Sealing of Joints: Common Problems and How to Avoid Them
Sealing of fittings in heating distribution systems: an introduction to the topic
Anyone who has been working in heating installation for more than a few years knows that a leaking fitting is one of the most common problems encountered during the maintenance of existing distribution systems. It is not only an issue of cheap or unsuitable materials – leaks can also appear in systems equipped with high-quality fittings, if the installation was not done correctly, if the size selection was underestimated, or if operating conditions have changed over the years. This article focuses specifically on direct reduced fittings, i.e., fittings that connect two pipes of different diameters, but most of the principles apply to heating fittings in general.
Leaks can take various forms: from slow, almost invisible seepage that only becomes apparent after several weeks as a water stain on the plaster, to a clear water leak that triggers the emergency shut-off alarm. In both cases, the consequences are unpleasant – damage to the building structure, increased water or antifreeze consumption in the system, localized pipe corrosion, and in the worst case, a failure requiring expensive repairs. Preventing them is possible through a combination of correct fitting selection, precise installation, and regular inspection.
Why leaks occur – main causes
A leak is never random. Behind every leaking fitting there is a specific cause or a combination of several factors. From practical experience, several recurring scenarios can be identified:
1. Incorrect fitting size compared to pipe diameter
This is by far the most common mistake, especially when making additional modifications or additions to existing distribution systems. It often happens that the installer is unsure of the actual outer diameter of the pipe and estimates the size "by eye". Copper pipes in sizes 15, 22, 28 mm have an outer diameter exactly matching the marking (in mm), but for example, plastic pipes (PE-RT, PEX) may differ. If the pipe is even slightly thinner than the fitting expects, the fitting may appear to fit at first glance, but under operating pressure and thermal expansion, a micro-leak can occur.
In the case of reduced fittings, the situation is even more complicated, as you are working with two different diameters at the same time – for example, when transitioning from 22 mm to 15 mm or from 28 mm to 22 mm. Each inserted end must fit precisely. If even one of them fits poorly, the fitting will not seal. Therefore, it is always necessary to verify the actual outer diameter of both pipes before ordering or installing the fitting. More about the correct size selection can also be found in the related article Dimensions of direct reduced fittings: which size do I need.
2. Insufficient pipe insertion into the fitting
Press-fit and compression fittings require a precisely defined depth of pipe insertion. If the pipe is not inserted deeply enough, the O-ring or sealing gasket does not fully surround the pipe, and a leak occurs at the point of insufficient insertion. This often happens when the installer is working in confined conditions, has insufficient space to maneuver, or simply does not consider it necessary to mark the pipe before insertion. In fact, all it takes is a marker and a measuring tape – before insertion, the depth of insertion is marked on the pipe according to the manufacturer's table, and after insertion, it is checked whether the mark has disappeared inside the fitting.
3. Damage to the sealing ring during installation
O-rings are made of soft elastomeric materials (EPDM, NBR) and are sensitive to mechanical damage. If the pipe has a sharp edge (not beveled before installation), it can cut or stretch the sealing ring to one side during insertion. Similarly, installation at extremely low temperatures, when the elastomer is stiff and less flexible, can damage the sealing ring. On construction sites where piping is installed in unheated spaces during the winter, it is advisable to pre-temper the sealing rings (store them in a warm place) before installation.
4. Excessive or insufficient pressing force
Press-fit fittings require precise pressing using certified tools with a calibrated jaw for the given type and size. Under-pressing – when the press does not complete the full stroke – leaves the sealing ring in an insufficiently compressed position. Over-pressing, on the other hand, can deform the body of the fitting or damage the sealing ring. With compression fittings, the problem is similar: too loose locking nuts do not press the sealing ring sufficiently, while overly tightened nuts deform the sealing ring or cause the fitting body to crack (typically with plastic fittings).
5. Thermal expansion and mechanical stress
Heating systems operate with temperatures ranging from about 20 °C (cold water after the system is turned off) to 70–90 °C (operating condition). A copper pipe of 10 meters in length will elongate by about 10 mm when the temperature rises by 60 °C. If the piping is not properly supported with expansion loops or sliding supports, these movements mechanically load the fittings. Repeated movement at the connection point can gradually loosen the sealing ring or cause material fatigue – even if the installation was originally flawless. The problem occurs most often on long straight sections without compensation and in places where the pipe is rigidly attached to the wall or ceiling without sufficient sliding.
