Common mistakes when installing PEX-AL-PEX pipes and how to avoid them
Common mistakes when installing PEX-AL-PEX pipes and how to avoid them
PEX-AL-PEX pipe is today one of the most common materials used in underfloor heating, hot water distribution, and heating systems. Its popularity is no coincidence – the combination of an aluminum layer and cross-linked polyethylene offers excellent thermal resistance, minimal thermal expansion, and a long service life. Nevertheless, the same mistakes keep appearing during installation, leading to leaks, premature failures, or at the very least an unnecessarily complicated installation.
In this section of the Knowledge Center, I have decided to summarize what I have seen most often over years of practice – and to explain why each mistake occurs and how to effectively avoid it. This article is intended primarily for installers and more experienced DIY enthusiasts who want to understand what they are doing, not just blindly follow a procedure.
1. Incorrect selection of pipe diameter for a given circuit
One of the most basic mistakes, yet one with long-term consequences. Many installers (and even more DIY enthusiasts) reach for whatever they happen to have in stock – or follow the simple logic of "bigger diameter = better flow." The reality is more complicated.
For underfloor heating, the most commonly used pipe is PEX-AL-PEX pipe 16 x 2, meaning an outer diameter of 16 mm and a wall thickness of 2 mm. The inner diameter is therefore 12 mm. This size is sufficient for most underfloor heating circuits and, at the same time, small enough that the bend radius is manageable without specialized tools.
For the main distribution pipe from the boiler to the manifolds or radiator circuits, PEX-AL-PEX pipe 20 x 2 (outer diameter 20 mm, wall thickness 2 mm) is commonly chosen. For more demanding main distribution lines, where flow needs to be higher or distances longer, PEX-AL-PEX pipe 26 x 3 or even PEX-AL-PEX pipe 32 x 3 comes into consideration.
The mistake occurs in both directions: too small a diameter causes excessive pressure drop and noise in the pipe, while too large a diameter increases installation cost and slows down the heating of water in the system (a larger volume of water needs to be heated each time). If you are interested in specific diameter calculations, refer to the article What PEX-AL-PEX pipe diameter do I need for underfloor heating, radiators, or water distribution in this Knowledge Center.
2. Bending without observing the minimum radius – pipe kinking
This is mistake number one that I see on projects even years after installation. PEX-AL-PEX pipe can be bent by hand or with the help of a spring or a bending tool, which is a great advantage – but it also tempts installers to make too sharp bends. When the minimum radius is exceeded, the middle aluminum layer becomes permanently deformed. The outer PEX layer remains visually undamaged, but the internal cross-section is narrowed. The result: increased pressure drop in that section, noisy flow, and premature wear at the kink point.
The minimum bend radius for standard PEX-AL-PEX pipe depends on the diameter:
- For 16 mm: minimum bend radius is 5× the outer diameter = min. 80 mm (for manual bending, 100 mm is recommended to be safe)
- For 20 mm: minimum bend radius is min. 100 mm, recommended 120–150 mm
- For 26 mm: minimum bend radius is min. 130–150 mm
- For 32 mm: minimum bend radius is min. 160 mm, here a bending tool is almost essential
Practical tip: when laying underfloor heating pipe using the "serpentine" method (meander), installers try to minimize space and therefore bend the loops as tightly as possible. Pipe spacing is usually 15–20 cm, but when turning at the end of a loop, keep in mind that the U-bend radius must be at least 8 cm (for 16 mm pipe). This means that the technological strip along the wall, where the turns are made, must remain free for at least 15–20 cm (half the circuit diameter near the wall). You can find more on this topic in the article Minimum bend radius of PEX-AL-PEX pipe and proper shaping technique without kinking.
3. Mistakes when pressing and preparing the pipe end
Press-fit joints are today the standard when installing PEX-AL-PEX. They are reliable, durable, and do not require soldering. But only if they are properly prepared. Mistakes made during pressing are among the most serious, because most of them are not immediately visible – they only appear during the pressure test or, worse, later during operation.
3.1 Uneven cut and inaccurate end preparation
The pipe must be cut perpendicularly – any slant will cause the seal in the fitting to seat unevenly. Never use a regular hacksaw for cutting – it leaves chips and an uneven cut. The correct tool is a special blade cutter designed for PEX-AL-PEX. After cutting, the end must be calibrated with a calibrating/rounding tool to restore the pipe's circular shape, which may have been deformed by the coiled storage form. Without calibrating the end, the press joint will leak.
3.2 Failing to insert the pipe fully to the stop
This is perhaps the most common mistake with press-fit joints of all. The installer inserts the pipe into the fitting, feels the resistance of the seal, and assumes it has reached the stop – but it has not. The seal creates resistance even before the pipe is properly inserted. Solution: before pressing, always mark the insertion depth on the end of the pipe with a marker or pencil (according to the fitting manufacturer's table). After insertion, check whether the mark is still visible just behind the front of the fitting. If so, the pipe is correctly inserted. If the mark has disappeared inside, something is wrong.
