What diameter of PEX-AL-PEX pipe do I need for underfloor heating, radiators or water pipes
What diameter of PEX-AL-PEX pipe do I need for underfloor heating, radiators or water pipes?
This is a question that almost every customer faces when planning heating or water plumbing systems. The seemingly simple choice of pipe diameter has, in practice, a much greater impact on the functionality of the entire system than it might appear at first glance. A pipe that is too narrow restricts flow and cannot cover the heat loss of a room, while a pipe that is too wide unnecessarily raises installation costs, makes bending more difficult, and takes up more space in the floor or in the grooves. In this article, we'll take a detailed look at choosing the right diameter, with real numbers and practical examples drawn from everyday installation practice.
What do the numbers in the PEX-AL-PEX pipe designation mean?
Before we get to the actual recommendations, it's important to understand what the numerical designation of the pipe actually represents. When we talk about, for example, PEX-AL-PEX pipe 16 x 2, the first number (16) is the outer diameter in millimeters and the second number (2) is the wall thickness in millimeters. The inner diameter – the so-called bore – is obtained by simple subtraction: 16 − 2 × 2 = 12 mm. It is precisely this bore that determines how much water the pipe can carry.
In practice, the most common PEX-AL-PEX pipe dimensions on the Slovak and Czech markets are:
- 16 × 2 mm – outer diameter 16 mm, bore 12 mm – the most popular choice for underfloor heating and thin distribution lines
- 20 × 2 mm – outer diameter 20 mm, bore 16 mm – standard for radiator distribution and stronger circuits
- 26 × 3 mm – outer diameter 26 mm, bore 20 mm – larger family houses, horizontal manifolds
- 32 × 3 mm – outer diameter 32 mm, bore 26 mm – main supply branches, holiday properties, larger installations
The aluminium layer in the middle of the "sandwich" is a key element – the pipe retains its shape even when bent and is not affected by oxygen diffusion, which is an issue with plain plastic pipe. You can find more about the structural advantages of PEX-AL-PEX over other materials in the article PEX-AL-PEX vs. other pipe types: comparison with copper, plastic and PEX-B.
Underfloor heating: why 16 × 2 mm is almost always the right choice
Underfloor heating is probably the most common application of PEX-AL-PEX pipe in Slovakia. Here the situation is fairly clear: for the heating loops themselves, laid in an anhydrite or cement screed, the 16 × 2 mm size is the industry standard and also the best choice for most cases. Why?
First – screed thickness. Underfloor heating is typically designed with a screed 65–80 mm thick above the insulation. A 16 mm pipe sits in this screed with sufficient coverage, whereas a 20 mm pipe would either require a thicker screed or could end up "protruding" closer to the surface finish, which is not desirable.
Second – bending radius. A 16 × 2 mm pipe has a minimum bending radius of about 5× the outer diameter, i.e. approximately 80 mm. This allows even tight loops with a spacing of 100–150 mm without kinking. With 20 mm pipe, the minimum bending radius is around 100–120 mm, which complicates work with dense loops in room corners.
Third – hydraulic balance. For a typical room of 15–25 m², a single loop of 16 mm pipe up to 100–120 metres long is hydraulically satisfactory. The pressure losses are manageable by calculation and the circulation pump can push the system through without problems.
When to use 20 × 2 mm even for underfloor heating?
There are situations where it makes sense to use a larger diameter even in an underfloor system. Specifically these include:
- Supply branches from the manifold – if the manifold serves multiple loops and is located further from the boiler, the supply branch should be 20 mm, sometimes even 26 mm, to minimise pressure loss on the supply line.
- Exceptionally long loops – in poorly insulated old houses or in irregularly shaped floor plans, where a single loop exceeds 120 metres, a 20 mm pipe can help reduce pressure losses and even out temperature distribution.
- Low-energy houses with large-area heating – where the supply water temperature is below 35 °C and the loops are long with a low temperature differential.
For more details on spacing, loop lengths and maximum temperature, see the separate article PEX-AL-PEX pipe for underfloor heating: spacing, loop length and maximum temperature.
Radiator distribution: diameter depends on output, not the number of radiators
For radiator systems, choosing the diameter is a bit more complicated, since it depends on the heat output that a given branch must carry and the length of the distribution line. A simplified rule that works in practice for most family houses:
- Supply to an individual radiator – 16 × 2 mm, or 20 × 2 mm for a longer run or a more powerful radiator (over 2 kW)
- Branch supplying 2–4 radiators – 20 × 2 mm as a minimum, 26 × 3 mm for higher outputs
- Main horizontal distribution in a family house – 26 × 3 mm or 32 × 3 mm depending on the total boiler output and the length of the pipework
A practical example – a typical 150 m² family house
Let's take a typical two-storey family house with a gable roof, a total heated area of 150 m², 12 radiators, and a condensing boiler with an output of 14 kW. The boiler is located in the utility room on the ground floor. What should the distribution diameters look like?
