Common faults and leaks in PEX-AL-PEX pipes: causes and repair

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Common faults and leaks in PEX-AL-PEX pipes: causes and repair

PEX-AL-PEX pipe is now one of the most widespread types of piping for underfloor heating, radiator circuits, and domestic water distribution. Its popularity is well deserved – the combination of layered plastics with an aluminum core provides excellent chemical resistance, formability, and a long service life. Nevertheless, from practical experience we know that problems do occur, and often precisely where we least expect them: buried in screed, hidden behind drywall, or embedded in the floor. This article deals with real faults, their causes, and – most importantly – how to detect them, repair them, or prevent them.

If you are still planning an installation or looking for guidance on how to correctly choose pipe parameters, we recommend first reading How to choose PEX-AL-PEX pipe: diameter, wall thickness, and pressure parameters and Installing PEX-AL-PEX pipe: bending, crimping, and correct joints step by step. This article builds on these topics and focuses on diagnosing and solving faults that can appear even after years of trouble-free operation.

Why do faults occur in PEX-AL-PEX pipes at all?

PEX-AL-PEX is a five-layer composite: an inner layer of cross-linked polyethylene (PEX), an adhesive layer, an aluminum core, another adhesive layer, and an outer PEX sheath. Each of these layers performs a specific function while also representing a potential point of failure. Faults can be divided into several basic categories:

Mechanical damage – rupture or deformation of one of the layers due to external force
Leaks at joints – insufficient crimping or incorrect fitting installation
Corrosion of the aluminum layer – chemical damage caused by unsuitable water or acids
Thermal damage – exceeding the maximum operating parameters
Material fatigue under cyclic stress – years of alternating thermal expansion and contraction
Damage during construction work – drilling, chiseling, nailing after pipe installation

In practice, we most often find that a fault is not caused by a single reason but by a combination – for example, a slight crimping error which, over time, is compounded by an unsuitable chemical composition of the water, resulting in a leak after three years of operation.

Leaks at joints: the most common problem in practice

Experience from the field shows that the vast majority of PEX-AL-PEX pipe faults are not found in the middle of a straight section of pipe, but precisely at the joints. This makes sense – a joint is a place where two different materials meet (the pipe and the fitting), where mechanical stress is concentrated, and where everything depends on the correct installation procedure.

Insufficient crimping – the most common hidden cause

A press fitting works by compressing a press sleeve (ring) around the pipe, creating a tight mechanical bond that holds the fitting's pipe stub firmly from the inside. If the press tool was not correctly positioned, if the pressing jaw was not properly secured, or if the operator stopped the compression before the press tool gave its characteristic completion click, the result may look completely fine from the outside – but it leaks.

This problem is treacherous because the fault may not appear until months or years later. In practice, for example, we handled a case where a complaint came in after two heating seasons – a fitting in the underfloor heating distribution manifold began to quietly seep through an incompletely crimped sleeve. We confirmed this by examining the profile of the crimped sleeve – the shape was not symmetrical around the whole circumference.

How to recognize it: Moisture, discoloration around the fitting, later mold appearing on the floor surface near the manifold, a gradual and hard-to-measure pressure drop in the system. For accessible fittings, it is enough to check the press tool's rotation and inspect the sleeve profile – it should be evenly compressed around the whole circumference.

Pipe not properly inserted into the fitting

Before a fitting is crimped, the end of the pipe must be inserted all the way to the stop. The "click" sound heard when inserting the pipe is not always a reliable indicator – especially with larger diameters such as PEX-AL-PEX 26×3 or PEX-AL-PEX 32×3, where resistance is higher and the installer may mistakenly believe the pipe has reached the stop, even though 5–8 mm is still missing.

The solution is simple: before every crimping operation, mark the end of the pipe with a felt-tip pen at a depth corresponding to the length of the fitting stub. After insertion, the mark must be right at the edge of the fitting sleeve. This simple habit prevents a large share of installation problems.

Damage to the seal or O-ring

Press fittings for PEX-AL-PEX use a combined system: the mechanical bond of the crimped sleeve plus a rubber O-ring on the stub section. The O-ring provides the primary seal and is made of EPDM rubber. Although EPDM is a chemically resistant rubber, it can be damaged by:

Breaking or falling out during handling before installation
Twisting during insertion (when the pipe is rotated instead of being pushed straight in)
Chemical attack – for example, if someone has added unsuitable corrosion inhibitors containing mineral oils to the system
Thermal aging after many years above 90 °C

If you have access to the fitting, inspect it visually. A damaged O-ring can be replaced – but this requires uncrimping or replacing the fitting.

Corrosion of the aluminum layer: the silent enemy

The aluminum layer in PEX-AL-PEX pipe performs several functions: it gives the pipe shape memory, acts as a diffusion barrier (preventing oxygen from penetrating through the wall), and reinforces the structure. However, aluminum is a metal, and therefore chemically reactive – under certain conditions it undergoes corrosion, which ultimately manifests itself as layer delamination or direct perforation.

