How to Choose PEX-AL-PEX Pipe: Diameter, Wall Thickness and Pressure Parameters
Introduction: why choosing the right PEX-AL-PEX pipe size matters so much
PEX-AL-PEX pipe is one of the most widely used materials today for home heating systems, underfloor heating and drinking water distribution. Its popularity is no accident – it combines the shape memory of the aluminium layer, the chemical resistance of cross-linked polyethylene, and relatively simple installation. Nevertheless, in practice we often see that choosing the wrong diameter or wall thickness causes problems that only become apparent after the screed has been poured, the ceilings covered, or the walls insulated – in other words, at the moment when correcting the mistake becomes very expensive.
I have seen jobs where the installer used 16mm pipe for the main boiler room distribution because "they had it in stock," and the result was constant noise in the pipe and uneven heating of the radiators. But I have also seen the opposite – an unnecessarily oversized 32mm pipe in an underfloor heating loop, resulting in slow heating, high pump losses and inefficient regulation. Both extremes cost money and nerves.
This article will give you a comprehensive overview of how to navigate PEX-AL-PEX pipe dimensions, what the individual parameters on the label mean, what pressure and temperature limits apply, and above all – how to choose the right size for a specific application. If you are also interested in a comparison with other materials, check out the article PEX-AL-PEX vs. other pipe types: comparison with copper, plastic and PEX-B in the same Knowledge Center.
What does the marking "16 x 2" or "26 x 3" mean – anatomy of a size designation
The first number in a designation such as 16 x 2 indicates the outer diameter of the pipe in millimetres. The second number indicates the total wall thickness. This is important to note right from the start – it is not the inner diameter! The inner diameter, i.e. the bore, is obtained by a simple calculation: outer diameter minus twice the wall thickness.
Example calculation of bore diameters for common sizes:
| Size | Outer Ø (mm) | Wall thickness (mm) | Bore / inner Ø (mm) | Cross-sectional area (mm²) |
|---|---|---|---|---|
| 16 × 2 | 16 | 2.0 | 12 | 113.1 |
| 20 × 2 | 20 | 2.0 | 16 | 201.1 |
| 26 × 3 | 26 | 3.0 | 20 | 314.2 |
| 32 × 3 | 32 | 3.0 | 26 | 530.9 |
You can see that going from 16 × 2 to 20 × 2 seemingly isn't a big jump – the outer diameter increased by only 4 mm, but the cross-sectional area nearly doubled. This has a direct impact on flow rate and pressure loss. That's why it's not wise to choose a size "by eye" based on what "looks similar" to what you've seen installed at a neighbour's place.
Pressure and temperature parameters: what you need to know before choosing
Every PEX-AL-PEX pipe has defined maximum operating conditions. These parameters are usually given as a trio: maximum continuous temperature, maximum pressure at that temperature, and short-term maximum temperature (safety margin). Typical values for quality PEX-AL-PEX pipe are:
- Maximum continuous operating temperature: 95 °C
- Maximum operating pressure at 20 °C: 10 bar (1 MPa)
- Maximum operating pressure at 70 °C: 6 bar
- Maximum operating pressure at 95 °C: 4 bar
- Short-term maximum temperature (emergency condition): 110 °C for a maximum of 1 hour
This has several practical consequences. In a typical low-temperature heating circuit (underfloor heating, temperature parameters 40/30 °C or 45/35 °C), these pipes are significantly overdesigned in terms of both pressure and temperature. In a circuit with a condensing boiler and radiators (typically 70–75 °C), we are still within a safe range. Problems only occur with old boilers without regulation, where the water temperature can briefly spike above 90 °C.
Wall thickness is directly related to pressure parameters – a stronger wall allows for higher pressure. That's why the PEX-AL-PEX Pipe 26 x 3 with a 3mm wall has higher pressure resistance than a hypothetical 26 × 2 pipe made of the same material. Manufacturers express this through the SDR parameter (Standard Dimension Ratio = outer diameter / wall thickness). The lower the SDR, the thicker the wall relative to the diameter, and the higher the pressure rating. For 16 × 2, SDR = 8; for 26 × 3, SDR = 8.67; for 32 × 3, SDR = 10.67 – meaning 32 × 3 has a relatively thinner wall relative to its diameter, which should be kept in mind for higher-pressure applications.
