Frequently Asked Questions about PEX-AL-PEX Pipes
Frequently Asked Questions about PEX-AL-PEX Pipes: Practical Answers You're Looking For
PEX-AL-PEX pipe is today one of the most popular materials for heating as well as cold and hot water distribution in apartment and family houses. Despite its widespread use, many questions still surround it – partly because there are several types of composite pipe on the market, and partly because installers and end customers encounter specific situations during installation that manufacturers only address generally in their catalogs. In this article, I've compiled the questions that actually come up – on construction sites, via email, during phone calls with customers – and answered them the way an experienced technician would, not marketing copy.
If you're looking for a basic overview of how to choose the right diameter or wall thickness, we also recommend other topics in our Knowledge Center: How to Choose PEX-AL-PEX Pipe: Diameter, Wall Thickness and Pressure Parameters or What Diameter of PEX-AL-PEX Pipe Do I Need for Underfloor Heating, Radiators or Water Distribution. Here we focus specifically on the questions that are missing from typical overviews.
What exactly does the abbreviation PEX-AL-PEX mean, and how does it differ from PE-AL-PEX?
This is one of the most frequently asked questions, and at the same time one of the most frequently misanswered. PEX stands for cross-linked polyethylene. AL is obvious – the aluminium layer. So the difference isn't in the AL, but in what's at the beginning and end: PEX versus PE.
PE-AL-PEX (for example VERME PE-AL-PEX 16 x 2 Pipe) has an outer layer made of non-cross-linked polyethylene and an inner layer of cross-linked PEX. PEX-AL-PEX has cross-linked polyethylene on both sides of the aluminium layer. In practice, this means PEX-AL-PEX has a more durable material on both the outer and inner wall – it's stiffer, holds its shape better after bending, has higher resistance to temperature cycles, and a better oxygen diffusion barrier.
In practice, this difference matters most for underfloor heating, where repeated temperature changes occur. For cold water distribution, the difference is less critical, but professionals usually prefer PEX-AL-PEX there too for its higher dimensional stability. However, both variants are fully valid solutions certified for both drinking and utility water distribution.
Why does PEX-AL-PEX have an aluminium layer? Wouldn't a pure plastic pipe be enough?
The aluminium layer in a composite pipe serves several key functions, which is precisely why PEX-AL-PEX is in many respects a better choice than a pure plastic pipe (PEX-B, PP, PE-RT).
Oxygen diffusion barrier: Plastics are to some extent permeable to oxygen. In heating systems, this causes oxygenation of the water in the system, leading to corrosion of metal components – circulation pumps, cast-iron radiator sections, steel manifolds. Aluminium is practically impermeable to oxygen, so PEX-AL-PEX serves as an oxygen barrier without the need for inhibitors.
Shape memory and stability: Unlike pure PEX-B, PEX-AL-PEX holds its shape after bending. You can bend it by hand or with a bending tool and it stays in the position you shaped it into. This is a huge advantage for distribution inside a floor, behind a radiator, or when routed under plaster.
Lower thermal expansion: PEX itself has a relatively high coefficient of thermal expansion (~0.15 mm/m·K). The aluminium jacket significantly reduces this coefficient to ~0.026 mm/m·K, which is close to the value for copper. This is crucial for long pipe runs – the pipe behaves predictably during temperature changes and doesn't create excessive stress at the joints.
Mechanical strength: Aluminium adds bending strength and resistance to external mechanical stress to the pipe.
What is the maximum temperature and pressure at which PEX-AL-PEX can be operated?
This is a question customers want a precise answer to – and rightly so. Standard PEX-AL-PEX pipe (for example PEX-AL-PEX 16 x 2 Pipe) is certified for the following operating conditions:
- Maximum continuous operating temperature: 95 °C at 6 bar pressure
- Maximum short-term temperature: 110 °C (short-term spike, not continuous state)
- Maximum working pressure at 20 °C: typically 10 bar
- Maximum working pressure at 95 °C: 6 bar
For most common heating systems (hot water heating at 55–70 °C, underfloor heating up to 45 °C), these parameters are more than sufficient with a large margin. Problems only arise with direct connection to a boiler without a mixing valve – there, outlet temperatures can briefly exceed 80 °C. PEX-AL-PEX can handle this, but in such an application, proper sizing and the use of fittings certified for the same conditions must be observed.
