How to properly install press fitting on steel pipe without leakage

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How to properly install a press fitting on a steel pipe without leakage

Press fittings on steel pipes today belong to the standard of professional heating installations. Compared to threaded connections, they offer speed, reliability, and long service life – but only if they are properly installed. And it is precisely here that most problems arise. Not because the technology is bad, but because the preparation is underestimated, the wrong tool is used, or the unsuitable pipe is chosen. I have seen dozens of projects where after commissioning and starting the system, leakage began exactly where the installer saved time or did not verify what they were actually pressing.

This article will guide you through the entire process – from selecting the right parts and tools, through pipe preparation, to the actual pressing and joint inspection. If you skip or neglect any phase, you risk leakage, corrosion, or in the worst case, a system failure. Let's proceed step by step.

What is a press fitting and how does the joint principle work

A press fitting (pressure-fit fitting) is a fitting that connects to the pipe by mechanical compression using a special pressing jaw. Inside the fitting, a sealing ring is placed – usually an EPDM O-ring for water-based heating systems, or NBR for gas lines or HNBR for oil-based media. When the pressing jaw compresses the outer profile of the fitting, it deforms the steel surface precisely enough so that the O-ring is firmly clamped between the pipe and the fitting, creating a permanently sealed joint.

It is crucial to understand that the sealing ring itself does not seal the pressure – it is the combination of a mechanically locked profile and the rubber ring that ensures the seal. Therefore, if the pressing is done incorrectly (insufficient, incomplete, with dirt in the groove), the O-ring has no chance of fulfilling its function in the long term.

The advantage of a double O-ring is precisely that if one slightly degrades (aging of the rubber, thermal cycles), the other holds. For heating systems with operating temperatures up to 110 °C and pressures up to 16 bar, this is an absolutely safe solution when installed correctly.

Material and type of pipe: not every steel pipe is suitable

This is one of the most important points that installers underestimate. A press fitting system is not compatible with any steel pipe. You must use a pipe that meets the exact tolerances of the outer diameter and wall thickness. If the pipe is not calibrated and rolled within the system's tolerance range, the pressing will not be correct – neither the jaw nor the O-ring can compensate for dimensional deviations.

For press fitting systems in heating, the following are used:

Black steel pipes according to EN 10255 (formerly DIN 2440/2441) – standard threaded pipes, but only in a calibrated version with an accurate outer diameter
Zinc-coated steel pipes – only if the system explicitly allows it; zinc can shorten the life of O-rings with certain media
Seamless steel pipes according to EN 10216 – for higher pressures and temperatures

Never use old pipe from a system, pipes with rust on the surface, mechanically damaged ends, or pipes whose diameter you have only estimated. Always check the outer diameter with a caliper at several points. Ovality over 0.5 mm at the insertion point into the fitting is a reason to cut off and re-prepare the end.

More about which diameter is suitable for your specific system can be found in the article Diameters of steel pipes and fittings: which dimension do I need for my system in the Knowledge Center.

Tools and their inspection before installation

The pressing tool (hydraulic pressing pliers or electric/battery-powered presser) must be certified for the given fitting system. Most manufacturers of press fitting systems issue a list of approved pressing tools – and this is not a formality. The jaws must have an exact profile corresponding to the profile of the fitting. If you use jaws from another system, even if they "can be mounted", the pressing will not have the correct geometry and the joint will be unreliable.

Before each installation, check:

Condition of the jaws – wear, cracks, deformations
Function of the pressing mechanism – pressure must reach full stroke (most modern pressers have an indicator of completed pressing)
Calibration of the tool – if the presser has not been calibrated for longer than the manufacturer prescribes (typically once every 1–2 years), it must not be used for pressure-loaded joints
Battery (for battery-powered pressers) – a weak battery can cause the pressing to not reach full pressure

Preparation of the pipe end – the most common source of problems

Preparing the end of the pipe is the place where most errors occur. Each step has its justification and each skipped step increases the risk of leakage.

Step 1: Cutting the pipe

Always cut steel pipe for press fitting systems perpendicular to the pipe axis. A slanted cut will cause the pipe to not be inserted sufficiently into the fitting around the entire circumference, which directly affects the sealing quality. Use an orbital saw, a disc saw with a metal blade, or a pipe cutter – never use a grinding disc, which leaves an uneven and thermally affected cut with burrs.

After cutting, always check the straightness of the cut – place a straightedge against the end of the pipe. Any deviation greater than 1° is a reason for a new cut.