6. Corrosion and scaling
In systems with poor water quality (high oxygen content, unsuitable pH, aggressive salts), gradual corrosion of the sealing surfaces occurs. Corrosion narrows or deforms the contact area and reduces the sealing ability of the fitting. Scaling (limescale) can settle directly on the sealing ring, deform it, and cause seepage. Therefore, regular inspection and replacement of sealing rings is recommended in hard water areas, or alternatively, water treatment before it enters the system.
Concrete examples from practice: when it actually happened
This is not theory – these scenarios are repeated in dozens of installations during service interventions every year.
Case 1 – Transition 22 × 15 in an apartment unit: The customer called due to a wet stain on the drywall ceiling in the bathroom. After opening the inspection hatch, it turned out that the direct reduced coupling 22 x 15 at the branch to the rising pipe of the bathroom drain was leaking at the insertion point of the smaller pipe. During installation, the plumber only beveled the larger pipe, while the smaller 15 mm pipe had a sharp edge from the cutting tool. When inserted, this edge cut the O-ring. The leak manifested only after three years, when corrosion expanded the micro-crack. The solution was to replace the coupling and bevel both pipe ends properly.
Case 2 – Dilation on a long section: A single-family house with floor heating and radiators. A direct transition from 28 mm copper pipe to 22 mm was installed using a direct reduced coupling 28 x 22. The pipe was fixed in two places without any sliding. With each heating cycle, the pipe expanded and contracted. After two seasons, the sealing in the coupling was torn out of the groove. The result: moisture in the corner of the wall and partial demolition of the tiling. The solution was to add an expansion loop before the coupling and replace the sealing.
Case 3 – Hard water and scaling: An apartment building in an area with hard water (total hardness over 3.5 mmol/l). After six years of operation, micro-leaks appeared on several compression fittings. The seals were literally overgrown with a layer of limescale 1–2 mm thick, which deformed their shape and prevented even seating. The solution was to replace all seals and install a water softener. The costs would have been negligible if seal maintenance had been performed every three years.
How to prevent leakage: a systematic approach
Preventing leakage is not a one-time action – it is a combination of correct decisions during selection, installation, and during operation. Here is a detailed procedure:
Selecting the correct coupling and dimensions
Always measure the outer diameter of both pipes with a caliper before ordering the coupling, not estimating based on nominal dimensions. For older systems, also verify the pipe material – steel, copper, or plastic pipes can have different outer diameters for the same nominal size. Direct reduced couplings are available in combinations such as 22 × 15 mm or 28 × 22 mm; ensure that both numbers exactly match the outer diameters of your pipes. More detailed information on this topic can be found in the article How to choose the correct direct reduced coupling for heating.
Also pay attention to the coupling material in relation to the pipe material. Copper couplings are joined with copper pipes by soldering, press fittings are intended for pipes with certified diameters. Compression sealing works within a wider range of tolerances, but it also has its limits. More about material combinations and their pressure parameters can be found in the article Material and pressure parameters of direct reduced couplings for central heating.
Preparing the pipe before installation
This is a step that is most often skipped in practice, yet it is crucial. Proper preparation includes:
- Cutting: always cut the pipe with a pipe cutter, not with a saw (saw teeth leave an uneven edge). If you don't have a cutter, the edge after sawing must be filed to remove burrs.
- Beveling: bevel the outer edge of the pipe at an angle of 15–20°. This seemingly small step prevents the sharp edge from damaging the O-ring or sealing during insertion.
- Removing dirt: clean the end of the pipe from grease, dust, and burrs. A greasy surface makes it difficult to evenly seat the sealing.
- Marking insertion depth: mark the insertion depth on the pipe with a marker. After insertion, check that the mark has disappeared inside the fitting and is evenly covered around the entire circumference.