For cold drinking water distribution, temperature requirements are naturally lower, but the pressure requirements are identical – the standard municipal water pressure of 3–6 bar is easily handled by PEX-AL-PEX.
What is the real difference between diameters 16, 20, 26 and 32 mm? When to use which?
This is a question where customers make the most frequent mistakes – either oversizing or undersizing. Each dimension has its own logic and ideal application.
16 x 2 mm – this is by far the most commonly used dimension. PEX-AL-PEX 16 x 2 Pipe is used as the basic pipe in underfloor heating – loop spacing 10–20 cm, single loop length up to 80–100 m. It's also used for connecting individual radiators in a two-pipe system and for distribution branches in a smaller house. The inner diameter is 12 mm.
20 x 2 mm – PEX-AL-PEX 20 x 2 Pipe is used as the primary circuit from the boiler to the manifold, for main branches of the heating circuit, or for supplying multiple underfloor heating loops at once. An inner diameter of 16 mm allows a significantly higher flow rate at a lower water velocity – which reduces both noise and pumping energy losses.
26 x 3 mm – PEX-AL-PEX 26 x 3 Pipe is the choice for larger buildings, family houses over 200 m², or for connecting the boiler room to the manifold over a greater distance. It's also used where you need a circulation loop for several zones at once, or for supplying a higher-output water heater.
32 x 3 mm – PEX-AL-PEX 32 x 3 Pipe is intended for plant rooms, larger boiler rooms, multi-purpose buildings, or for storage tank connections with high flow rates. In a family house, this pipe is almost never used – if an installer proposes it as the distribution pipe in the floor, it's probably oversized.
The specific choice should always be based on a hydraulic calculation – if you'd rather not calculate, read the article What Diameter of PEX-AL-PEX Pipe Do I Need for Underfloor Heating, Radiators or Water Distribution, where you'll also find practical tables and scenarios.
Is PEX-AL-PEX suitable for drinking water, or only for heating?
Yes, PEX-AL-PEX is also certified for drinking water distribution – but with one condition: it must be a pipe with the appropriate certification (e.g. DVGW, W270, ACS). Quality pipe from our range holds these certificates. The aluminium jacket is not in contact with the water – it's separated from the water by the inner PEX layer, so there is no risk of water contamination by aluminium.
In practical terms: PEX-AL-PEX is commonly used in apartments and family houses for both cold and hot water distribution (CW and HW). It's resistant to limescale (smooth inner wall), doesn't corrode, and doesn't affect the taste or smell of water. It's also approved for hot water distribution up to 60 °C (up to 70 °C briefly for anti-legionella flushing), which meets standard hygiene requirements.
One practical note from experience: if you're installing both drinking water and heating distribution from the same pipe, consistently distinguish what's what using color coding or tape marking. Customers who have their floor opened up years later often struggle to identify the circuits. Mark it and document it.
How are PEX-AL-PEX pipes connected? Which method is best?
PEX-AL-PEX can be joined in several ways, each with its own advantages and disadvantages. This topic deserves its own article (see Installing PEX-AL-PEX Pipe: Bending, Pressing and Proper Joints Step by Step), but here's a brief overview:
Press fittings: This is the standard in professional installation. The fitting (brass or stainless steel) is inserted into the pipe, and a press tool (TH, U or M profile) deforms the stainless steel ring so it seals the joint mechanically. The result is strong, reliable, and requires no special training or open flame. Downside: you need a press tool, which isn't cheap – but most installers have one today.
Compression fittings (screw-type): Suitable for repairs or accessible locations. The pipe is inserted, and you tighten the nut. Advantage: detachable, no tool required. Disadvantage: not suitable for embedding in concrete or plaster – these joints must remain accessible. During long-term operation with hot water, slight loosening may occur, requiring retightening.