3.3 Using the wrong pressing jaws
Every fitting manufacturer (Fränkische, Rehau, Herz, Viega, and others) has a defined pressing profile shape. The jaws of the pressing tool must exactly match the fitting's profile. Using the wrong jaws – for example, a TH profile on a fitting designed for an M or U profile – results in a joint that looks fine but leaks. Always verify that the jaw profile matches the fitting before starting work.
3.4 Pressing in an unsupported position
Another common problem: the installer presses the jaws onto a pipe that is "hanging in the air" or under tension. The jaws shift slightly during pressing, and the pressing profile becomes asymmetric. The pipe must be firmly supported in a straight line during pressing, and the jaws should be aligned perpendicular to the pipe axis.
4. Failure to account for expansion and proper pipe fastening
PEX-AL-PEX has a very low coefficient of thermal expansion compared to pure plastic pipe – thanks to the aluminum layer, expansion is only around 0.025 mm/m·K. For comparison, plastic pipe (PPR, PE) has 5–10 times higher expansion. Nevertheless, expansion still needs to be addressed even with PEX-AL-PEX, especially on long straight sections in installations with significant temperature changes (for example, distribution lines from the boiler, where the temperature fluctuates between 20 °C and 80 °C).
4.1 Fastenings that are too tight or too sparse
Recommended spacing of mounting clamps depends on pipe diameter and direction of the run:
- For 16 mm: horizontal runs every 40–50 cm, vertical runs every 70–80 cm
- For 20 mm: horizontal every 50–60 cm, vertical every 80–100 cm
- For 26 mm and 32 mm: horizontal every 60–80 cm, vertical every 100–120 cm
Fastening that is too tight, without room for movement, prevents expansion and creates mechanical stress in the pipe and joints. Fastening that is too sparse, on the other hand, causes the pipe to sag between clamps, which is both an aesthetic and a functional problem – stagnant air or debris can collect at the sagging points.
4.2 Missing expansion loop or compensator
For straight sections longer than 6–8 meters, it is recommended to include an expansion loop (lyre). For 16 mm pipe with a temperature difference of 60 °C (20 → 80 °C), the elongation over 6 m is only about 9 mm – which sounds negligible, but with rigid fastening and repeated heating/cooling cycles, it is enough to cause material fatigue at the joint.
5. Laying in a screed slab without a protective sleeve (conduit)
This mistake concerns primarily laying pipe in a concrete screed slab for underfloor heating. PEX-AL-PEX pipe, laid directly in concrete without protection, may react over the long term with the alkaline environment of the concrete, and also has no room for any expansion. Where the pipe crosses expansion joints in the floor (between rooms, in doorways), it must be protected with a plastic conduit for at least 20–30 cm on each side of the joint.
The same applies if the pipe enters a wall or passes through a ceiling – it must be run in a protective sleeve along the entire length of the passage. Without this, the pipe is mechanically stressed by the edge of the structure, and over the long term the outer layer may become cut through or cracked.
6. Mixing different fitting and pipe systems without checking compatibility
There are many manufacturers of PEX-AL-PEX pipe on the market, along with their corresponding press and screw fittings. Not all of them are mutually compatible, even if the nominal diameter matches. A difference in wall thickness (e.g., pipe 16 × 2 vs. 16 × 1.8) can mean that a fitting designed for one type does not hold properly on another type – the seal is either too loose, or the pipe cannot be inserted all the way to the stop.
In practice, I have seen projects where three different fitting brands and two different pipe brands were used on a single installation. Each combination happened to work – some joints were tight, others started dripping after a few months. The rule is simple: for one installation, stick to one system (pipe manufacturer + fitting manufacturer), ideally a verified combination. For example, VERME PE-AL-PEX pipe 16 x 2 has precisely defined dimensions for which the manufacturer guarantees compatibility with the corresponding fittings of its system.
7. Insufficient pressure test before covering the installation
This point is perhaps the most important in the whole article, because neglecting it leads to the most expensive repairs. The pressure test (leak test) must always be carried out before insulating, covering, or pouring concrete. This is a requirement set by standards (STN EN 14336 for heating, or the relevant national standards for plumbing) and is an absolute condition for the warranty of most fitting manufacturers.
7.1 How to properly carry out the pressure test
For heating systems, the test is performed with water at 1.5 times the operating pressure, but at least 6 bar; installations are usually tested at 6–10 bar. The test lasts at least 30 minutes (60–120 minutes recommended). During this time, the pressure must not drop by more than 0.2 bar. If it drops faster, the leak must be located and repaired before covering the installation.
