.
- Select certified fitting products – Choose fittings tested and certified for standards like ASTM F1960 and NSF 14.
- Match fittings with the right PEX tubing type – Make sure fittings match the specific PEX tubing material (PEX-A, PEX-B, etc).
- Check manufacturer’s instructions – Carefully follow all instructions for proper installation.
- Consider metal for critical joints – Use reinforced metal fittings for the main water supply line connections.
- Inspect o-rings on push-fittings – Make sure o-rings are present and undamaged before inserting tubing.
- Allow proper curing time – Let installed fittings cure for the recommended time before pressurizing system.
Reputable brands like Viega, Zurn, SharkBite, and Uponor make quality fittings engineered specifically for PEX tubing. Getting the right PEX fittings ensures reliable joints that will last.
Conclusion
Both plastic and metal fittings can provide secure, leak-free connections for PEX plumbing systems. Plastic PEX fittings are lightweight, corrosion resistant, and easy to install. Metal fittings add more structural strength and adaptability.
When properly installed, either plastic or metal fittings are generally acceptable for residential PEX plumbing. For critical connections, reinforced metal fittings may provide more confidence and support very heavy vertical piping. But plastic fittings have improved greatly in durability and are warrantied for decades of use.
The most important factors are using PEX-specific fittings, following manufacturer guidelines, and installing fittings correctly. With quality fittings made for PEX tubing, plastic or metal can both create reliable joints that will last the lifetime of the plumbing system.