Causes of aluminum corrosion in the piping system

The most serious cause is incorrect pH of the heating or domestic water. Aluminum is an amphoteric metal – it is attacked by both acidic (pH below 6.5) and strongly alkaline (pH above 8.5) environments. In heating systems, a pH in the range of 7.0 – 8.5 is recommended. Problems arise, for example, with:

The use of unsuitable corrosion inhibitors (some products intended for steel radiators can be aggressive toward aluminum)
Introducing high doses of antifreeze mixture (propylene glycol itself is fine, but incorrect dilution ratios or mixing different products can lead to unwanted reactions)
Locations with extremely soft water (pH below 7.0)
Chemical descaling of the system with strong acids without neutralization

How corrosion manifests itself: A characteristic sign is the appearance of white powder (aluminum oxide) or a grayish sludgy substance at the lowest point of the system or in strainer filters. Later, a visible bulging of the outer PEX sheath may occur at the point where the aluminum layer has completely deteriorated. Pressure tests may still pass at this stage – as long as the inner PEX layer holds. However, the loss of the diffusion barrier means oxygen enters the system, and oxidative corrosion of aluminum and steel components (e.g., radiator bodies) accelerates dramatically.

Galvanic corrosion with mixed materials

Another problem is galvanic corrosion, which occurs when aluminum comes into direct contact with copper in the presence of an electrically conductive liquid. In practice, this situation is encountered, for example, when connecting PEX-AL-PEX to copper piping in an older installation without a dielectric separator. Copper ions (Cu²⁺) dissolved in the circulating water then deposit on the aluminum surface and locally accelerate corrosion. It is therefore important to use dielectric fittings when transitioning between materials.

Mechanical damage and the most common construction "accidents"

In our field experience, we have dealt with dozens of cases where a pipe routed in the floor or wall was damaged by someone during later construction work. The most common scenario: an electrician arrived a month after the floor was finished and drilled in a spot where, according to the documentation, there should not have been a pipe – but there was.

Drills, nails, and other surprises

Mechanical puncturing of the pipe is a dramatic but relatively easy fault to diagnose. Water leaks out immediately or shortly after the injury. Partial injuries are worse to deal with – a drill that merely scratched or weakened the pipe wall without fully breaking through. In such a case, the fault may not appear until there is increased pressure or thermal stress.

Real-world example: On one job, we dealt with a situation where the customer reported a slow pressure drop in an underfloor heating circuit. Using a thermal camera and monitoring moisture, we localized the spot near a wall. We dug it up and found a PEX-AL-PEX 16×2 pipe into which a nail for corner trim had been driven during masonry work. The nail was still in place and effectively acted as a seal – until it was removed while searching for the cause.

Damage during bending – kinking

PEX-AL-PEX pipe is bent by hand or with the help of bending springs and bending mandrels. If it is bent with too small a radius without a bending tool, the aluminum layer can locally kink. The result is visible – the bend has a sharp fold instead of a smooth curve. In practice, such a damaged spot may remain leak-tight for years, but the inner diameter is significantly narrowed (the recommended minimum bending radius is 5× the outer diameter; in practice, for common sizes: for a 16mm pipe, at least 80 mm). At the same time, the material at the kink point is weakened, and temperature fluctuations can cause a crack.

You can find more on this topic in the article Minimum bending radius of PEX-AL-PEX pipe and the correct shaping technique to avoid kinking in our Knowledge Center.

Thermal damage: what happens when limits are exceeded

PEX-AL-PEX pipe has a declared maximum continuous operating temperature of 95 °C at a pressure of 6 bar. In the short term (so-called peak operating conditions), it can withstand up to 110 °C. These values apply to all standard diameters – from PEX-AL-PEX 16×2 intended for underfloor heating, through PEX-AL-PEX 20×2 for radiator circuits, up to larger distribution sizes.

What happens if the temperature is permanently exceeded?

PEX is cross-linked polyethylene – cross-links between the chains give it thermal resistance that ordinary polyethylene does not have. However, prolonged exposure to temperatures above 95 °C causes gradual degradation of the polymer structure: the material loses elasticity, becomes more brittle, and in combination with pressure surges, may crack. The outer sheath of the pipe may look intact, but the inner layer is already significantly weakened.

Real-world example: In an older house with a solid-fuel boiler without temperature regulation, we found that the PEX-AL-PEX piping had been exposed for a long time to temperatures of 105–115 °C when the boiler was running at full output. The outer PEX sheath showed significant yellow discoloration and a slightly cracked surface. The pipe passed a 6 bar pressure test, but after a bending test, the outer sheath slightly delaminated from the aluminum layer. The piping had to be replaced as a preventive measure.