Which diameter for which application: a practical guide
This is the core of the entire decision-making process. PEX-AL-PEX pipe sizes are not designed at random – each has its typical domain of use, although they naturally overlap.
16 × 2 mm – the king of underfloor heating and final connections
The PEX-AL-PEX Pipe 16 x 2 is by far the most commonly used size on Slovak construction sites. This is mainly because it is excellently suited for underfloor heating loops. A typical underfloor heating loop is 60–120 metres long with a flow rate of 0.05–0.15 l/s. For these parameters, a 12 mm bore is optimal – the pressure loss per metre is acceptable and pump requirements are low.
Typical uses of 16 × 2:
- Underfloor heating loops (each room a separate loop)
- Final section to the radiator (feeding one radiator, or possibly two in series)
- Drinking water distribution to individual fixtures (sink, toilet)
- Thin walls, limited space within the floor construction
What 16 × 2 cannot handle: don't try to supply water to an entire floor with multiple bathrooms through it. A flow rate of 0.3 l/s or more would cause a pressure loss requiring a very powerful pump. In addition, the noise of water flowing through the pipe would become annoying.
Interestingly, our category also includes VERME PE-AL-PEX Pipe 16 x 2 – a different manufacturer, same size, but there may be differences in the quality of the aluminium layer, the type of adhesive between the layers, and drinking water certifications. We will look at this in more detail later.
20 × 2 mm – universal distribution and radiator systems
The PEX-AL-PEX Pipe 20 x 2 is the second most commonly used size. Its 16 mm bore and 201 mm² cross-sectional area make it an ideal material for radiator circuits, where 2–4 radiators are fed from a single branch, or for horizontal distribution in apartments and family houses.
Typical uses of 20 × 2:
- Horizontal distribution on a floor (supplying a group of radiators or bathrooms)
- Connecting underfloor heating manifolds to the riser
- Hot/cold water distribution in a family house (horizontal branches)
- Supplying a storage water heater
In two-pipe radiator heating systems, 20 × 2 is the standard for branches supplying a group of 3–6 radiators with a total output of up to 8–10 kW. For smaller houses with simple distribution, many people manage with just a combination of 20 × 2 (horizontal distribution) and 16 × 2 (branches to individual radiators). I have seen this work flawlessly dozens of times, even after 15 years of operation.
26 × 3 mm – main distribution lines and boiler room
The PEX-AL-PEX Pipe 26 x 3 comes into play where a larger heat output needs to be transferred or several branches need to be supplied simultaneously. With a 20 mm bore, it can carry up to 0.35–0.40 l/s at a reasonable flow velocity (up to 1.5 m/s). The thicker 3 mm wall also provides better resistance to mechanical damage during handling and when passing through building structures.
Typical uses of 26 × 3:
- Main branches of the heating system in a larger family house or villa
- Connecting the boiler room to the manifold (buffer, hydraulic separator)
- Riser pipe in an apartment building (small and medium-sized buildings)
- Connecting the boiler to a storage tank
- Main hot water distribution lines in a hotel or guesthouse
32 × 3 mm – large buildings and primary circuits
The PEX-AL-PEX Pipe 32 x 3 is the largest commonly available size in this category. With a 26 mm bore and a cross-sectional area of 530 mm², it can carry 0.6 l/s or more at standard flow velocity. This corresponds to a heat output transfer of around 30–40 kW at a temperature difference of 12–15 K, which is sufficient for a larger family house or a small apartment building.
