Direct coupling seal: why the joint leaks and how to fix it
Leakage in straight fittings: why the connection leaks and how to fix it
A leaking connection in the boiler room, a wet trail under the floor heating or a slight dampness around the fitting, which you notice only during the annual inspection – these are situations we encounter in practice much more often than we would like. A straight fitting is one of the most numerous elements in every heating distribution system, and precisely for this reason it is statistically the most common place where a sealing issue appears. This article will explain in detail where the leak occurs, why it happens and – most importantly – how to reliably eliminate it without unnecessarily disturbing the entire installation.
The topic of leaking connections is broader than it seems at first glance. It is not just about something being "poorly assembled". Behind a leaking connection may be an incorrectly selected size, an unfortunate combination of materials, an unsuitable assembly procedure, long-term pipe tension or simply the wear of the sealing ring. Each of these cases requires a different approach and a different solution. Let us proceed systematically.
How sealing works in a straight fitting – basic principle
Before identifying the cause of the problem, it is necessary to understand how sealing should work in a properly assembled connection. Straight fittings of the Hepworth system operate on the principle of mechanical sealing – when crimping or tightening the thread, the sealing insert (EPDM ring or another elastomer) deforms so that it tightly fills all micro-irregularities on the pipe surface and inside the fitting.
The sealing ring achieves a contact pressure of several tens of bars during crimping – much more than the operating pressure of the heating system (typically 1.5 – 3 bars). Therefore, a properly crimped connection will not leak even a single drop under normal conditions. If it still happens, something in this chain has failed.
Most common causes of leakage in a straight fitting
In practice, I have seen hundreds of cases of leaking connections. The vast majority can be categorized into one of six categories. Almost always, it is a combination of several factors at once.
1. Incorrect pipe or fitting diameter
This is the most common cause – and at the same time the easiest to prevent. If you use a Straight fitting 15 x 15 on a pipe with an outer diameter of 16 mm, the sealing ring will be compressed incorrectly or not at all. The opposite case – an oversized fitting – causes the sealing ring to lack sufficient contact pressure and the medium finds a path through micro-leaks.
In practice, people often confuse the outer diameter of copper and the outer diameter of plastic. A 15 mm copper pipe has an exact outer diameter of 15 mm. A 16 mm PEX/PE-RT plastic pipe has an outer diameter of 16 mm. Hepworth fittings for these two types of pipes are not interchangeable. Straight fitting 16 x 16 is specifically designed for plastic pipes with an outer diameter of 16 mm – and if you mistakenly use it on a copper 15 mm pipe, you will get a connection that will not even pass a basic pressure test.
This topic is discussed in more detail in the article What diameter of a straight fitting do I need for my heating system in the same Knowledge Centre.
2. Incorrect insertion depth of the pipe into the fitting
With crimp fittings, there is a so-called insertion depth – the minimum length the pipe must protrude beyond the edge of the sealing ring for the crimping to be effective. For 15 mm fittings, this depth is typically 18 – 20 mm, for 22 mm it is about 23 – 25 mm, and for 28 mm it is up to 28 – 30 mm.
If the installer does not insert the pipe deep enough – for example, because they are working in a confined space or simply forget to check – the pliers will compress only the body of the fitting without the sealing ring being in the correct position relative to the pipe. The result is a connection that looks fine at first glance but starts to leak as soon as it is pressurized.
3. Damaged or improperly installed sealing ring
The EPDM sealing ring is durable but not indestructible. If the ring falls out, slips or twists during installation (which often happens during fast installation in a cold environment), the seal will not be uniform. The ring may also have a manufacturing defect – a small crack or cut – and in that case, even precise installation will not help.
When working with plastic pipes in winter, the pipes are stiffer and the sealing ring is less flexible. I recommend always inspecting the ring before installation and replacing it if there are any doubts. Replacement rings for Hepworth fittings are commonly available and inexpensive – compared to the consequences of a leak, this precaution is well worth it.
4. Pipe surface – scratches, ovality, dirt
The sealing ring works with microscopic tolerances. If the pipe surface has a deeper scratch (e.g. from a cutting tool that slipped), a groove from a pipe bender, or a thick layer of oxidation (in old copper pipe systems), the seal will leak through this defect.
Another issue is pipe ovality. Plastic pipe coiled for storage tends to retain an elliptical shape rather than a circle after unrolling. When such an oval pipe is inserted into a round fitting, the seal does not seat evenly and "windows" appear where the medium can escape. A solution is to use a pipe calibrator to reshape the diameter, or briefly heat the pipe with a hot air gun before inserting it.
5. Improper crimping – bad pliers or incorrect jaws
The crimped joint is designed for a specific profile of the crimping pliers jaw. If the installer uses pliers from another manufacturer, a different jaw size or worn-out pliers, the geometry of the crimped joint will not be accurate. The fitting may deform asymmetrically, so that the sealing ring achieves the correct contact pressure on one side and leaves a gap on the other.