Always prefer a water test over an air or nitrogen test – an air test is more dangerous (risk of injury if a joint bursts) and less reliable at detecting small leaks. An air test is only used where it is not possible to fill the system with water before installation.
7.2 Documenting the pressure test
In family homes, many installers carry out the pressure test but do not document it. Later, if a failure occurs and the customer files a complaint, there is no evidence that the test took place. Proper documentation includes: the date of the test, the pressure at the start and end, the ambient temperature, the name of the responsible person, and the customer's signature. In Slovakia, this may not be a legal requirement for every installation, but it protects the installer.
8. Faulty joints with screw (compression) fittings
In addition to press-fit joints, screw (compression) fittings are also used in practice. These have the advantage of being reusable, but if installed incorrectly, they are a source of leaks. The most common mistakes:
- Over-tightened nuts: Excessive force when tightening the nut deforms both the seal and the fitting body. Hand-tightening plus about 1.5 turns with a wrench is usually enough. There is no need to tighten "with maximum force."
- Insufficiently tightened nuts: The opposite extreme – the fitting looks tightened, but during the pressure test or after a few thermal cycles it starts to drip. Every compression fitting should be checked again after the first setting and after the first heating cycle.
- Missing insert liner: Most screw fittings for PEX-AL-PEX require a rigid metal liner to be inserted into the end of the pipe. This liner prevents the pipe from narrowing when the compression nut is tightened. If you forget the liner, you tighten the nut and everything looks fine – but the internal diameter at the joint is narrowed, and at higher flow rates, noise or increased resistance appears.
9. Forgetting thermal insulation of the distribution pipe
The main distribution pipe from the boiler to the manifolds, or between floors, must be thermally insulated. Without insulation, heat losses occur – in unheated spaces (utility room, cellar, intermediate ceiling), the loss can reach 10–20% of the system's total heat output. In addition to the energy loss, there is a risk of moisture condensation on the cold pipe in summer, which accelerates corrosion processes on metal parts (clamps, fittings).
The thickness of the insulation depends on the pipe diameter and where it is routed. For 16–20 mm pipe in an unheated space, at least 20 mm foam insulation (λ ≤ 0.04 W/m·K) is recommended; for larger diameters, 25–30 mm. In underfloor heating floor structures, the pipe is usually not insulated individually – here the entire base layer (polystyrene or other boards) serves as insulation.
10. Ignoring maximum operating parameters
PEX-AL-PEX is a durable pipe, but it has defined limits. For most common products, the following applies:
- Maximum continuous operating temperature: 95 °C
- Maximum short-term temperature: 110 °C (e.g., during a boiler fault condition)
- Maximum working pressure at 70 °C: 6–10 bar (depending on diameter and manufacturer)
The mistake occurs when PEX-AL-PEX is used in a direct connection to a hot water storage tank with a temperature above 95 °C without an expansion or protective element, or when it is used for the primary circuit of a heat pump with very low temperatures without verifying whether the given type of pipe is suitable for it.
More on selecting the right pipe according to temperatures and pressures can be found in the article How to choose PEX-AL-PEX pipe: diameter, wall thickness, and pressure parameters.
11. Mistakes in laying underfloor heating – spacing and circuit lengths
Underfloor heating deserves a separate mention, as this is where mistakes that directly affect heating comfort tend to concentrate.
11.1 Circuits that are too long
The maximum recommended length of a single underfloor heating circuit made from PEX-AL-PEX pipe 16 × 2 is usually 80–100 meters. Longer circuits result in an excessive pressure drop, causing uneven temperature distribution – the beginning of the circuit is noticeably warmer than the end. If you have a large area, it is better to create two shorter circuits than one that is too long.
11.2 Uneven spacing
When laying the pipe, installers in corners and near walls tend to make bends that come too close together. The result is local overheating. Standard pipe spacing is 10, 15, or 20 cm, depending on the room's heat load calculation. In areas with weaker insulation (perimeter walls, windows), a smaller spacing should be used, while a larger spacing is fine in the rest of the room. This variability is called the "edge zone," and when properly designed, it significantly improves thermal comfort.
11.3 Laying without securing to mounting rails or clips
Pipe laid loosely on the insulation board without fastening tends to shift and lift when the concrete is poured, resulting in an uneven laying depth. The minimum concrete thickness above the pipe is 30–45 mm (depending on the screed mix manufacturer). If the pipe is too high in some places, the screed slab may crack right above the pipe, or the surface temperature of the floor may become uneven. Secure the pipe with clips or to mounting rails every 50 cm.
More detailed information on this topic can be found in the article PEX-AL-PEX pipe for underfloor heating: spacing, circuit length, and maximum temperature.
12. Storage and handling before installation
PEX-AL-PEX pipe is wound onto coils, which is practical for transport, but if the coil is handled improperly, problems can arise even before installation itself.
