PRESS Inserts 16, 20 and 26 mm - Differences in Usage and Length Calculation

ALPEX PRESS crimping systems have become the gold standard in professional heating, but the correct choice of pipe diameter and accurate calculation of pipe length are critical for the functionality of the entire installation. The difference between 16, 20 and 26 mm is not only in the price of the material, but primarily in hydraulic performance and the spatial requirements for installation.

Differences in cross-section and flow capacity

The pipe diameter determines the maximum possible heat flow that the system can transfer, with 16 mm primarily serving as distribution lines to individual radiators or floor heating in small areas. On the contrary, 20 mm is a universal size for main distribution lines in standard apartments and smaller family homes, where it ensures sufficient pressure even over longer sections.

• 16 mm: Average flow rate approx. 0.5 l/min, suitable for short branches.
• 20 mm: Average flow rate approx. 1.2 l/min, standard for main lines.
• 26 mm: Average flow rate approx. 2.5 l/min, intended for main supply lines and large buildings.

Length calculation and pressure loss

An error in measuring the pipe length before crimping is the most common cause of failure, as the crimped joint requires an exact pipe feed without any gap. For each diameter, there is a different minimum length required to achieve tightness, which depends on the depth of insertion into the fitting.

For longer sections over 10 meters, it is necessary to account for pressure losses, which increase exponentially with length and decrease in pipe size. In practice, we have recorded cases where an inaccurate length calculation for a 16 mm branch led to the pump being unable to sufficiently circulate the coolant through the last radiators in the row.

Critical dimensions for crimping

PRESS technology requires that you always measure the length so that the end of the pipe extends beyond the crimping edge by the required section. This section varies for different diameters, and ignoring it will result in either a leak or damage to the O-ring at the inlet.

• For 16 mm pipe, the required insertion length is approx. 15 mm into the body of the fitting.
• For 20 mm pipe, the required insertion length is approx. 17 mm into the body of the fitting.
• For 26 mm pipe, the required insertion length is approx. 20 mm into the body of the fitting.

Application in practice and specific situations

In real projects, we often use a combination of these three sizes, where 26 mm forms the main ceiling distribution, 20 mm runs into the floor, and 16 mm leads directly to each valve. When using threaded PRESS transitions, it is essential to consider that larger diameters require more robust mounting and greater distance from the wall.

For example, when connecting a massive cast iron radiator, we recommend using PRESS - external thread - 3/4"Mx26, because smaller diameters would not have sufficient cross-section for the high volume of water in older systems. On the contrary, for modern thin-walled panel radiators, PRESS - external thread - 1/2"Mx16 is often sufficient.

Mounting and system stability

The pipe size directly affects the frequency of supports, which must be followed according to standards to prevent the route from bending due to thermal expansion. Wider pipes have greater stiffness, but also higher weight, which requires secure anchoring using special brackets.

To set the correct distance from the wall for wall-mounted panels, we use mounting template for wall mounting of PRESS, which ensures that all fittings will be parallel and easily accessible for service. Incorrect mounting can lead to tension in the joints, which is a risk to long-term tightness.

Maintenance and leak tightness check

The PRESS system is designed to be permanently tight, but regular inspection of the O-rings is necessary with every intervention in the system. If it is necessary to dismantle part of the installation, always check the condition of the rubber, as a worn O-ring can cause a leak even in a properly crimped joint.

Always have spare O-rings for brass PRESS fittings - 26x3 on hand, since their quality is critical to the function of the entire system. An incorrect type of O-ring, for example, used for another material, can lead to rapid degradation and coolant leakage.

Hydraulic balance and diameter regulation

In practice, we often encounter the problem when an installer uses the same diameter for all branches, even though the distance from the heat source is different. In the PRESS system, achieving hydraulic balance is key. If you have a long branch with a 16 mm pipe and a short branch also with 16 mm, the water naturally chooses the shorter path of less resistance, thus "choking" the heating in the long branch.

For this purpose, it is advisable to use a combination of sizes to achieve as balanced hydraulic resistance as possible. For main risers in family homes, it is standard to use 20 mm or 26 mm, while 16 mm should be left only for end sections to radiators. In floor heating, it is even more critical: if you have one loop on 16 mm and another on 20 mm in the same zone, you will have temperature differences in the floor regardless of thermostat settings.

Example of calculating the optimal length for 16 mm vs. 20 mm

When designing distribution lines, we must realize that the maximum recommended length for a single 16 mm branch is usually up to 15 meters (including all bends and connections). Longer routes require an immediate switch to 20 mm. Here is a clear table to help you quickly determine the correct diameter:

Installation procedures and safety margins

Along with the pure calculation of length, there are several factors in practice that must be included in the measurement to avoid unsuccessful crimping. The first is the so-called "heating reserve". Metal pipes expand during operation, and if they are mounted too tight between fixed points, microcracks may appear in the joint after the first heating period.

The second aspect is handling the pipe before crimping. When working with 26 mm pipe, you need to consider that the pipe is stiffer and harder to bend than 16 mm. Therefore, it is necessary to allow a slightly larger space for manual shaping before the final fitting is applied. If you are working in tight spaces where you have to bend the pipe at a 90-degree angle, we recommend using a bending spring to prevent the pipe from flattening and reducing the cross-section, which would subsequently increase hydraulic resistance.

Measurement errors and their impacts

A common mistake among beginners is measuring the length only after reaching the fitting, forgetting that the crimping tool itself has its thickness. If there is not enough space to insert the pipe into the fitting before crimping, you may damage the inner surface of the pipe or the O-ring. Therefore, it is always better to measure 2 to 3 mm more and cut the pipe after checking, rather than leaving it too short.

• Pipe too short: The joint will not be tight, as the pipe does not reach the lower boundary of the crimping zone.
• Pipe too long: The pipe may protrude from the fitting, which will prevent the crimping tool from properly closing the joint, or damage the surrounding insulation.
• Ignoring bends: With each bend, the effective length of the pipe increases. For a 90° bend in a 16 mm pipe, add approximately 10 mm to the total material length.

Bend geometry and minimum radius

When working with the PRESS system, it is critical to respect the minimum allowed bend radius, which increases proportionally with the increasing pipe diameter.

Frequently asked questions

Why can't I push the pipe all the way in?

This is usually due to an incorrect pipe diameter or a damaged edge that prevents free movement. Check whether you are using a pipe that exactly matches the standards for the given diameter.

Can I use 20 mm instead of 26 mm for the main line?

This is physically possible, but with long runs and multiple branches, there will be significant pressure losses and reduced heating performance at the most distant points.

How often do I need to replace O-rings during normal use?

O-rings are not replaced during normal operation, only when disassembling joints or if visible damage occurs during installation.

The correct choice between 16, 20 and 26 mm is key to the energy efficiency and lifespan of your heating system, so never skimp on quality fittings or the accuracy of measurements before crimping.

Do you have a question about this topic?

Struggling to decide or dealing with a specific situation in your home? Write to us - we are happy to help.