Typical uses of 32 × 3:
- Primary circuit of a heat pump
- Connecting solar collectors to storage
- Main riser in an apartment building with up to 8–10 units
- Supplying a larger underfloor heating manifold (10 or more loops)
- Technological distribution in industry (process water)
Wall thickness: why looking at the diameter alone is not enough
When we compare 26 × 3 with a hypothetical 26 × 2 (if it existed), both have the same outer diameter – meaning they take up the same amount of space in the structure, need the same size wall openings, and the same size clamps. But the inner diameter would be different: 20 mm vs. 22 mm. A seemingly small difference, but the cross-sectional area would increase by almost 21%. In addition, a thicker wall (3 mm) means:
- Higher permissible operating pressure – especially at higher temperatures
- Better resistance to mechanical stress – for example when the pipe passes through a concrete structure or is embedded in a screed
- Greater rigidity – less prone to kinking when bent
- Better resistance to puncture or backfill load
In practice, a 3mm wall occurs specifically in larger diameters (26 × 3 and 32 × 3), because at larger diameters, maintaining roundness and mechanical stability is more important. For smaller diameters (16 × 2 and 20 × 2), a 2mm wall is fully sufficient when used correctly.
Watch out for one detail: some suppliers also offer 16 × 2.2 or 20 × 2.25 – i.e. with a slightly thicker wall. This is not a mistake, quite the opposite – such pipe has a slightly higher operating pressure, but also a slightly smaller bore. Check the technical data sheet of the specific product before making your choice.
Maximum flow rate and flow velocity: how to size correctly
Experts agree on one basic rule: the flow velocity of water in a pipe should not permanently exceed 1.5 m/s for hot and cold water distribution and 1.0 m/s for heating. There are two reasons: erosion (at high velocity, water mechanically damages the pipe wall, especially at joints and bends) and noise (flow above 1.5 m/s is audible and annoying).
From the velocity and diameter, we can calculate the maximum flow rate:
| Size | Max. flow at 1.0 m/s (l/s) | Max. flow at 1.5 m/s (l/s) | Approx. heat output at ΔT=10K (kW) |
|---|---|---|---|
| 16 × 2 | 0.113 | 0.170 | 4.7 – 7.1 |
| 20 × 2 | 0.201 | 0.302 | 8.4 – 12.6 |
| 26 × 3 | 0.314 | 0.471 | 13.1 – 19.7 |
| 32 × 3 | 0.531 | 0.796 | 22.2 – 33.3 |
The heat output is approximate and depends on the temperature difference (ΔT) in the system. For low-temperature underfloor heating with ΔT = 5 K, these figures are half as high; for a classic radiator system with ΔT = 20 K, they are correspondingly higher.
Practical example: you have a family house with a total heat demand of 14 kW. You heat with radiators at a temperature difference of 70/55 °C (ΔT = 15 K). The required flow rate is Q = P / (c × ρ × ΔT) = 14,000 / (4186 × 1000 × 15) ≈ 0.22 l/s. The table shows that 20 × 2 can handle this flow rate at a velocity of around 1.1 m/s – borderline, but still acceptable. If we have a long run (more than 20 m), we recommend going with 26 × 3 instead, to have sufficient margin.
Certifications, standards and marking: what to trust on the label
PEX-AL-PEX pipe should comply with the European standard EN ISO 21003 (multilayer pipe systems for hot and cold water installations) or EN 15348 (for heating). These standards define requirements for material properties, dimensional tolerances, and testing of pressure resistance, temperature resistance and length expansion.
Pipe intended for drinking water distribution must also have certification according to relevant hygiene standards – in Europe this is typically WRAS (UK), KTW (Germany) or DVGW certification. Pipe without these certifications can be used for heating, but not for drinking water distribution, where undesirable substances could migrate into the water.
On the pipe label, you should always be able to see:
- Size (e.g. 16 × 2)
- Maximum operating temperature and pressure (e.g. Tmax 95 °C, Pmax 10 bar)
- Application (water / heating)
- Manufacturer and country of origin
- Batch number or production date
- Relevant certificates (EN ISO 21003, DVGW, etc.)
Be cautious with very cheap pipe from unknown sources – the aluminium layer may be thinner than stated by the manufacturer, or the adhesive between the layers may be of poor quality, leading to delamination under thermal load. A delaminated pipe loses its shape memory and increases the risk of leaks at the joints.