Damage during installation – hot work near the pipe

A special category is damage during construction or trade work. Working with a torch, angle grinder (sparks), or hot air gun near PEX-AL-PEX piping can cause local melting of the outer PEX sheath. Such a spot may not leak immediately, but it is mechanically weakened. In practice, we recommend protecting piping with a covering foil or mineral wool whenever hot work is carried out in an area with installed piping.

Expansion and material fatigue under cyclic stress

PEX-AL-PEX pipe expands when heated and contracts when cooled. The coefficient of thermal expansion is approximately 0.025 mm/(m·°C) – significantly more than copper (0.017 mm/(m·°C)), but thanks to the aluminum layer, significantly less than pure PEX-B (0.15 mm/(m·°C)). For a specific calculation: 10 meters of pipe heated from 20 °C to 70 °C will elongate by approximately 12.5 mm. For a typical underfloor heating circuit of 100 m with a temperature difference of 40 °C, we are talking about a 100 mm elongation per circuit.

If the pipe is not laid so that it can expand freely (a missing expansion loop or protective sleeve where it passes through solid material), repeated heating and cooling cycles put mechanical stress on the fittings and joints. This can result in gradual loosening of crimped joints or microcracks in the bend areas. This is typically a fault that only appears after five to ten years of operation.

More details on properly designing circuits and expansion measures can be found in the article PEX-AL-PEX pipe for underfloor heating: spacing, circuit length, and maximum temperature.

How to detect a fault and leak: diagnostic methods

Detecting faults in PEX-AL-PEX piping is technically the most demanding part of the issue – especially if the piping is hidden in the floor or wall. Here are the most commonly used methods, from the simplest to the most sophisticated:

Water pressure test

A pressure test is a basic and mandatory test for every new installation, but it also helps with diagnostics. The system is pressurized to 1.5 times the maximum operating pressure (typically 6 bar for systems rated at 4 bar), and the pressure drop is monitored for 30 minutes (leak test) or longer (strength test according to STN standards). A pressure drop greater than 0.1 bar over 30 minutes indicates a leak.

The problem is that a pressure test tells you a leak exists – but not where. To locate it, you need additional methods.

Thermal camera (infrared thermography)

A thermal camera detects differences in surface temperature. With the heating system active, it shows the piping in the floor as warmer areas. A leak appears as locally scattered heat outside the expected pipe route. This method is reliable and non-invasive – it does not require demolition. Disadvantage: it works well only with screed floors without thick thermal insulation above the layer, which would absorb the temperature difference.

Acoustic detection (correlator)

Professional diagnostic devices measure acoustic vibrations propagating through the pipe from the point of the leak. By measuring from two points and using mathematical correlation, they can calculate the distance to the leak. This method is less common for PEX-AL-PEX than for steel piping (plastic dampens sound propagation), but modern correlators can also work with plastic pipes.

Gas detection (nitrogen + tracer gas)

The system is drained, dried, and filled with a mixture of nitrogen and a tracer gas (usually hydrogen – 5% H₂ in N₂). The gas escapes through the leak and is detected by a sensitive sensor above the floor. This method has excellent spatial resolution (down to a few centimeters) and works for all types of pipes, including plastic ones.

Repairing a fault: what is realistic and what is not

Once you have located the fault, the question of repair arises. Options are limited – PEX-AL-PEX cannot be welded or glued like a simple plastic. There are essentially three repair methods:

Replacing a section of pipe

For accessible piping (in a utility room, behind drywall), replacing the damaged section is the most elegant solution. The damaged section is cut out, two compression or press fittings are installed, and a new piece of pipe is inserted. When choosing the diameter of the replacement pipe, it is important to keep the same type – for example, if you have a distribution circuit made of VERME PE-AL-PEX 16×2 pipe, the replacement piece must be of the same or a compatible diameter with the same wall thickness.

Bypassing the damaged section

For pipe in the floor or wall where demolition would be extensive or impractical (historic buildings, expensive flooring), a bypass solution is sometimes used: the damaged section is taken out of service, fittings are installed at both ends (where the pipe emerges from the floor), and a new section of pipe is routed along an alternative path – e.g., in a conduit along the wall. This solution is functionally correct but less aesthetically pleasing.

Sealing with a compression fitting – an emergency solution

In the event of a failure (puncture), the first step may be an emergency seal using a compression fitting without crimping tools. This solution is TEMPORARY and is not intended for long-term operation under pressure and temperature – it should be followed by a proper service intervention using a press fitting.

What is NOT a repair: wrapping the leak point with plumber's tape, epoxy putty, or other improvised methods. Such "repairs" never provide sufficient pressure and temperature resistance and are the seed of another future fault. In our regular practice, we have seen such "repairs" from other workshops – and they always ended, sooner or later, in another leak.