- Freezing: The pipe should not be exposed to extreme frost (-20 °C and below) during long-term storage, although brief freezing usually will not damage it.
- UV radiation: Long-term (weeks and months) exposure to direct sunlight degrades the outer PE layer. Store it under a shelter or in the shade.
- Dragging the coil along the ground instead of unwinding it: When an installer pulls the pipe from a coil lying flat on the ground, residual internal stresses get wound into the pipe. The coil should stand upright, and the pipe should be unwound "from the side."
- Kinked pipe upon unpacking: If you find a kink (a sharp bend) on the coil, that section is damaged and must not be built into the installation. Cut out the affected section.
Overview of the most common mistakes and their consequences
| Mistake | Immediate consequence | Long-term consequence |
|---|---|---|
| Incorrect pipe diameter | Noise, weak flow | Over-/under-sized heating |
| Kink at a bend | Narrowed flow | Failure, leak, section replacement |
| Not inserting pipe fully during pressing | Leak (not always immediate) | Flooding, structural damage |
| Missing pressure test | Undetected leak | Failures after pouring concrete |
| Circuits that are too long | Uneven heating | Comfort complaints, adjustments |
| Missing insulation on distribution pipes | Heat losses | Increased operating costs |
| Mixed fitting systems | Apparent seal | Gradual leaks, complaints |
Frequently Asked Questions (FAQ)
Can I bend PEX-AL-PEX by hand, or do I need a bending tool?
For pipe diameters of 16 mm and 20 mm, bending by hand with a spring, done slowly and carefully, is quite possible if you observe the minimum bend radius (80 mm for 16 mm, 100–120 mm for 20 mm). For diameters of 26 mm and 32 mm, a bending tool is almost essential – without it you risk kinking the aluminum layer. A spring inserted into the pipe when bending helps significantly to prevent flattening.
How many bar must the installation withstand during the pressure test?
The standard pressure test for heating distribution systems is performed at 1.5 times the operating pressure, but at least 6 bar. Most home heating systems operate at 1.5–3 bar, so a test pressure of 6 bar is a realistic baseline. The test lasts at least 30 minutes, during which the pressure must not drop by more than 0.2 bar.
What happens if I forget to insert the metal liner in a screw fitting?
Without the liner, when the compression nut is tightened, the outer PE layer and the aluminum deform inward. The joint may look tight, but the internal diameter of the pipe at that point is narrowed, which increases the pressure drop and causes noise during flow. Under higher pressures or thermal cycles, this spot may later crack. Liners are part of every compression fitting – never skip them.
How long can a single underfloor heating circuit be with 16 × 2 pipe?
The maximum recommended length of a single circuit with 16 × 2 pipe is 80–100 meters. Shorter circuits (50–70 m) are even more favorable from a hydraulic standpoint. If 100 meters is exceeded, the pressure drop becomes too large and the temperature distribution in the circuit will be uneven – the beginning will be noticeably warmer than the end. It is better to split a large area into two circuits.
Do I need to insulate the underfloor heating pipe laid in a screed slab as well?
Pipe in a screed slab is not insulated individually – the heat flow upward into the room is desirable. The insulation layer beneath the screed slab (polystyrene, foam board) ensures that heat is directed upward and not down into the floor structure. Only insulate the main supply pipe leading to the manifold, especially if it passes through unheated spaces.
Is PEX-AL-PEX pipe suitable for cold drinking water as well?
Yes, most PEX-AL-PEX products are certified for both cold and hot drinking water distribution. It is important to verify the relevant product certification (e.g., certification for contact with drinking water according to EN 1275 or national standards). For example, PEX-AL-PEX pipe 20 × 2 is also suitable for water. However, for drinking water distribution it is equally important to use fittings certified for drinking water – not every fitting designed for heating has this certification.
Conclusion: practice and precision determine the long life of an installation
PEX-AL-PEX is a great material that, when properly installed, will last for decades without any problems. Most of the failures and leaks I encounter in practice are not caused by poor pipe quality, but by installation mistakes – and these are, in the vast majority of cases, preventable. It is enough to use the correct tools, observe the minimum bend radii, not neglect calibration and checking the insertion depth when pressing, and never leave a job site without a pressure test.
If you are interested in a detailed step-by-step installation procedure, read the article Installing PEX-AL-PEX pipe: bending, pressing, and proper joints step by step. For solving specific problems after installation, refer to Common failures and leaks in PEX-AL-PEX pipe: causes and repair. And if you are still just planning an installation and are unsure about your choice, PEX-AL-PEX vs. other pipe types: comparison with copper, plastic, and PEX-B will help you.
You can find the complete range of pipes in the category PEX-AL-PEX heating pipe – from the smallest PEX-AL-PEX 16 × 2 for underfloor heating up to larger dimensions for main distribution lines.
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