Push-to-connect fittings: The fastest installation, suitable for repairs or temporary connections. Professionals don't prefer them for permanent distribution runs, although manufacturers guarantee their reliability.
Important: PEX-AL-PEX is not suitable for welding or gluing, unlike PP pipes. Always use fittings designed specifically for PEX-AL-PEX – not for PEX-B or PP, as these are not interchangeable without an adapter.
Can I bend PEX-AL-PEX by hand, or do I need a bending tool?
It depends on the diameter. For 16 mm and 20 mm diameters, hand bending is possible (optionally with a simple hand bending spring), provided you maintain the minimum bending radius. For 16 mm this is typically 5x the outer diameter = at least 80 mm; for 20 mm approximately 100 mm. A smaller radius can cause "kinking" – flattening of the cross-section at the bend – which significantly increases hydraulic resistance and can lead to failure.
For 26 mm and 32 mm diameters, a bending tool is almost a necessity – these diameters are difficult to bend by hand and the risk of kinking is high. Professional bending tools (spring, lever, or hydraulic) ensure a constant radius without damaging the wall.
Another thing: PEX-AL-PEX behaves differently from copper when cold-bent. It has greater "springback," meaning it straightens slightly after release. You therefore need to bend it a bit more than the required final angle. Experienced installers do this intuitively, but newcomers may find it tricky. More technical details can be found in the article Minimum Bending Radius of PEX-AL-PEX Pipe and Proper Shaping Technique Without Kinking.
How long does PEX-AL-PEX last? Is it a reliable material?
The lifespan of PEX-AL-PEX according to manufacturers and European standards is at least 50 years under proper operating conditions (temperature, pressure, installation). In practice, if the pipe is correctly installed and not exposed to mechanical damage or UV radiation (UV degrades PEX under long-term direct exposure), the actual service life is even longer.
Where does PEX-AL-PEX fail? Almost never in the pipe itself, but at the joints – whether due to an incorrectly pressed joint, an unsuitable fitting, a joint exposed to vibration without fixation, or damage from cutting (incorrect cut using a tool meant for a different material). The pipe itself is remarkably durable – it doesn't corrode, doesn't scale internally, and doesn't crack.
From a customer-experience perspective: in recent years, problems with PEX-AL-PEX have appeared mainly with cheaper products from unknown brands with uneven aluminium layer thickness or poor-quality adhesive between layers. Under temperature cycling, the aluminium can delaminate from the PEX, leading to bubbles and deformations. Buy pipe from proven manufacturers with EN ISO 21003 certification.
Can PEX-AL-PEX be welded or repaired without replacing the whole section?
No, PEX-AL-PEX cannot be welded or fusion-joined. In case of failure (cracking, mechanical damage), the damaged section is cut out and replaced with a fitting – either a press fitting for runs embedded in concrete (which requires access), or a compression fitting for visible runs.
If the pipe is embedded in a concrete screed, the situation is more complicated. You need to locate the point of failure (pressure test, thermal camera), open the floor locally, replace the damaged section with press fittings at both ends, and pour the screed back. This is why thorough documentation of the pipe routing during installation is so important – mark the pipe positions on the floor plan or photograph them before pouring the screed. It will save an enormous amount of trouble.
Practical tip: some installers place entire sections in a protective conduit (sleeves) when embedding in concrete, which allows the pipe to be pulled out and replaced in case of failure without breaking up the floor. We recommend this solution especially for long straight runs.
Why do some people say PEX-AL-PEX is problematic? Is there any truth to it?
This skepticism has some basis – but it mostly stems from two sources: installation of cheap, uncertified products, or installation mistakes. Let's address this honestly.
Layer delamination: With low-quality pipe (weak adhesive between the PEX and AL layers), the layers can separate under temperature cycling. This leads to deformation, water leaks, or increased resistance. With quality pipe certified to EN ISO 21003, this problem doesn't occur.