Thermal expansion and why it matters
One of the key properties of PEX-AL-PEX compared to pure plastic (e.g. PEX-B) is significantly lower thermal expansion. While pure PEX-B has a coefficient of linear thermal expansion α ≈ 0.15–0.20 mm/(m·K), PEX-AL-PEX, thanks to its aluminium layer, achieves α ≈ 0.026 mm/(m·K) – almost the same as copper (0.017 mm/(m·K)).
What does this mean in practice? 10 metres of PEX-AL-PEX pipe, when the temperature changes by 50 °C (e.g. from 20 °C to 70 °C), will elongate by:
ΔL = L × α × ΔT = 10 × 0.026 × 50 = 13 mm
The same calculation for pure PEX-B: ΔL = 10 × 0.18 × 50 = 90 mm. The difference is enormous – 90 mm vs. 13 mm. This is why PEX-AL-PEX doesn't require as many thermal expansion compensators and why it is more stable within a structure. Nevertheless, for long straight runs (over 8–10 m), we recommend using expansion bends or loops.
A larger pipe diameter does not automatically mean a larger absolute length expansion – it depends on the length of the section and the temperature range. However, the thicker wall of a larger diameter can create higher forces on anchor points during thermal expansion, which must be taken into account when designing pipe fixings.
Differences between manufacturers: what to watch out for
The PEX-AL-PEX pipe market is quite fragmented – there are dozens of manufacturers from Germany, Italy, Poland, China and other countries. A 16 × 2 size from different manufacturers may have the same outer dimensions but differ in these critical details:
- Aluminium layer thickness: Ranges from 0.15 mm to 0.4 mm. A thinner layer means lower rigidity and a worse barrier effect (oxygen, diffusion).
- Type of Al layer welding: Butt-weld vs. overlap-weld. Overlap welding is more resistant to opening up when bent.
- Type of adhesion: A chemically resistant adhesive between the PEX and Al layers is crucial – poor adhesion leads to delamination at temperatures of 70–80 °C.
- Drinking water certifications: Not every pipe is certified for drinking water – always check this if you plan to use the pipe for cold or hot water.
- Dimensional tolerances: Quality pipe has narrow outer diameter tolerances (±0.1 mm), which ensures reliable press fitting of joints.
Therefore, when choosing between different manufacturers, price should not be the only consideration. You'll pay 20–30% more for a verified product with certifications, but you'll save yourself potential warranty-claim stress five years down the line. Personally, I always recommend asking the seller for a technical data sheet with specific values for the Al layer thickness and certifications.
Practical scenarios from the field: how decisions are made on site
Scenario 1 – Renovating radiators in an older apartment building. The client has a 75 m² apartment, 3 radiators, and a wall-mounted boiler with a 14 kW output. The installer is considering what to use instead of the old steel pipes. Answer: from the boiler to the first branch point, use 20 × 2 (about 3 m); from the branch point to the individual radiators, use 16 × 2. Nothing more is needed – the system is small and the output modest.
Scenario 2 – New family house combining underfloor heating and radiators. A 180 m² house with a 24 kW boiler, 6 underfloor heating loops (480 m of pipe in total), and 4 radiators. The manifold is located in the central boiler room. Solution: from the boiler to the manifold, 26 × 3 (5 m); from the manifold to the radiator branch, 20 × 2 (8 m); individual radiators, 16 × 2; all underfloor heating loops, 16 × 2.
Scenario 3 – Heat pump and storage tank. A 12 kW air-to-water heat pump, a 500 l storage tank, and further distribution to a mixing unit. The primary circuit between the heat pump and tank: 32 × 3. Reason: with a low-temperature heat pump, ΔT is small (5–8 K), meaning a high flow rate is needed for the same output. Without a sufficient diameter, the flow would need to be faster, causing noise and pump wear.
Scenario 4 – Garden cabin. A single-room 30 m² cabin, one radiator, one 6 kW electric boiler. The entire installation uses 16 × 2 – from the boiler to the radiator and back, with a total length of 8 m. No complications, a simple choice.
