how to choose the correct diameter of crimp fitting for PEX pipe
When selecting the correct crimp fitting for PEX pipe, it is not only the material brand that is decisive, but primarily the exact matching of the pipe's outer diameter with the coupling's internal dimension. An incorrect ratio between the pipe and the fitting leads to inevitable sealing failure, which can cause expensive property damage in a pressurized system.
Distinguishing standard PEX pipe dimensions
In practice, we most commonly encounter two main sizes, which account for 90% of the market: pipe with an outer diameter of 17 mm and 20 mm, with 17 mm being the standard for regular radiator branching and hot water supply in apartments. Smaller pipes such as 14 mm or larger 25 mm are specific and require special attention when ordering components to avoid installation errors.
The key rule is that the internal diameter of the crimping part must exactly match the outer diameter of your pipe, not its nominal diameter DN, which is only an approximate equivalent. For a pipe marked as PEX 17 mm, look for a fitting marked 17 mm, because if you use a 20 mm fitting, the pipe will be loose and the crimping ring will not have sufficient grip to create a seal.
Why neither a gap nor excessive pressure is acceptable
If the fitting is one size larger than the pipe, for example, a 17 mm pipe into a 20 mm fitting, the crimping tool will not be able to create the necessary pressure on the metal ring, resulting in a permanent gap and leakage. Conversely, if you try to fit a larger pipe into a smaller fitting, the pipe will be damaged, the plastic will deform, and it will eventually crack during the first heating of the system.
- Incorrect size will cause the crimping ring to not be damaged, but it will not tighten around the pipe.
- With excessive pressure, the pipe will "tear" directly at the joint, which is an irreparable error.
- A system with a gap will leak even after years of operation, as the seal is not functional.
Importance of manufacturer and material tolerances
Different manufacturers of PEX pipes (Wavin, Uponor, Rehau) may have slightly different production tolerances, so it is always better to use fittings of the same brand as the pipe itself, or to check compatibility in technical data sheets. If you use universal crimp fittings, you must realize that their internal space is designed for an average value of a given diameter, which may not be sufficient for extremely thin-walled or thick-walled pipes.
In practice, I have seen cases where a customer mixed a pipe of one brand with fittings of another and eventually had sealing issues, because the internal surface of the pipe was differently processed. Therefore, when ordering, check whether your selected Elbow PRESS 90° - for PEX pipe - 17x17 is intended for the specific type of pipe you have in stock.
Interpreting labels on the packaging and fitting
The label on the fitting is key: the number before the slash indicates the diameter of the pipe the fitting is intended to accept, and the number after the slash indicates either the second pipe diameter (for T-pieces and elbows) or the type of threaded connection (for inserts). For the average customer, it is most important that the first number always matches their pipe.
When purchasing T-piece PRESS - for PEX pipe - 17x1/2"Fx17, note that the two numbers 17 mean that both arms are intended for PEX pipe with a diameter of 17 mm, while 1/2" indicates the external thread for connecting another type of fitting. This is a common confusing situation that leads to errors if the customer only follows the threaded part.
Practical procedure for measuring and checking
Before you start installation, always take a caliper and measure the outer diameter of the pipe at two points to rule out ovality or local deformations caused by transport. If the measured value is 17.2 mm instead of a clean 17 mm, it still belongs to the 17 mm category, but you cannot use a 14 mm fitting.
- Measure the pipe at the point where it will be crimped, not at the end, where it may have been damaged.
- Compare the result with the manufacturer's table, where tolerances for the given diameter are listed.
- Try to slide the pipe onto the fitting without crimping; it should fit snugly with slight resistance, not float freely.
This simple step will save you hours of searching for the cause of a leak later. If the pipe is 17 mm, look for Insert PRESS - for PEX pipe - 17x17 in our range, which guarantees the correct size for this type of pipe.
Role of the crimp ring and pressure deformation
The compression ring is not just a passive element, but actively changes its shape under the pressure of the tool to surround the outer surface of the pipe. If the fitting diameter is too large, the ring "collapses" into the center and creates only a small point contact instead of an even cylindrical compression, which leads to leakage.
Frequently asked questions
Can I use a 20 mm fitting on a 17 mm pipe?
No, it is technically impossible to achieve a seal, because the compression ring cannot reach the pipe and the system will leak.
How can I tell if a fitting is for PEX or PE-RT?
Most modern compression fittings are universal for both materials, but always check the technical data sheet to confirm it is not specified for only one type of polymer.
What if my pipe has a diameter of 17.5 mm?
Check the manufacturer's tolerances, but if the value is outside the standard, the pipe should likely be rejected, as the compression system does not work with half-tolerances.
Choosing the correct diameter is the foundation of the long-term functionality of the entire heating system, so never save on quality fittings and always adhere to exact dimensions. A properly selected and installed compression joint will then serve you for years without the need for intervention, which is the goal of every professional installation.
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