This problem is insidious – the joint may look "crimped" and the installer considers it complete. The leak will only appear during a pressure test or later during operation. Therefore, it is essential to work exclusively with pliers and jaws certified for the specific system. The topic is discussed in more detail in the article Difference between a crimped and threaded straight coupling: when to use which.
6. Thermal stress and pipe tension
Heating pipes expand with each heating and cooling cycle. Copper expands by approximately 1.2 mm per meter at 70 °C. Plastic PE-RT/PEX pipe expands much more – up to 10 – 15 mm per meter with the same temperature difference. If the piping is not properly secured and expansion joints do not absorb the movement, this tension is transferred directly to the joints, where it acts as a constant tensile force.
As a result, even a perfectly assembled joint will begin to loosen after a year of operation, the sealing ring will fatigue and the joint will start to leak. This is a typical case when a customer calls saying that "the installation was great for a year and a half, and now it suddenly leaks."
How to diagnose where and why a joint is leaking
Before starting the repair, you must precisely determine the cause. Repairing without diagnosis is a game of chance – it is usually a temporary patch that does not solve the problem permanently.
Visual inspection and leak location
The first step is simple: pressurize the system (if it is not already under pressure) and carefully inspect all joints with a clean dry cloth or paper towel. A slight drip may be hard to see, but moisture on a white towel will reveal it immediately. Do not forget to check the bottom of the fitting – water will run down and may mislead you about the actual leak location.
With floor heating circuits hidden in the screed, diagnosis is more difficult. In such cases, a thermal camera (rent it or order a thermographic inspection) helps – moisture in the screed is significantly colder than the surrounding area and appears as a dark spot on the image. Alternatively, you can use a pressure test: fill the system with water, pressurize it to about 6 bar and monitor whether the pressure drops. If it does, there is a leak somewhere.
Determining the cause based on the leak characteristics
The characteristics of the leak can reveal a lot about its cause:
- Leaking immediately after the first filling: most likely an installation error – insufficient insertion depth, damaged sealing ring or incorrect coupling size.
- Leaking that appears after a month – a year of operation: typically thermal stress, sealing ring fatigue or gradual pipe surface corrosion.
- Leaking only at higher temperatures (heating season), not in summer: almost certainly expansion – the pipe is not properly secured.
- Leaking fluid is cloudy or has an odor: likely a faulty seal combined with corrosion or microbial growth in stagnant water.
Repair procedure: step by step
Repairing a leaking straight coupling depends on its type (crimped vs. threaded), accessibility of the location and the cause of the failure. The most important rule is: never permanently seal a leaking joint with silicone, sealant or tape without first removing the cause. Such solutions last only a few days to weeks – and then the problem returns, usually in a worse form.
Repair of Threaded Fitting
The threaded fitting is the simplest to repair. The first step is to drain the section (or the entire system), pressurize the pipe and loosen the nut part. Remove the old sealing material (Teflon, hemp fiber with paste, rubber gasket) completely – with a scraper, cloth or solvent. Never apply new sealing material over the old one!
After cleaning the threads, apply a new Teflon tape (3 – 5 layers in the direction of the thread) or anaerobic (liquid sealant), tighten the thread to the specified torque – for DN 15 it is usually 25 – 35 Nm, for DN 22 about 40 – 50 Nm. Do not over-tighten! A cracked fitting body is much worse than an under-tightened thread.
Repair of Compression Fitting
A compression fitting cannot be opened and closed again after compression. An incorrectly compressed or leaking compression fitting must be removed and replaced with a new one. Procedure:
- Drain the corresponding section of the system and isolate the joint from pressure.
- Cut the pipe as close as possible to the fitting with a hacksaw or special cutter (on both sides).
- Measure how much pipe remains – you must have enough length to insert into the new fitting (minimum 20 – 25 mm on each side depending on the diameter).
- If the length is insufficient, use an extension piece or a combination of fitting and sleeve.
- Insert the new fitting to the correct depth (check with a scribe on the pipe), calibrate the pipe and compress it with the correct pliers.
For larger diameters – for example, for Straight Fitting 22 x 22 or Straight Fitting 28 x 28 – the pliers and jaws are larger and heavier. For these diameters, manual pliers for electro-compression are not recommended – use battery-powered or hydraulic pliers, which ensure even pressure around the entire circumference of the fitting.
Solving the Cause of Thermal Expansion and Contraction
If you have found that the leak is caused by thermal expansion, replacing the fitting alone will not solve the problem. It is necessary to add pipe supports at the specified intervals (for Cu 15 mm max. 1.25 m horizontally, for PEX 16 mm max. 1.0 m), or incorporate an expansion loop into the pipe route. Without this adjustment, the new joint will loosen again after some time.