FAQ: the most common questions about choosing PEX-AL-PEX diameter and parameters
Can I use 16 × 2 as the main pipe from the boiler to the manifold?
It depends on the boiler output and the length of the run. For a boiler up to 8–9 kW and a run of up to 5 metres, it can technically handle it, but it's borderline. For higher output or a longer run, 16 × 2 will cause too much pressure loss and flow noise. In practice, we recommend at least 20 × 2 for most boiler rooms, and 26 × 3 for outputs above 15 kW.
Why does 26 × 3 have a thicker wall than 16 × 2 and 20 × 2, which have only 2 mm?
This is to maintain mechanical rigidity at a larger diameter. A larger diameter pipe with the same wall thickness is less resistant to flattening and deformation – mathematically, the stiffness of the cross-section decreases as the ratio of diameter to wall thickness increases. Manufacturers therefore increase the wall thickness for larger diameters to maintain mechanical integrity and sufficient pressure parameters.
What is the maximum pressure in a typical home heating system, and is PEX-AL-PEX safe?
In a typical home heating system, the operating pressure ranges between 1.5 and 3 bar. The expansion vessel and safety valve are set so that the pressure does not exceed 3–4 bar. The maximum operating pressure of PEX-AL-PEX at 70 °C is 6 bar – so we have a twofold safety margin. The system is safe as long as the safety valve and expansion vessel are correctly designed.
Can I use heating pipe for drinking water as well?
Not automatically. It depends on the specific product and its certifications. The pipe must be certified for contact with drinking water (e.g. according to EN ISO 21003-2 or KTW/DVGW). Some products in the category are dual-use (heating and water), others are for heating only. Always check the certifications in the technical data sheet of the product before using it for drinking water distribution.
Is 32 × 3 suitable for a typical family house?
For most family houses up to 200 m², 32 × 3 is oversized as the main distribution line. An unnecessarily large diameter causes slower system heat-up (more water in the pipe), higher heat losses through the pipe surface, and greater demands on pump size. Exceptions include houses with a heat pump (where ΔT is small and flow rate is high), large houses over 250 m², or houses with combined systems (underfloor heating + radiators + water heating).
What is more important – diameter or wall thickness?
Both parameters are important, but for different purposes. Diameter (i.e. bore) determines flow rate and pressure loss – i.e. the hydraulics of the system. Wall thickness determines pressure and mechanical resistance. For most typical home installations, the priority is choosing the correct diameter – pressure parameters are always met with sufficient margin for standard sizes. Wall thickness becomes critical in special applications (industry, high-pressure systems, pipe in aggressive environments).
Conclusion: choosing the PEX-AL-PEX size as an investment in reliability
Choosing the right diameter and wall thickness for PEX-AL-PEX pipe is not just a technical formality – it's a decision that will affect the everyday comfort of your heating system for decades. Oversized pipe unnecessarily increases installation costs and worsens regulation properties. Undersized pipe jeopardises hydraulic balance, causes noise, and leads to excessive pump wear.
Basic rules to help you decide: for underfloor heating loops and the final section to a radiator, 16 × 2 is almost always the right choice; for horizontal distribution and groups of radiators, 20 × 2; for main distribution lines in a larger house or boiler room, 26 × 3; and for primary circuits of heat pumps or larger buildings, 32 × 3. Always verify these choices with a flow rate and pressure loss calculation, or by consulting a designer.
If you're interested in how to properly install, bend and press-fit the pipe, read the article Installing PEX-AL-PEX pipe: bending, press-fitting and correct joints step by step. For questions related to the minimum bending radius and shaping without kinking, there's also the article Minimum bending radius of PEX-AL-PEX pipe and correct shaping technique without kinking. And if you're not sure what specific diameter you need for your application, you'll find the answers in the article What diameter of PEX-AL-PEX pipe do I need for underfloor heating, radiators or water distribution.
The entire range of pipes including 16 × 2, 20 × 2, 26 × 3 and