Why do heating pipe faults occur in the first place?
Anyone who has worked in heating system installations for more than a few years knows that leaks and faults on pipework are not a rarity – they are part of reality. A heating system operates under extremely demanding conditions: temperature cycles from cold to nearly one hundred degrees, overpressures of around 1–3 bar, a chemically aggressive medium (water with or without inhibitors), mechanical stresses from expansion... All of this simply shows up somewhere over years and decades of operation.
This page is not about what we sell. It's about what actually happens in the pipework and what to do about it before a small leak turns into a wet ceiling, moldy walls, or a failure of the entire heating circuit. We will systematically go through all common types of faults – by pipe material, location, and cause – and for each one explain how to correctly identify and repair it.
Topics such as choosing the right pipe, comparing copper and multilayer pipe, or what the values PN10 and T=95°C mean, are covered in detail in other articles of this Knowledge Center (for example How to choose the right heating pipe: copper, multilayer, or plastic? or Pipe temperature and pressure: what do PN10, T=70°C and T=95°C mean in practice?). Here we focus purely on diagnosing and repairing existing problems.
Map of typical leak locations in the heating system
Before we start discussing individual causes, it is useful to know where leaks most commonly occur. In practice, the vast majority of leaks do not originate from the pipe itself (i.e. from its straight sections), but from these locations:
- Joints and fittings – compression fittings, press joints, threaded adapters
- Bend locations – especially on plastic and multilayer pipes, where a cold bend was made with too small a radius
- Material transitions – copper to plastic, steel to copper, various zinc or brass fittings
- Seals and O-rings – aging of rubber or silicone after long-term operation
- Radiator connections – thermostatic heads, drain valves, angle valves
- Moving components – pumps, control valves, dirt separators
If you're searching for a leak and don't know where to start, begin with these locations. A visual inspection with dry hands and a paper towel (not a cloth!) is a reliable first step – water leaves a trace.
Leaks at compression fittings: the most common problem in practice
A compression fitting (EK fitting) is the most popular way to join pipes without the need for soldering or pressing. It is used massively – when connecting copper pipe to valves, manifolds, pumps, and tanks. That's exactly why it's also the most common leak location.
Causes of leaks at compression fittings
1. Under-tightened nut – by far the most common cause. The installer tightened the fitting "by hand," without a torque wrench. During the first heating cycle, the pipe expanded, the seal deformed slightly, and a dripping joint resulted. Solution: tighten the nut by another sixth of a turn (about 10–15°) with the system cold, then test.
2. Over-tightened nut – less common, but possible. Excessively tight tightening of the compression ring (ferrule) on soft copper pipe causes deformation. The resulting irregularity then prevents the seal from seating evenly. Sometimes replacing the ferrule and re-tightening from scratch helps; other times the entire section needs to be cut out and a new fitting installed.
3. Damaged ferrule or nut – the ferrule (compression ring) gets damaged or deformed when repeatedly disassembled. A strict rule applies: any compression fitting that is disassembled must receive a new ferrule. No "I'll put it back, it'll hold." It won't.
4. Wrong pipe diameter vs. fitting – a 15 mm fitting on 15 mm outer diameter pipe is straightforward. But for multilayer pipe (PEX-Al-PEX), the wall has different dimensions than copper of the same outer diameter. In that case, you need either a special fitting designed for PB/multilayer pipe, or an adapter. For example, the 15 x 1 EK compression fitting for copper or PB pipe is designed for exactly both types.
5. Contaminated sealing surface – shavings, metal filings, or dirt from the pipe get caught on the sealing surface in front of the ferrule. The result is a micro-leak that cannot be fixed by re-tightening, only by disassembly and cleaning.
Step-by-step repair of a leak at a compression fitting
The repair procedure is relatively straightforward, but the correct steps must be followed. First, drain the system (at least partially, to a level below the leak point), and place absorbent material under the fitting. Disconnect the nut by turning counterclockwise. Inspect the ferrule – if it is cracked, deformed, or you have already reassembled it once, replace it with a new one. Check the pipe end: it must be straight (a perpendicular cut), without dents or shavings. Apply a thin film of sealing paste or PTFE tape to the sealing surface (not on the thread!). Insert the pipe fully, tighten the nut by hand, then add 1 and 1/4 turns with a wrench. Refill the system and check.