Installation of the coupling
The installation procedure depends on the type of coupling (press, compression, soldered), but some rules apply universally. A step-by-step guide can be found in the article Installation of a direct reduced coupling step by step. For the most common types, the following applies:
- Press (press-fitting): use certified pressing tools exclusively for the system. Check that the tool jaws match the fitting size. One pressing stroke must be complete – never stop the press mid-stroke. After pressing, visually check the deformation of the profile (correct pressing deformation is even around the entire circumference).
- Compression: tighten the nut with a wrench, not by hand. The recommended tightening is usually 1.5–2 turns after manual tightening. Do not over-tighten – cracking of plastic fittings due to over-tightening is a common problem.
- Soldered: both joined surfaces must be clean and covered with flux. The soldering temperature must be sufficient for capillary penetration of the solder around the entire circumference.
Pressure test after installation
After completing the installation and before closing the piping into the structure (pouring into the floor screed, sealing installation shafts, mounting drop ceilings), a pressure test is mandatory. For heating systems, it is standard to perform the test at 1.5 times the maximum operating pressure, but at least 6 bar, for a minimum of 30–60 minutes. During the test, all connections are visually inspected. Only connections that have passed the pressure test may be covered. This seemingly time-consuming formality will save enormous costs in the future.
Regular inspection and maintenance
Even properly installed connections require regular attention. The recommended inspection interval is 1–2 years, ideally before the heating season. The inspection includes:
- Visual inspection – traces of scale or rust are the first signs of a slow leak
- Checking the tightening torque on compression fittings
- Checking the pH and hardness of the water in the system
- Checking the expansion tank and system pressure (a pressure drop may indicate a micro-leak)
More detailed procedures for regular inspections can be found in the article Maintenance and inspection of fittings in heating systems.
Pressure and temperature parameters – what to watch out for
Each fitting has a maximum operating pressure and a maximum temperature at which it maintains its seal. Typical operating temperatures for central heating are 70–90 °C and pressures of 2–4 bar. Most quality fittings (including products from the Hepworth series) are dimensioned for PN 16 (16 bar) at 20 °C, which means a safe reserve for standard heating applications. However, as the temperature increases, the maximum operating pressure decreases. For example, a fitting with PN 16 at 20 °C may have an allowable pressure of only 6–10 bar at 90 °C – which is still sufficient, but it is important to know this if the system operates at higher pressures (e.g., in district heating or industrial applications).
Compression fittings with rubber seals (EPDM) are resistant to standard heating media including mixtures with additives. NBR seals are more suitable for oils and some special media. Always use a seal intended for the specific medium – this seemingly minor detail can cause premature degradation and leakage. An overview of materials and parameters is provided in the article Materials and pressure parameters of direct reduced fittings for central heating.
When a leak appears: how to react properly
Any detected leak always requires a quick response. The steps are as follows:
- Shutting down the system: close the appropriate shut-off valves and determine the extent of the leak. If the leak is at multiple locations, it is necessary to shut down the entire circuit.
- Identifying the cause: a leak is not repaired without determining the cause. If you simply tighten the nut without eliminating the cause, the problem will return.
- Replacing the seal or the entire fitting: in most cases, replacing the seal and nut is sufficient for compression fittings. For crimped fittings, the entire fitting must be replaced, as crimping is an irreversible process. For soldered fittings, it is necessary to open the joint, clean the surfaces, apply new solder and flux, and resolder.
- Pressure test after repair: even after a local repair, perform a short pressure test (at least 15–30 minutes at operating pressure) before closing the structure.
- Recording in the service log: document each leak, its cause, and the method of repair. Repeated leaks in the same section indicate a systemic issue (e.g., poor water quality, incorrect expansion).
Special situations: combining different materials and systems
In practice, we often encounter the need to connect piping made of different materials – for example, an older steel pipe with a new copper or plastic one. When combining materials, the risk of leakage increases for several reasons: different thermal expansion of materials, different tolerances of outer diameters, and the risk of galvanic corrosion when dissimilar metals (e.g., copper and steel without a protective element) are in contact. In such cases, it is necessary to use transition fittings specifically designed for the particular combination of materials, or to install a dielectric separator. A more detailed look at this issue is provided in the article How to properly connect pipes of different diameters without water leakage, as well as in the article When to use a reduced fitting and when to use another type of reducing fitting, where alternative fittings (reducers, transitions) are discussed.
