How to connect PEX pipe with copper piping without corrosion

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Switching from copper piping to modern PEX tubing is a common situation during heating system renovations or when connecting new floor heating systems to older installations, but incorrect handling can lead to rapid electrochemical corrosion. However, the correct choice of connecting fittings and adherence to installation procedures completely eliminate this risk and ensure the long-term durability of the entire system.

Corrosion risk when connecting different metals

Combining copper and aluminum (which is found in some types of multi-layered piping) or direct contact with steel parts creates a galvanic cell, where the less noble metal begins to corrode. In the case of PEX tubing, which is itself inert plastic, it is critical that the metal part of the fitting does not come into direct contact with the water in the system without proper insulation from copper. Directly screwing a copper fitting onto PEX tubing without an intermediate material is technically unacceptable.

In practice, we have seen cases where carelessness in the selection of fittings led to water leaks after three years of operation, as corrosion progressed from the inside out. The key is to use special reducing fittings that have an internal polyethylene insert or are made from materials that do not create a galvanic reaction with the water in the system. These components are standard in professional installation and their cost is negligible compared to the cost of wall repairs.

Choosing the correct connecting fitting and type of PEX

The most reliable solution is to use crimp or threaded fittings specifically designed for the transition between copper and plastic piping, with the PEX pipe having an oxygen diffusion barrier. If you are working with high-quality PEX pipe, such as PEX Pipe - 20x2; PN6-T=+90° C, PN10-T=+60° C - 200m, you must realize that it is a thermoplastic that requires a precise tool for crimping or the correct screwing method with a spring washer. An incorrect type of fitting, such as a standard brass fitting without sealing, will immediately cause a leak.

Use only fittings marked "for PEX" and "copper transition" that have an integrated O-ring.
Avoid cheap combined fittings without a certificate for potable water and heating technology.
Check whether the fitting is compatible with your specific type of PEX (PEX-a, PEX-b or PEX-c).
For larger diameters, for example in the main distribution line, we recommend PEX Pipe - 25x2,3; PN6-T=+90° C, PN10-T=+60° C - 200m, which requires more robust connecting elements.

Technicians often make the mistake of trying to bypass the problem by using tape or sealing compounds for direct contact between copper and plastic, which is in violation of all applicable standards. The correct approach is through specialized components designed to physically separate the two different metals while also ensuring watertightness and pressure resistance. When using smaller diameters, such as PEX Pipe - 17x2; PN6-T=+90° C, PN10-T=+60° C - 120m, it is important to ensure precise cutting and removal of sharp edges before inserting into the fitting.

Installation procedure and technological control

The joining process must begin with a precise cutter that ensures a perpendicular cut without deforming the pipe, followed by ensuring the internal smoothness and then inserting it into the fitting. When using crimping tools, it is essential to follow the exact crimping torque according to the manufacturer's instructions for the fitting, as insufficient crimping causes a leak, while excessive crimping can damage the sealing ring. This process is critical even with longer coils, such as PEX Pipe - 17x2; PN6-T=+90° C, PN10-T=+60° C - 240m, where it is necessary to ensure proper fixation before the actual connection.

In practice, we have encountered situations where installers forgot to clean the end of the pipe from plastic burrs, which subsequently damaged the T-shaped sealing rubber. This simple oversight leads to a slow leak that only becomes apparent after months of operation and causes damage to tiles or plaster. A thorough inspection of each connection before sealing the wall is therefore essential and does not require any complex measuring instruments, only careful observation and touch.

Pressure testing and safety limits

After completing the installation, it is necessary to pressure test the system, as PEX piping can withstand much higher values than copper, but the connection must be tested at the same level as the rest of the system. Normally, a test pressure of 1.5 times the operating pressure is recommended, meaning for a system with PN10 this means a test at 15 bar, but for transitions between materials it is advisable to maintain lower values during the first 24 hours. For long runs, such as PEX Pipe - 17x2; PN6-T=+90° C, PN10-T=+60° C - 600m, it is critical to properly dimension the length of the coil and minimize the number of connections to reduce the risk of leaks.

Test the system gradually, first with cold water and then with warm water.
Visually and acoustically check all connections to listen for air hissing.
Be careful with thermal expansion, which can affect the tightness if the system is not sufficiently fixed during sudden heating.
Ensure that pressure gauges are calibrated and show accurate values.

A practical example from our practice shows that a system properly installed and tested at 10 bar operated without problems for ten years, while a system with insufficiently tightened connections had problems already after the first heating period. The difference was in the thoroughness during installation and inspection, not only in the quality of the pipe itself. Therefore, it is always better to invest time in proper testing than to later deal with failures.

Maintenance and long-term system stability

The long-term stability of the transition between copper and PEX piping depends on the quality of the material used and the correct setting of the temperature in the system. If the water in the system is too hot or contains a high level of chlorides, it may cause degradation of sealing materials in the fittings, which requires regular inspection. It is important to monitor for leaks at transition points, especially if the system is located in hard-to-reach areas.

We recommend an annual inspection of the system, especially for older installations where changes in the chemical composition of the water or mechanical stress on the piping may have occurred. Always use original parts and fittings from verified manufacturers, as non-original components may have different tolerances and cause premature failure. Proper maintenance will extend the life of the system by decades and save you a lot of stress.

Principle of function of the expansion ring

The expansion ring (expander ring) is a key element when crimping PEX piping, ensuring that the plastic does not return to its original shape after cooling and maintains constant pressure on the sealing rubber.

Common questions

Can I use standard brass fittings for PEX?

Standard brass fittings are not suitable for direct transition to PEX, as they do not have a special sealing ring and may cause corrosion or leakage. Use only special fittings designed for this type of piping.

How long will the connection between copper and PEX piping last?

With proper installation and use of quality fittings, this connection should last at least 20 to 30 years, which is equivalent to the lifespan of the PEX piping itself.

What should I do if I have already installed a faulty connection?

Immediately turn off the system and relieve the pressure, then replace the connection with the correct type of fitting. If the connection is in concrete, it will be necessary to break part of the plaster to access the repair.

To conclude, connecting PEX piping with copper piping is technically simple if basic principles are followed and the correct components are used. Thoroughness during installation and checking the quality of materials are key to the long-term functionality of your heating system.

Do you have a question about this topic?

Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.