For quality copper pipework, we recommend using proven components such as the compression fitting for copper pipe 15x1 EK – these parts are made of brass, and their ferrules retain their shape even after long-term operation at higher temperatures.
Faults on multilayer pipe: micro-cracks, delamination, and poor bends
Multilayer pipe (PEX-Al-PEX, or commercially IVAR Turatec and similar) has earned its place in every modern boiler room and underfloor heating system over the last two decades. It is flexible, long-lasting, and does not corrode. Nevertheless, it has its own specific types of faults.
Micro-cracks from improper bending
Multilayer pipe can be bent by hand or with a spring bender. The minimum bend radius depends on the diameter: for 16x2 mm multilayer pipe, the minimum bend radius is usually around 5–6× the outer diameter, i.e. about 80–100 mm. For 20x2 mm multilayer pipe, it is at least 100–120 mm.
When the pipe is bent to a shorter radius – for example while hot, or with excessive force – the aluminum layer breaks (crumples) at the bend point. This may not be immediately visible from the outside, but with cyclic heating, a micro-crack forms at that point, through which water eventually starts to leak. Characteristic symptom: a damp spot on the pipe that is not at a fitting.
The solution is always the same: cut out the affected section and insert a replacement piece with two new joints. Attempting a patch or squeezing with a hose clamp is a short-term and dangerous fix – at 80–95°C, the pressure at the cracked point increases.
Layer delamination
Delamination is the separation of layers in the PEX-Al-PEX construction. Typical cause: prolonged contact with aggressive, low-pH water (acidic boiler water), or thermal shocks from improperly regulated systems (frequent switching between cold/hot water without a gradual transition). It manifests as bulging of the outer layer – the pipe looks "swollen." The affected section must be replaced entirely.
Leaks at press joints on multilayer pipe
Press joints (press fittings) are not used with compression fittings, but they are common on larger diameters of multilayer pipe. A leak at a press joint occurs when:
- The pipe was not inserted all the way before pressing (the most common cause – the installer failed to make a control mark)
- Unsuitable pressing jaws were used for the given fitting type
- The O-ring was twisted or made of an unsuitable material (for heating, EPDM is required, not NBR)
Corrosion of copper pipe: causes and solutions
Copper is a traditional and highly durable material for heating pipework. However, it has one Achilles' heel: under certain conditions it can corrode faster than you would expect.
Pitting corrosion
Pitting corrosion manifests as small pits on the inner or outer wall of copper pipes. The causes are:
- Hard water with high chloride content – chloride ions attack the passivating copper oxide layer
- Acidic water (pH below 7.0) – in such water, copper corrodes noticeably
- Microbially induced corrosion – certain bacteria (sulfate-reducing) excrete acids that locally attack the pipe wall
- Excessive flow velocity (above 2 m/s with soft water) – erosive corrosion at bends
Pitting corrosion on a copper pipe cannot be repaired – the affected section must be replaced. Short-term patching with epoxy is only postponing the inevitable. Proper prevention involves maintaining the boiler water pH in the range of 7.5–9.0 and annually checking the corrosion inhibitor (when boiler water isn't tested every season, this gets ignored for 15 years until a pipe bursts behind the drywall).
Galvanic corrosion at material transitions
Copper and steel are galvanically very different materials. When joined in a wet environment, steel (the less noble metal) corrodes preferentially. This is why steel pipe connected directly to copper without a dielectric union gradually rusts at the point of contact.
Correct solution: always insert a dielectric union (isolator) or use a brass fitting, which acts as a "galvanic bridge" with an acceptable potential difference. Never join copper directly to galvanized steel with a plain fitting.
Leaks at radiator joints and valves
Radiator valves – thermostatic, angle, straight – are moving components with seals that wear out. Typical faults:
Dripping valve packing
The packing (gland) is the seal around the valve spindle that prevents water leaking along the stem. On old valves (15+ years), the rubber dries out and cracks. Symptom: dripping directly from the middle of the valve body, from where the adjustment spindle exits. Solution: on some valves, simply tightening the packing nut (by 1/8 of a turn!) is enough; on old valves, replacing the entire valve is faster and cheaper than searching for a replacement seal.