Step 2: Removing burrs and chamfering the edge

Every cut – even the highest quality one – leaves burrs on the inner and outer edge of the pipe. The outer burr can damage the O-ring when inserting the pipe into the fitting. The inner burr increases hydraulic resistance and can become loose over time and damage system components (pumps, valves, regulators). Both must be removed.

Use a chamfer (bevel) of less than 15–20° on the outer edge. For the inner edge, a burr remover (so-called Entgrater) is sufficient. This step takes 20 seconds and can save the replacement of a fitting or the repair of a pump.

Step 3: Calibration of the pipe end

Even though the pipe appears round, after cutting it may have a slightly oval profile – especially in thin-walled pipes. If you have a calibrator that is part of the tool kit for the given system, always use it. The calibrator returns the end of the pipe to a round cross-section and at the same time creates an accurate outer chamfer.

Step 4: Marking the insertion depth

Each fitting has a specified minimum insertion depth (insertion depth). This value is stated in the technical data sheet of the fitting and typically ranges from 18–35 mm depending on the diameter. Before inserting, mark this exact value on the pipe with a marker or pencil. After insertion, the line must be visible just at the edge of the fitting – if it disappears inside, the pipe is inserted too deep; if the line is far from the edge of the fitting, it is not inserted enough.

This is one of the most common reasons for leakage – a pipe inserted to 70–80% of the specified depth appears "well seated", but the O-ring is not in the correct position relative to the pipe and after crimping the joint will be leaky.

Cleanliness and O-ring: silent killers of press connections

The O-ring in the fitting is pre-lubricated with a special neutral lubricant at the factory, which facilitates insertion and also helps in seating. Never replace this lubricant with anything else – not silicone spray, not vaseline, not thin oil. These substances can damage the rubber or distort the sealing geometry during crimping.

Before installation, visually inspect each O-ring:

It is not cracked, deformed, or missing from its groove
There is no solid debris on it (metal chips from the cut, construction dust, sand particles)
The color of the rubber corresponds to the medium (EPDM = black or light brown for heating with water)

Chips from the cut are particularly insidious – an inattentive installer cuts the pipe, removes the burrs, but does not remove the chips from the O-ring groove, into which they fall during insertion. After crimping, one point on the O-ring is damaged and the joint starts to leak during the first pressure test or later at operating temperature.

Therefore, always visually inspect the O-ring groove before inserting the pipe into the fitting – ideally with a flashlight. And never crimp the pipe on the construction site without covered ends – even one stone in the groove is enough.

The crimping itself: step by step

Now that the pipe is prepared, the fitting is checked and the tool is ready, we can proceed to crimping. There is also a correct procedure here.

Mounting the jaws on the fitting

Mount the jaws of the crimping tool precisely into the crimping profile of the fitting – this is the part with the protrusion (boss), which is deformed into a "groove" profile after crimping. If you mount the jaws in the wrong position (e.g. shifted by 5 mm), the deformation will occur at the wrong place and the O-ring will not be blocked. The crimping head must be perpendicular to the pipe axis – a skewed mounting causes asymmetric crimping.

Crimping

Start the crimping cycle. Modern electric and battery-powered crimping tools have an automatic cycle that evaluates the completion of crimping on its own. With manual hydraulic pliers, you must press until the jaws are fully closed. Never interrupt crimping in the middle – a partially crimped joint cannot be completed and must be disassembled (which requires the entire fitting to be cut off in the case of press joints).

After crimping, visually inspect the joint profile – the crimping must be even around the entire circumference. If the profile is significantly deeper on one side than on the other, the jaws were not mounted perpendicularly.

Checking the insertion depth after crimping

After crimping, check the insertion depth mark again. If the pipe has shifted during crimping (which can happen with larger diameters) and the mark has moved away from the fitting, the joint must be disassembled (cut off) and the entire process repeated with a new fitting.

Connecting pieces with internal thread: connection to fittings

In practice, press fittings are often combined with fittings (valves, thermostats, manifolds) that have an internal thread. This is where connecting pieces with internal thread come into play – a special type of fitting that has a press connection to the pipe on one side and an internal thread for screwing in the fitting on the other side.

For example, to connect a valve or heating branch to a DN15 pipe (outer diameter 15 mm) with a fitting with a Rp1/2" thread, you use a connecting piece with internal thread 15mm-Rp1/2". If the pipe is 15 mm but the fitting has a larger Rp3/4" thread, you reach for a connecting piece 15mm-Rp3/4".

For a 18 mm pipe with a connection to a Rp1/2" valve, the correct choice is connecting sleeve 18mm-Rp1/2" or for a Rp3/4" thread connecting sleeve 18mm-Rp3/4". For a larger 22 mm pipe connected to a R1" thread, the connecting sleeve 22mm-R1" is available.