Concrete Examples from Practice
Case No. 1: New Installation, Dripping During Pressure Test
The installer installed a floor heating distribution with PEX pipe 16 mm. During the pressure test at 6 bar, one joint at the manifold started to drip. After opening the system, it turned out that the pipe was inserted into the fitting only 12 mm instead of the required 20 mm – in a tight position near the wall, the installer simply did not push the hose fully. The joint was replaced, the pipe was inserted correctly, and the second pressure test was completed without problems.
Lesson: Always mark the insertion depth on the pipe with a scribe before compression. Ten seconds of work will save you hours of repair.
Case No. 2: Dripping After One Year of Operation on Cu 22 mm
The customer called saying that after two heating seasons, a joint on the main supply pipe in the boiler room started to drip. The joint was a compression Straight Fitting 22 x 22 on a copper pipe, visually in good condition. After uncovering the pipe, it turned out that the 6-meter section between two fixed supports had no expansion compensator. Copper at operating temperature of 75 °C expanded by about 7 mm – and this force was concentrated exactly into one joint. We added an expansion loop, replaced the joint, and since then there have been no problems.
Case No. 3: Dependence of the Joint on Pipe Calibration
The installer called with a complaint: "The fitting is leaking, it is poorly manufactured." After inspection, it turned out that the PE-RT 16 mm pipe came from a spool with significant ovality – the difference between the maximum and minimum diameter was almost 1.2 mm. Therefore, the sealing did not seat evenly. After calibrating the pipe before installation and using a new fitting, the joint was tight. The fitting had no manufacturing defect.
Case No. 4: Threaded Fitting with Incorrect Sealing
An older distribution with copper pipes 28 mm. The installer used only Teflon tape during a quick repair, where a pasted Teflon tape (Loctite 577 or similar) was required due to the larger thread clearance on the old fitting with worn thread grooves. The joint started to drip after the first pressurization. Solution: cleaning the threads, using pasted sealing + Teflon, tightening to the specified torque. A permanent solution in such a case would be the replacement of the fitting.
Prevention: How to Prevent Leaks from the Start
An experienced technician who invests 20% more time in preparation will save 80% of repair time. Here are specific preventive measures that have proven effective in practice:
- Always check the pipe diameter before purchasing a fitting – do not buy fittings "by eye". Measure the outer diameter with a caliper. For more on how to correctly choose the diameter, read the article How to Choose the Right Straight Fitting for the Pipe: Diameter, Material and Pressure Parameters.
- Calibrate the pipe before installation – especially for plastic pipe from a spool. A calibrator is a cheap tool that saves expensive complaints.
- Mark the insertion depth on the pipe – with a pencil or scribe. This one measure eliminates by far the most common cause of leaks.
- Visually check the sealing before each installation – replace a dislodged, twisted or cracked EPDM ring.
- Use the correct pliers and jaws – do not combine different manufacturers and always check that the jaw is the correct size for the given diameter.
- Secure the pipe correctly – at the specified intervals and with expansion compensators where the pipe route does not allow natural compensation.
- Perform a pressure test before embedding and covering – at least 1 hour at 1.5 times the maximum operating pressure. Detect the problem while the joint is still accessible.
The topics of correct installation and preventive maintenance are also discussed in detail in the articles Step-by-step installation of a direct coupling on copper pipe and Maintenance and inspection of direct couplings in home heating in this Knowledge Center.
Special situations: repairs in confined spaces and hidden piping
Not always do you have the luxury of easy access to a leaking joint. A large portion of problems in practice comes from joints that are embedded in walls, cast in floor screed, or hidden behind tiles. Different rules apply here.
If the joint is hidden in a drywall panel, the situation is relatively simple – drywall can be easily cut open and just as easily restored. It is worse with ceramic tiles or concrete screed. In such cases, always start with a thermographic inspection or acoustic diagnostics to precisely locate the leak and minimize the area to be broken open.
When repairing in confined spaces (behind a radiator, in an installation shaft, between the ceiling and a drywall partition), it is very important to work with compact tools. Some systems allow the use of shortened pliers or adapters in such spaces. If the space does not allow this either, sometimes it is simpler to cut the pipe at an accessible location further from the joint, create a repair bypass through an accessible location, and leave the leaking joint "dry" (without flow).
This solution is a compromise, but in some situations (historic buildings, exceptionally confined shafts), it is the only realistic option without extensive breaking open.
Material aspects: why the combination of metals matters
A subtle but important topic: electrochemical corrosion. If a brass or bronze direct coupling is connected to a steel or galvanized pipe in an environment with soft water and low pH, galvanic corrosion can occur. The result is gradual surface damage in the joint area – and after years of operation, micro-cracks can form through which the medium leaks.