When comparing different manufacturers, we also recommend paying attention to whether the fittings are certified for your system. Different manufacturers use slightly different sealing groove profiles or different hardness levels of O-rings, which can cause incompatibility. Connecting fittings from different systems without verifying compatibility is one of the underestimated causes of leaks. More on this topic Hepworth fittings vs. other brands: what to watch out for when selecting can be found in the eponymous article in the knowledge center.
Most Frequently Asked Questions (FAQ)
Why does my compression fitting leak immediately after installation, even though I tightened it properly?
Over-tightening is just as problematic as under-tightening. With plastic compression fittings, cracking of the fitting or deformation of the gasket occurs precisely due to over-tightening. The correct procedure is to tighten by hand until resistance is felt, then another 1.5–2 turns with a wrench. If the fitting still leaks after proper tightening, check whether the pipe is inserted only shallowly and whether the gasket is damaged or has popped out of the groove.
Can I use only a new gasket to repair a leaking fitting, or must I replace the entire fitting?
This depends on the type of fitting. With compression fittings, replacing only the gasket (or also the nut) is a standard procedure – provided the body of the fitting is not corroded or cracked. With press-fit (pressed-in) fittings, it is not possible to replace the gasket – the entire fitting must be cut out and replaced with a new one. With soldered fittings, the joint must be opened, cleaned, and re-soldered.
How long do gaskets in heating system fittings last?
The lifespan of gaskets depends on the quality of the water, thermal cycles, and the quality of the gasket itself. EPDM gaskets in standard heating systems typically last 15–25 years under normal conditions. In systems with hard water or aggressive pH levels, the lifespan may be significantly shorter – 5–10 years. Regular annual inspections and preventive gasket replacement in older installations are more economical than dealing with a failure and subsequent repairs.
Can a pressure test detect every potential leak?
A pressure test detects current leaks and major installation errors. It will not, however, detect leaks that develop after a longer period of operation due to expansion, corrosion, or material aging. Therefore, a successful pressure test after installation is not sufficient – the system also requires regular visual inspections during operation.
What should I do if a fitting leaks in an inaccessible location (embedded in the floor, behind tiling)?
This is unfortunately a situation that requires mechanical access to the location – there is no safe long-term alternative to sealing the leak with sealing compounds in pressurized heating system lines. First, shut down the circuit. Second, locate the exact leak location (using a thermal camera, moisture meter). Third, locally expose the fitting, repair it, and insulate it. The only way to avoid this situation is to perform a pressure test before closing the structure.
Can I use 22×15 and 28×22 fittings in solar systems or systems with antifreeze mixtures?
Most high-quality fittings with EPDM gaskets are compatible with heating system antifreeze mixtures based on ethylene glycol or propylene glycol up to a concentration of approximately 50%. However, always check the specifications of the specific product – some cheaper gaskets (especially NBR) react unfavourably with glycol mixtures and degrade more quickly. For solar systems with higher operating temperatures, also verify the maximum temperature resistance of the fitting.
Conclusion: Tightness is not a coincidence, it is the result of systematic care
Tightness of heating system piping is not a matter of luck or just the quality of the materials used. It is a predictable result of correct selection, precise installation, and regular inspection. From practice, we know that the vast majority of leak cases were preventable – it would have been enough to use the correct size of fitting, chamfer the pipe edge, mark the insertion depth, or perform a pressure test before closing the structure.
When selecting specific products, we recommend looking at the full range of direct reduced fittings – for the most common installation situations, sizes such as direct reduced fitting 22 x 15 for the transition from the distribution line to the radiator connection, or direct reduced fitting 28 x 22 for typical vertical lines in apartment buildings are available. In case of further questions about selecting the appropriate reducing fitting for your specific situation, the article Common Questions about Direct Reduced Fittings for Heating will also help you.
Investing in the right fitting and thorough installation pays off many times over in the form of peace of mind, lower service costs, and a long lifespan of the entire system.
Do you have a question about this topic?
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