Leak between the valve body and the radiator
This is the point where the valve is screwed into the radiator. The seal is usually made of hemp/PTFE or metal. After many years, or after improper removal of the radiator, the seal can fall out or crumble. Repair: shut off the branch (or the system), remove the valve, clean the thread, apply new PTFE tape (8–12 turns in the direction of the thread), and reinstall without overtightening (the valve thread is usually G1/2" or G3/4").
Leaks in hidden pipework and underfloor circuits
This is a category that scares every homeowner. Pipe hidden in the floor, wall, or drywall – and something is dripping somewhere. The good news: most modern underfloor multilayer pipe installations are made without joints in the embedded section (laid in one piece from manifold to manifold). The bad news: if the installation was done poorly and there is a joint there, finding it is a whole procedure.
Identifying a leak in a floor or wall
The first step is always a pressure test: fill the system, shut off the expansion tank, and monitor the pressure gauge for 30–60 minutes. If the pressure drops, a leak exists. A larger leak usually creates a wet spot on the floor; a smaller (capillary) leak shows up as dampness after a longer time or salt efflorescence on the wall. A thermal imaging camera is commonly used today to precisely locate a leak in a walled-in or embedded installation – at the leak point, the thermal field is disturbed because hot water spreads elsewhere than it should.
Repairing a hidden leak
There are two options: mechanical (open the floor/wall, replace the damaged section) or chemical injection with a sealing agent. Chemical injection (so-called self-sealing additives) works reliably only for micro-leaks up to about 0.05 mm. For larger cracks in multilayer or copper pipe, it has no chance. Mechanical repair is always more reliable and, in the long run, cheaper.
Leaks caused by water hammer and system surges
Water hammer (hydraulic shock) is a phenomenon that occurs when a valve is closed abruptly or a pump is switched off, sending a shock pressure wave through the pipe. In residential heating systems, it's not as dramatic as in industrial systems, but repeated micro-surges can, over several years, fatigue the material at compression fittings and fittings, or loosen them from their mounts.
Symptom: regular banging in the pipes, especially when the pump starts. Long-term effect: loosened fittings, cracked ferrules on compression fittings. Solution: install shock absorbers (small membrane vessels on problematic branches), quality pipe mounting brackets with sufficient spacing (for 16 mm multilayer pipe, every 30–40 cm; for 20 mm copper pipe, every 100–120 cm).
Leaks from the expansion tank and safety valve
These two components are not part of the pipework itself, but they are directly connected to the system, and their faults manifest similarly to pipe faults.
Safety valve: If the safety valve regularly releases water (sometimes several liters with each heating cycle), it means either a ruptured membrane in the expansion tank, or an improperly sized expansion tank (too small for the system volume). This is a topic of its own – but the water released by the safety valve needs to be drained somewhere, and if that's not addressed, it will drip onto the boiler room floor and can look like a pipe leak.
Expansion tank: A ruptured expansion tank membrane causes the system pressure to fluctuate excessively (0.8 bar when cold, easily 3.5 bar or more after heating up). This leads to repeated opening of the safety valve and mechanical stress on all joints. Check: with the system cold, press the expansion tank valve stem (like on a car tire) – if water comes out, the membrane is ruptured. Solution: replace the tank or membrane.
Preventive maintenance: how to avoid most faults
Decades of practice show that the most expensive repairs are always the ones that could have been prevented. A basic annual inspection of the heating system includes:
- Visual inspection of all visible joints, fittings, and valves – dry hands, paper towel
- Reading the pressure in the cold system (should be 1.0–1.5 bar, depending on building height)
- Checking the expansion tank membrane (valve stem)
- Taking a boiler water sample and testing pH (test kits are available for a few euros)
- Topping up corrosion inhibitor according to manufacturer's instructions (not "once every 5 years," but annually)
- Checking pipe mounting – loose brackets cause vibration and stress on joints
A more detailed overview can be found in the article Maintenance and inspection of heating pipework: what to watch for every season?, which is part of this Knowledge Center.
What to do in case of an active leak: emergency procedure
If you have discovered an active leak (dripping, spraying water), proceed in this order:
1. Minimize damage: Place a container underneath, switch off the power supply to the boiler (if water is getting close to electrical equipment), open the drain valve, and reduce the system pressure.
2. Identify the location: The drip isn't always where the source is. Water runs down. Trace it upward and against the direction of slope.