When installing the connecting sleeve, the press side is mounted in the same way as any other press fitting – the preparation of the pipe and crimping procedure is identical. The threaded side is sealed in the classic way: Teflon, hemp fiber with paste, or a threaded gasket according to the type of medium and the manufacturer's instructions for the valve. Be careful not to transfer torque to the press side during tightening – the fitting must be firmly held with a wrench, otherwise you risk damaging the freshly crimped joint.

For more information on the correct selection of combinations of pipe diameters and threads, see the article How to choose the correct combination of pipe diameter and fitting thread (e.g. 15mm-Rp1/2" vs. 18mm-Rp3/4") in the Knowledge Center.

Pressure testing after installation

After completing all crimped joints, pressure testing follows. This is not a formality or bureaucratic requirement – it is the only way to detect possible leaks before embedding the pipe into the wall or covering it with insulation.

Standard procedure for pressure testing heating systems:

Fill the system with water and bleed it – open all bleed valves until a continuous stream of water without air bubbles is released
The test pressure is usually 1.5 times the maximum operating pressure, but at least 6 bar for heating systems (according to STN EN 14336 and the manufacturer's recommendations for fittings)
Maintain the pressure for at least 30–60 minutes – longer for larger systems
During the test, visually inspect each joint – water droplets, condensation, or wet traces
Before recording the test, check whether the pressure gauge has lost pressure; a pressure drop is a clear sign of a leak somewhere in the system

If you discover a leak in a press joint – and yes, it can happen even to experienced plumbers – the only correct repair is to cut off the fitting and reinstall it. A press joint cannot be "re-sealed", re-crimped, or repaired in any other way. It is more expensive than repairing a threaded joint, but it is the only reliable solution.

Typical errors from practice and how to avoid them

Over the years of practice and monitoring customer projects, I have repeatedly seen the following errors:

Missing pipe end calibration: The plumber cuts, cleans, and inserts without calibration. At first glance, everything looks fine, but the ovality of the pipe end causes the O-ring to not be evenly compressed around the entire circumference. During thermal expansion during the heating season, the joint will loosen.
Using sheet metal or grinding discs for cutting: Leaves a rough, non-perpendicular cut with heat-affected edges – the walls are "melted" at the edge, and after beveling, hidden burrs remain.
Ignoring the insertion mark: "It looks deep enough" is not sufficient. Without the mark, you have no control and are relying on the manufacturer's tolerances.
Crimping one fitting at a time and moving on: More experienced plumbers sometimes get into the habit of assembling the entire run first and then crimping everything. If someone moves the uncrimped part before crimping (e.g., during bending the run), the insertion depth changes. Best practice: insert, check the mark, and crimp immediately.
Using the wrong O-ring for the medium: Different O-rings (NBR, yellow color) are used for natural gas and compressed air than for water (EPDM, black). Mixing them is dangerous. Always check the fitting marking before installation.
Installation at very low temperatures: The rubber of O-rings loses elasticity at temperatures below –5 °C and can crack or deform during insertion. When working in unheated areas during winter, store fittings in a warm place and allow them to acclimate to +10 °C for at least 2 hours before installation.

Special situations: repairing existing pipe systems and combining with threaded installation

In practice, heating projects often do not involve a completely new pipe system, but rather the renovation or expansion of an existing steel threaded system. This raises the question: can I combine the old threaded system with new press fittings?

Yes, but you must follow the rule: the transition point must be realized via a transition piece – for example, a connecting sleeve with an external or internal thread, where one side is threaded (connecting to the existing system) and the other side is press (continuing with new pipe). Never try to directly crimp a fitting onto old pipe without verifying its diameter and surface condition.

The old steel system may have:

Rust on the outer surface – rust scale that changes the outer diameter and damages the O-ring
Scaling inside, which narrows the diameter (this is not an issue for press fittings, but it is information for future projects)
Mechanical damage or deformations from previous repairs

Always measure the outer diameter of the existing pipe at the location where you plan to make a press joint. If the pipe does not meet the tolerances, the only solution is to cut it off and insert a new piece with precise dimensions.

A more detailed comparison of both technologies can be found in the article Press fittings vs. threaded fittings: when to use which solution in the Knowledge Center.

Pipe expansion and placement of press joints

Steel pipes expand when heated – they stretch along the axis. This expansion must not exert direct tensile force on the press joint. Press fittings are designed for operating pressure and vibrations, but not for axial tensile force that would be caused by a freely inserted pipe section without expansion compensators.