Preventive measures: how to avoid faults

Proper prevention begins at installation and continues throughout the entire service life of the system:

Quality fittings and calibrated press tools – do not combine fittings from different manufacturers, always use the prescribed crimping procedure
Protective sleeve at transitions through solid structures – the pipe must not be in direct contact with concrete or masonry without a protective sleeve
Expansion loops and compensators – for long straight sections, every ten or fifteen meters
Regular check of pH and heating water quality – ideally once every 2 years; if pH is found outside the recommended range, immediately circulate and refill with demineralized water and a suitable inhibitor
Pressure test for every installation and repair – without exception, before embedding in the floor or closing up in the wall
Documentation of the pipe routing – photo documentation before embedding, ideally marked on a drawing
Correct installation without kinking – following minimum bending radii, discussed in the article Minimum bending radius of PEX-AL-PEX pipe and the correct shaping technique to avoid kinking

You can also find a detailed guide on preventing installation errors in the article Common installation mistakes with PEX-AL-PEX pipe and how to avoid them.

What to do if the pressure drop continues despite a repair

It happens that after locating and repairing an apparent leak, the pressure drop continues. The reasons may be:

There is another, undetected leak elsewhere in the system
The repair fitting was not correctly crimped, or the new piece of pipe has its own defect
Pressure is dropping due to air vents releasing air (and with it a small amount of water) – this is not a fault
The expansion tank is undersized or incorrectly set, and the system is actually venting through the safety valve

In such cases, we recommend systematically separating sections of the system with shut-off valves and testing each section individually. This will help you locate which part of the system the fault persists in.

Frequently Asked Questions (FAQ)

Can I repair a PEX-AL-PEX pipe fault in the floor without breaking up the floor?

In some cases, yes – if the fault is located near the manifold or near a wall where the pipe emerges from the floor, it may be possible to repair the fitting or a short section without extensive demolition. For a fault in the middle of a circuit deep in the screed, minimal destruction is almost unavoidable – or a bypass of the damaged section along an alternative route on the wall may be used.

Why is there a pressure drop in the system when I don't see any visible water leak?

A leak can be very slow (drops per hour) and located where water is quickly absorbed by the screed or thermal insulation under the floor. The system can lose pressure even without visible moisture. In such cases, we recommend using a thermal camera during full operation or tracer gas detection. A pressure drop can also have other causes – an incorrectly set expansion tank or a malfunctioning safety valve.

Is it possible to repair a kinked pipe without replacing it?

Not reliably. Kinking damages the aluminum layer and narrows the inner diameter – these changes are irreversible. The kinked spot can be temporarily reinforced with a rigid protective sleeve slipped over it, but the correct solution is always to replace the damaged section. A kinked spot is mechanically weakened and may crack at an inconvenient moment under thermal stress.

I have found corrosion of the aluminum layer in several places. Do I need to replace the entire system?

It depends on the extent. If corrosion has affected the aluminum layer along most of the pipe length (the outer PEX sheath is bulging, there is white powder in the system), a complete replacement is more appropriate than a series of spot repairs. If it is an isolated spot (e.g., one transition through an aggressive environment), it is enough to replace the damaged section and eliminate the cause of corrosion – usually by correcting the water pH and eliminating galvanic contact with copper.

Can I use compression fittings as a permanent repair instead of press fittings?

Compression fittings are a fully valid permanent solution for accessible locations (manifold, riser, utility room). They are not recommended for locations embedded in screed or enclosed in a wall – not because they are less reliable, but because they require periodic inspection and possible retightening of the nut, which cannot be ensured in an inaccessible location.

How long does PEX-AL-PEX pipe last before faults occur?

With correct installation, under recommended operating conditions (temperature up to 95 °C, pressure up to 6 bar, pH 7.0–8.5), and undamaged installation, the declared service life is 50 years. The faults we have encountered in practice have always resulted from one of the factors described – installation errors, chemically aggressive environments, or mechanical damage. Quality pipe – such as PEX-AL-PEX 20×2 from proven manufacturers – really does last decades without problems when operated correctly.

Conclusion: PEX-AL-PEX faults are usually solvable – if you know where to look

PEX-AL-PEX pipe is a reliable and proven material that, with correct installation and operation, shows no serious problems for decades. Most of the faults we encounter in practice have a clear and traceable cause: an installation error, chemically unsuitable conditions, or mechanical damage during construction work. The key to solving them is a systematic approach – from a pressure test, through thermography, to a targeted repair of the specific spot.

If, while reading this article, you recognized symptoms matching your situation, take a look at other topics in our Knowledge Center: Maintenance and inspection of PEX-AL-PEX piping: what to regularly check and Frequently asked questions about PEX-AL-PEX pipe – where you will find answers to further practical questions about this type of piping.

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