3. Shut off the branch: Most modern systems have a shut-off valve on each branch. Shut off only the affected one – the rest of the system can keep running.
4. Temporary solution: Special hose clamps or epoxy patches are legitimate short-term (24–72 hours) solutions that buy you time for a proper repair. They are not a permanent solution.
5. Permanent repair: Depending on the location and cause of the leak (see the scenarios described above).
Pipe replacement: when is it time?
Copper pipe, with proper maintenance and neutral water, lasts 50+ years. Multilayer pipe (PEX-Al-PEX, such as IVAR Turatec 18x2 multilayer pipe) has a declared service life of 50 years at PN10, T=70°C parameters. In practice, three factors are decisive: water quality, frequency and amplitude of temperature cycles, and installation quality.
Signs that a pipe is at the end of its service life and needs to be replaced entirely:
- Repeated leaks at different locations in a short time – that's not bad luck, that's a systemic problem
- Visible pitting corrosion on multiple sections of copper pipe
- Chronically reduced wall thickness (measured by ultrasound on steel pipes)
- The pipe is plastic (PP or PE without an Al layer) in a system with temperatures above 60°C and is older than 20 years
Frequently Asked Questions (FAQ)
Can I repair a leak at a compression fitting myself, or do I need to call a plumber?
A handy DIYer can handle most compression fitting repairs, provided they have the right size wrenches, a new ferrule, and sealing paste. The key is to shut off the branch, drain the water from the affected section, and proceed systematically. It gets complicated if the joint is in a wall or under the floor – there, a professional is more appropriate, because an incorrect repair means more demolition.
Why does the pressure in my boiler drop every winter, and I have to top up water?
A small pressure drop (0.1–0.2 bar over the season) is normal – water degasses and shrinks slightly. If you have to top up water every week or after every cooling cycle, a leak exists. Either it's visible somewhere in the system, or – worse – it's a hidden leak in the floor or wall. A pressure test, observing the pressure gauge over an hour without heating, will reveal whether the problem is real.
What is the service life of a ferrule (compression ring) on a compression fitting?
A ferrule is a single-use component. After one correct installation, it should last the entire lifetime of the installation (decades). The problem arises with every disassembly – as soon as you open the fitting, the ferrule must be replaced. Don't try to save money on a ferrule; compared to an hour of a plumber's work and leak damage, it's negligible.
Can I use the same compression fitting for multilayer pipe as for copper?
Not always. It depends on the type and manufacturer of the fitting. Some compression fittings are designed exclusively for copper, others are compatible with both materials. The reason is the difference in wall stiffness – multilayer pipe is softer, and the ferrule must be shaped differently. Always check the fitting's specification. There are, for example, combi-compression fittings such as the 15 x 1 EK compression fitting for copper or PB pipe, which are explicitly designed for both types.
What should I do when the safety valve drips with every heating cycle?
This is not a fault of the pipe itself, but of the expansion tank or the system's set pressure. Check the expansion tank pre-charge (usually 0.5–0.75 bar with the system cold) and the membrane's integrity. If the membrane is ruptured, water will leak from the tank's valve stem. A temporary fix – topping up water and releasing it through the safety valve – accelerates corrosion of the whole system and is not an acceptable long-term solution.
Why is water leaking from double-sided fittings (couplings, T-pieces) even though I installed them only recently?
The likely reason is thermal expansion. Multilayer pipe expands significantly more when heated than copper – at 80°C and a length of 10 m, this can be as much as 8–10 mm. If expansion is not accommodated (loop compensators, free mounting), the pipe tries to expand and pulls on the fittings. Solution: proper mounting with expansion zones. Details on designing pipework are in the article How many meters of pipe do I need for a heating circuit: calculation and planning of pipework.
Conclusion: diagnosis before repair
Most heating pipe faults can be repaired at low cost if detected in time. The key habit is a systematic visual inspection once a season and pressure measurement. When you know where and why leaks commonly occur, you'll find them much sooner, before they cause damage. If you're planning the installation of new pipework or replacing part of the system, also pay attention to the articles Installing multilayer pipe step by step: tools, fittings, and compression fittings and How to correctly join copper pipe using a compression fitting without water leaks? – preventing faults starts with proper installation.