Aluminium corrosion with unsuitable pH water: Aluminium is sensitive to extreme pH. If the water in the heating system is too acidic (pH below 6.5) or too alkaline (pH above 9), the aluminium layer can corrode from the inside. Typical heating water has a pH of 7–8.5, which is safe for PEX-AL-PEX. If you use inhibitors or antifreeze mixtures, always check their compatibility with aluminium.
Fitting problems: This is the most common real-world problem. Incorrect cutting (angled, jagged PEX), incorrect pressing (wrong press jaw profile, incomplete pressing), using fittings meant for a different pipe type – these are the causes of leaks. Not the pipe itself. More in the article Common Installation Mistakes with PEX-AL-PEX Pipe and How to Avoid Them.
Can I combine PEX-AL-PEX with other materials (copper, PP, steel)?
Yes, with a suitable transition fitting, you can combine PEX-AL-PEX with copper, steel, PP, or PE-RT. This is a common situation during renovations, where existing distribution is made of a different material and you just want to add new sections of PEX-AL-PEX.
When combining with copper or steel, make sure to properly use transition fittings (brass or stainless steel with the appropriate thread or press end). Avoid direct contact between aluminium and copper in an environment containing an electrolyte (water) – aluminium is more noble in the galvanic series, but contact with copper can, under unfavorable conditions (acidic water, high conductivity), cause galvanic corrosion. A proper transition fitting prevents this contact.
Practical example: during renovation of an older apartment building with existing copper riser pipes, customers typically run horizontal distribution within the apartment using PEX-AL-PEX and connect it to the vertical copper via a transition union. This works reliably for decades if done correctly.
Is PEX-AL-PEX suitable for heat pumps and condensing boilers?
Yes, PEX-AL-PEX is ideal for both heat pumps and condensing boilers. Heat pumps operate at low temperatures (35–55 °C), where PEX-AL-PEX excels. Condensing boilers also operate with low return temperatures, with the outlet temperature usually not exceeding 70–75 °C – well below the PEX-AL-PEX limit.
Important detail: for a heat pump with a direct evaporator or a cooling circuit (reversible heat pump), check whether the proposed pipe is also suitable for refrigerants or glycol mixtures. Standard PEX-AL-PEX is compatible with propylene glycol-water mixtures (common antifreeze for heat pumps), but always check pH and aluminium compatibility with propylene glycol.
Do I need to thermally insulate PEX-AL-PEX? Where yes, where no?
It depends on the application. For hot water distribution within the building structure (in a wall, floor, but not underfloor heating), thermal insulation is mandatory according to STN EN standards and the regulation on the energy performance of buildings. Without insulation, undesirable heat losses occur, increasing energy costs.
For underfloor heating loops embedded in screed, insulation on the pipe itself is not necessary – the point of this system is for heat to be released into the floor. Insulation belongs under the entire screed (EPS or mineral wool), not on the pipe.
For indoor cold water distribution, insulation is not mandatory but is recommended where surface condensation could occur (cold spaces, cellars) or where the pipe is close to heat sources.
Pipe routed outdoors or in unheated spaces (attic, garage) must be protected from freezing – below 0 °C, frozen water causes PEX-AL-PEX to crack, because although PEX is elastic, the aluminium layer is not. Use durable insulation or electric heating cables.
How does PEX-AL-PEX behave in a fire? Is it safe?
PEX-AL-PEX does not support combustion on its own – it is self-extinguishing. Under direct flame exposure (e.g. a welding torch nearby), it melts and can burn, but fire does not spread on its own along the pipe. From a fire safety standard perspective, it is a commonly accepted material for installations in residential buildings.
Practical warning: when working with a hot-air gun or near open flames (soldering copper pipes in the same room), protect PEX-AL-PEX from direct heat. PEX begins to soften at approximately 120–130 °C, a temperature easily reached by a hot-air gun or proximity to a flame.