Practical rule: for steel heating in an apartment building, calculate an expansion of about 1 mm per meter of pipe at a temperature change of 80 °C (from 20 °C to 100 °C). For a 10-meter section, this is 10 mm of movement. If you have a straight 10-meter section without a compensator or expansion bend, it can pull the joint out or permanently compress it during full system operation.

Always place press fittings on pipe sections that are not exposed to direct axial force. In expansion areas, use flexible compensators or ensure a classic expansion bend made from bent pipe.

Installation Documentation and Traceability

In a professional environment – during the renovation of apartment buildings, industrial halls or public buildings – press fitting installation documentation is not only an industry practice, but also a legal obligation. Many manufacturers of press fitting systems require proof that a certified and properly calibrated plier with the correct dies was used in the case of a warranty claim.

Implementing an installation record protects you not only against the customer, but also against the insurance company. The record should include:

Type and serial number of the plier used
Type and size of the dies
Date and scope of the installation
Result of the pressure test (pressure, duration, result)
Name of the responsible plumber and his certification for the system

Most Frequently Asked Questions (FAQ)

Can I install a press fitting on a wet pipe or a filled system?

Most manufacturers do not recommend this for standard press fittings, as water in the pipe during pressing can hydrostatically prevent the O-ring from seating properly and hinder even pressing. There are special "wet press" fittings designed for installation under water – but these must be explicitly labeled as such. If you are unsure, always drain the system.

What to do if the pressing was completed and the joint is leaking?

There is no reliable fix for a leaking press joint without completely replacing it. Sealing with thread sealant, tape or re-pressing does not work in the long term. The only correct solution: drain the system, cut out the damaged fitting together with a piece of pipe (at least 5 cm behind the fitting), prepare the new pipe end from scratch (measuring, beveling, calibration) and install a new fitting. It is inconvenient, but it is the only reliable fix.

Which plier do I need and do I need a manufacturer's certification?

For commercial installations, certification from the system manufacturer is mandatory – without it, the warranty is only valid for material defects, not for installation. For home use, it is sufficient to have a plier with the appropriate profile and diameter dies. Most reputable manufacturers (Viega, Geberit, Oventrop and others) have partner programs for plumbers. In any case, it is important to note: a layperson without experience should not press heating pipes – the risk of leakage and property damage is real.

Can I combine press fittings from different manufacturers?

No. Each press fitting system has a patented pressing profile – the shape of the groove and the O-ring pocket are dimensionally specific. Dies from manufacturer A will not press fittings from manufacturer B correctly, even if they look geometrically similar. In addition, compatibility is not certified and not recognized by insurance in the case of a damage incident. Always use a complete system from one manufacturer, from the pipe through the fittings to the tools.

How can I determine if a fitting is properly pressed without a pressure test?

Visual inspection is the first step: the pressing profile must be even around the entire circumference, without jumps, cracks or deformations in the wrong place. Check that the insertion mark is still at the edge of the fitting. Modern pliers have an electronic indicator of a completed cycle – if the plier indicates OK, it is a good sign. But visual inspection and pressure testing are the only guarantees. No "test pressing" or other methods can replace them.

Can a press fitting be removed and reused?

No. A press fitting is a one-time, non-removable joint. After pressing, the fitting is permanently deformed and its O-ring and sealing geometry are set to the exact dimensions of the pipe. Disassembly is only possible by cutting – the fitting is discarded and the pipe must be shortened by 5–8 cm where the fitting was. That is why thorough preparation and inspection before pressing are so important – replacing a fitting takes time and money.

Conclusion: Press installation is a system, not just a tool

Press fitting technology is excellent – fast, reliable, without open flame (which is often required or very practical in many buildings) and extremely durable when installed correctly. But it is precisely this word "correctly" that carries all the weight. The technology does not tolerate shortcuts: every skipped step in pipe preparation, every unverified insertion depth or every plier with the wrong profile will show up – sometimes during the pressure test, sometimes only after the first heating season.

If you are doing press installation for the first time, proceed methodically: tool preparation, pipe preparation (cutting, deburring, calibration, insertion mark), O-ring check, insertion, insertion mark check, pressing, visual inspection, pressure test. Each of these steps exists for a reason you have now read about.

Other related topics that will help you with professional planning of the piping system can be found in our Knowledge Center: Common faults in press fittings on steel pipes and how to fix them, Corrosion of steel pipes and fittings: causes, prevention and solutions and Maintenance and inspection of steel pipes with press fittings in heating.

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.