How to choose the correct diameter for compression fittings

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Choosing the correct diameter for press fittings is not only about mathematical calculations, but primarily about the safety and longevity of the entire heating system. An incorrectly selected pipe size leads to increased pressure loss, noise in the system, or even damage to pumps and radiators.

Difference between outer and inner diameter

In the ALPEX PRESS system, the values of the outer diameter of the pipe are used for designation (e.g. 16 mm, 20 mm, 26 mm), but the actual pressing process depends on the exact wall thickness and inner diameter. The fitting must be designed so that the sealing O-ring (such as O-ring - for PRESS brass fittings - 26x3) perfectly surrounds exactly this specific material layer without deformation.

Many plumbers make the mistake of thinking that if they have a pipe with an outer diameter of 20 mm, any "twenty" will do, while there are different types of walls (S1, S2, S3). Pressing technology requires absolute compatibility between the fitting profile and the pipe profile, because even a millimeter deviation will cause a water leak under pressure.

ALPEX S1 has a thinner wall and a smaller inner diameter for the same outer dimension.
ALPEX S2 and S3 have specific thicknesses that affect the flow of liquid.
Pressing tools are calibrated precisely to these standards, not to general measurements.

Flow calculation and pressure loss in the system

When selecting the diameter, it is critical to consider not only the length of the pipe, but also the volume flow required to heat the space. If you choose a too narrow pipe, the water will flow too fast, causing turbulence, noise and high pressure loss, which the pump must compensate for.

Experience shows that for longer branches (> 30 meters), you should choose a diameter one size larger than you would use for short connections. For example, when supplying more than five radiators in one loop, it is advisable to switch from 16 mm to 20 mm to avoid problems with uneven heating of distant bodies.

For standard apartment heating, a diameter of 16 mm is often sufficient for individual loops.
For family homes and larger areas, a combination of 20 mm and 26 mm is standard.
Press-through fittings with external thread, such as press-through 1/2"Mx16, must match the diameter of the input pipe, otherwise hydraulic resistance will occur.

Integration with other components and radiators

System compatibility means that the pipe diameter must logically continue from the boiler outputs and the input into the radiators. We often encounter situations where the plumber uses a strong 26 mm pipe, but connects it to a small radiator with a 15 mm output without proper reduction, which creates an inefficient transition.

Quality installation uses precise reducing elements, such as press-through 3/4"Mx20 or press-through 3/4"Mx26, which ensure a smooth flow transition without vortex formation. These components are key when transitioning from the main pipe to side branches to consumers.

An incorrect diameter in combination with an incorrect shape of the transition can lead to the fact that the water will not reach sufficient pressure to open the thermostatic heads, which causes the radiators to heat only partially. Therefore, it is necessary to always check the technical documentation of the radiator and adjust the pipe diameter accordingly.

Always start from the heat source and proceed towards the most distant point.
Make sure that the transitions are intended for a specific pipe material (ALPEX).
Use mounting templates for proper mounting of brackets to prevent mechanical stress on the connections.

Technological limits and maximum distances

Each diameter has its physical limits that determine the maximum distance to which the pipe can be stretched without significant performance loss. For the ALPEX PRESS system, the narrower the pipe, the shorter one loop can be before the effect of thermal loss and pressure drop becomes apparent.

In practice, we have seen cases where the plumber failed to heat a floor of a family house because they used a 16 mm pipe for a distance of 40 meters. The correct choice in this case would have been 20 mm or 26 mm, which would reduce the flow speed and allow even heat distribution.

These data are approximate and also depend on the insulation properties of the pipe and the temperature difference between the supply and return. However, for long routes, it is always safer to choose a larger diameter than to risk an undersized system that will be non-functional.

Long runs require a larger diameter to maintain hydraulic potential.
Temperature drop (difference between supply and return) affects the optimal diameter.
Always consider future system expansion when planning new piping layouts.

Safety and Leak Prevention

Incorrect diameter is one of the main causes of leaks in press fittings, because the crimping pliers may not achieve the correct compression level if the fitting is too small or too large in relation to the pipe. Crimping is a process that works only with precise component alignment.

Therefore, it is essential to use only certified fittings and to check their condition before installation. If you have any doubts about the quality of O-rings, always replace them with new ones, for example O-ring for PRESS brass fittings - 26x3, which ensures sealing for the given diameter. Neglecting this step can lead to costly property damage.

Regular inspection and maintenance of the crimped piping, including the replacement of worn parts, is key to the long-term functionality of the system. The risk of leaks increases over time if incorrect components were used or if the diameter was incompatible with the pipe.

Always verify that the fitting is compatible with the specific brand and type of pipe.
Use high-quality crimping tools calibrated for the given diameter.
Always perform a pressure test after installation before embedding in plaster.

Frequently Asked Questions

Can I use a smaller fitting on a larger pipe?

No, you should never attempt to install a smaller fitting on a larger pipe, as this will immediately damage the O-ring and cause a pressure leak.

How can I determine if I have chosen the correct diameter for my project?

We recommend consulting a specialist for a hydraulic calculation that takes into account thermal load, pipe length, and the number of appliances for an optimal diameter.

What should I do if I have mixed diameters in my system?

Mixed diameters are acceptable if properly sized reducing couplings are used to smoothly connect different sections without causing hydraulic resistance or turbulence.

Choosing the correct diameter for press fittings is the foundation for the safe and efficient operation of your heating system, so you should not hesitate to consult with experts if you have any uncertainties. Adherence to technical standards and the use of high-quality components will ensure many years of trouble-free operation.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.