Frequently Asked Questions (FAQ)
Can I use PEX-AL-PEX for utility water (toilet, washing machine, dishwasher)?
Yes, PEX-AL-PEX is fully suitable for both utility and drinking water distribution. For drinking water, the condition is using certified pipe (certified for contact with drinking water – e.g. DVGW, ACS). The pipe in our range meets these certifications. For utility water (toilet, laundry, garden), there are no special requirements – standard PEX-AL-PEX for water and heating is sufficient.
How long an underfloor heating loop can I make from 16 mm PEX-AL-PEX?
The recommended maximum length of a single loop for PEX-AL-PEX 16 x 2 is 80–100 m (including the supply and return sections from the manifold). Longer loops have a high pressure drop, are supplied unevenly with flow, and are poorly regulated by thermostatic heads. More on this topic can be found in the article PEX-AL-PEX Pipe for Underfloor Heating: Spacing, Loop Length and Maximum Temperature.
What happens if PEX-AL-PEX freezes in the floor?
Freezing is a critical situation. Water expands by ~9% in volume when it freezes, creating enormous pressure. PEX-AL-PEX – unlike copper – has some elasticity thanks to its PEX layers, but the aluminium jacket limits this elasticity. The result is usually cracking of the aluminium layer, leading to a leak once it thaws. Prevent this with proper thermal protection of the building, and if long-term draining is planned, add suitable antifreeze additives to the system.
Can I install PEX-AL-PEX myself (DIY), or do I need a certified installer?
From a technical standpoint, PEX-AL-PEX is not a material that is prohibited from DIY installation. For heating distribution in a family house, a handy person can manage it themselves, especially with compression fittings. For drinking water distribution, certain requirements for inspection and commissioning apply in Slovakia, but the installation itself is not legally restricted to certified installers only (unlike gas installation). Nevertheless, when embedding in concrete or using press joints, we strongly recommend an experienced installer – a mistake in embedded pipe is expensive to fix.
What is the difference between PEX-AL-PEX and PEX-B (without aluminium)?
There are three main differences: PEX-AL-PEX has an aluminium diffusion barrier (protects the system against oxygen), holds its shape after bending (shape memory), and has a lower coefficient of thermal expansion. PEX-B (pure PEX) is cheaper, more flexible, but straightens back out after bending and oxygen diffuses through it. A detailed comparison can be found in the article PEX-AL-PEX vs. Other Pipe Types: Comparison with Copper, Plastic and PEX-B.
Is a pressure test required when installing PEX-AL-PEX?
Yes, a pressure test is standard practice for every installation, and for underfloor heating it is absolutely mandatory before pouring the concrete screed. Typically, the system is pressurized to 1.5 times the operating pressure (at least 6 bar) and monitored for 30–60 minutes. A pressure drop indicates a leak. Before pouring, this is the last chance to detect a fault without demolition. Perform the test at ambient temperature and, if possible, also at elevated media temperature – some leaks only appear under thermal expansion.
Conclusion: PEX-AL-PEX is reliable – if used correctly
After years of experience with customers and installers, it can be summarized as follows: PEX-AL-PEX is an excellent material for both heating and water distribution, which, with correct installation and a quality product, lasts the entire lifetime of the building without problems. Most complaints and failures we encounter share a common denominator – not the material, but an installation mistake or a poor-quality product.
Invest in proven products, follow the correct installation procedure, document the pipe routes, and regularly monitor the pressure in the system. If recurring problems occur, read the articles Common Faults and Leaks in PEX-AL-PEX Pipe: Causes and Repair and Maintenance and Inspection of PEX-AL-PEX Distribution: What to Monitor Regularly – there you'll find concrete procedures, not just general advice.
Our range includes proven diameters from PEX-AL-PEX 16 x 2 for underfloor heating and individual radiators, through PEX-AL-PEX 20 x 2 and PEX-AL-PEX 26 x 3 for main distribution, up to PEX-AL-PEX 32 x 3 for larger buildings. You can find the full range in the category PEX-AL-PEX heating pipe.
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