Hepworth fittings are primarily designed for copper and compatible plastics (PEX, PE-RT, PERT/Al/PERT), where this problem does not arise. However, if the system combines different materials (e.g., existing steel distribution pipes + new copper branches), it is necessary to either insert a dielectric separator or adjust the chemical treatment of the water in the system (corrosion inhibitors, pH adjustment). A more detailed discussion of this topic is provided in the article Hepworth direct couplings vs. other brands: what they differ in and what to choose.
Pressure parameters and their influence on tightness
Hepworth direct couplings are dimensioned for a maximum operating pressure of typically 16 bar at 20 °C, or 10 bar at 110 °C (depending on the specific product and diameter). Standard home heating systems operate at a pressure of 1.5 – 2.5 bar – which is a huge reserve.
Problems can arise in the following cases:
- Improperly set expansion tank – if the pre-charge pressure is too low, the system can exceed the set pressure of the safety valve when heated and repeated pressure surges occur.
- Water hammer – for example, when a thermostatic valve is opened quickly or when a pump fails. These short-term pressure peaks can reach 5 – 8 bar and repeated occurrences can cause fatigue of the sealing.
- Overfilled system – if too much water is added to the system without an expansion space, dangerous overpressure can occur when heated.
Therefore, if a leak repeatedly occurs in multiple places without an obvious installation error, check the pressure parameters of the system and the condition of the expansion tank.
Frequently asked questions (FAQ)
Can I tighten or repair a leaking compression joint without replacing it?
No. A compression joint is a one-time use joint – after compression, the deformation of the fitting is permanent and it is not possible to open and reseal the joint. Any attempt to "tighten" will result in either asymmetric deformation or cracking of the fitting. The only correct repair is to cut off the old joint and install a new coupling. Therefore, it is crucial to do the installation correctly the first time.
The joint only leaks when the heating is turned on – not in summer. What does this mean?
It is almost certainly thermal expansion. The pipe elongates when heated, and if it is not properly secured or lacks an expansion compensator, this force is transferred to the joints. In summer, when the system is cold and without pressure, the joint holds. Solution: correct the pipe securing, add expansion bends or compensators, and replace the damaged joint.
Can I use silicone or epoxy mixture as a temporary patch?
As a truly temporary solution (for example, overnight until a replacement part arrives), yes – external sealing can stop dripping for several hours. But never leave it as a permanent solution. Silicone does not transmit pressure, degrades in hot water, and when it fails, the joint can leak much faster than before the "patch." In addition, it can contaminate the heating fluid and damage seals in other parts of the system.
When is it appropriate to choose a 28 x 28 coupling instead of a 22 x 22 one?
The coupling size is selected according to the outer diameter of the pipe you are connecting. If you have a Cu pipe with an outer diameter of 28 mm (typically used for main supply pipes or distribution branches for larger boilers), use the Direct Coupling 28 x 28. For standard radiator branches with Cu 22 mm or Cu 22 mm plastic-copper pipe, the correct one is Direct Coupling 22 x 22. A detailed guide to sizing is available in the article What diameter of direct coupling do I need for my heating system.
Why is it important to use pliers specifically designed for the Hepworth system?
The geometry of the compression profile is different for each system. Pliers or jaws from another manufacturer may look similar, but the groove profile will not exactly match the fitting – the result is incomplete or asymmetric compression. The warranty conditions of the Hepworth system also usually require the use of certified tools. Saving money on tools is not advisable in this case.
Can I connect an old 15 mm copper pipe with a new 16 mm PEX pipe using a direct coupling?
No, not with a direct coupling – Cu 15 mm and PEX 16 mm have different outer diameters and different sealing. For such combinations, special transition fittings are available. More on this scenario is covered in the article Can I connect pipes of different diameters with a direct coupling and also Direct couplings during heating system renovation: most common mistakes when measuring and purchasing.
Conclusion: the tightness of a joint is the result of a correct process, not chance
A direct coupling appears to be a simple component – a piece of metal with a seal that holds two pipes together. But this apparent simplicity often leads to underestimation. In practice, it turns out that most leakage problems are a direct result of someone skipping a step in the process – not calibrating the pipe, not checking the insertion depth, using incorrect pliers, or forgetting to check the securing in a long run.
The good news is that these errors are 100% predictable and 100% avoidable. It is enough to know their causes – which you have now done – and systematically verify before each installation that all conditions for a tight joint are met. A pressure test before covering is the last safety net you should never forget.
If you are unsure about choosing the correct coupling for a specific situation, browse the entire category of direct couplings or other articles in this Knowledge Center – for example, Common questions about Hepworth direct couplings, where you will find answers to the most practical questions from everyday plumbing practice.
Do you have a question about this topic?
Not sure how to proceed or dealing with a specific situation in your home? Write to us – we are happy to help.