Main branch from the boiler to the horizontal manifold: 26 × 3 mm – this branch must carry the entire boiler output, and any unnecessary restriction here causes increased pressure losses and noise in the system.
Risers to the floors or to individual loops supplying 3–4 radiators: 20 × 2 mm – this is sufficient for most cases with a temperature differential of 10–15 °C.
Final branches to individual radiators: 16 × 2 mm – the classic "last mile" of the installation, where the pipe is bent around corners, routed through grooves in the walls, or under skirting boards.
Hydraulic calculation in brief – why diameter really matters
Pressure loss in a pipe increases with the square of the flow velocity and decreases with the fourth power of the inner diameter. In practice this means that doubling the diameter (at the same flow rate) reduces the pressure loss to one sixteenth! That's why choosing the correct diameter is especially crucial for long runs and high-flow circuits.
The recommended water velocity in a heating pipe is 0.3–1.0 m/s. If the velocity is lower, there's a risk of air bubbles settling and poor system balancing. If it's higher, noise occurs and fitting wear increases.
Cold and hot water distribution: different priorities, similar dimensions
PEX-AL-PEX is increasingly used for drinking water distribution as well – both cold and hot. Here slightly different rules for choosing the diameter apply, since with water we need to consider not only pressure losses but also comfort (outlet pressure) and hygienic aspects (heating time, stagnation).
Cold water
For cold drinking water distribution in an apartment or family house, 20 × 2 mm is the standard choice for the main horizontal distribution, while branches to individual fixtures (sink, toilet, shower) are usually 16 × 2 mm. For larger buildings or long distribution runs, 26 × 3 mm is recommended as the main riser.
An important factor with water is the network's inlet pressure. In Slovakia, public water supply pressure typically ranges from 2.5–6 bar. If your inlet pressure is 3 bar or less and the distribution run is long (over 20 m), it's advisable to choose one size larger in diameter so the pressure at the last outlet remains comfortable (at least 1.5–2 bar at the outlet).
Hot water – hygiene also needs to be considered
With hot water there's an additional hygienic aspect. Standards recommend that the volume of water in the pipe between the tank and the outlet point should not exceed 3 litres – otherwise there's a risk of stagnation and growth of Legionella bacteria in the pipe, which does not get heated up sufficiently with each draw-off. Therefore, for hot water the rule is: better a smaller diameter and shorter run than an unnecessarily large diameter.
For hot water in a typical apartment: 16 × 2 mm for branches to outlet points, 20 × 2 mm for the main distribution from the tank if the tank is far from the bathroom (for example in the basement). If you have a circulation pipe, you can safely use 20 × 2 mm even for longer runs.
Overview of recommended diameters by application
| Application | Recommended diameter | Note |
|---|---|---|
| Underfloor heating – heating loops | 16 × 2 mm | Standard for most applications, max. 100–120 m/loop |
| Underfloor heating – supply from manifold | 20 × 2 mm | Depends on number of loops and run length |
| Radiator – final branch | 16 × 2 mm | Sufficient for standard radiators up to 2 kW |
| Radiator branch – 2–4 units | 20 × 2 mm | Depends on total branch output |
| Main distribution in a family house (boiler) | 26 × 3 mm | For boiler output 10–20 kW, length up to 15–20 m |
| Main supply – large house, apartment building | 32 × 3 mm | Output above 20 kW or long run |
| Cold water – branches to fixtures | 16 × 2 mm | Sink, toilet, kitchen sink – standard outlet points |
| Cold water – main distribution in a house | 20 × 2 mm | Consider 26 × 3 mm if inlet pressure is low |
| Hot water – distribution in an apartment | 16 × 2 mm | Hygienic consideration – limit volume in pipe |
Wall thickness: why it's not just about diameter
Besides the outer diameter, wall thickness also plays a role. Notice that 26 × 3 mm and 32 × 3 mm pipes have a wall thickness of 3 mm instead of 2 mm as with the smaller sizes. This is not a coincidence – as the diameter increases, so does the mechanical stress from internal pressure, and therefore the wall must be thicker in order to maintain the same pressure and temperature parameters.
Typical pressure parameters of PEX-AL-PEX pipe are:
- Maximum operating pressure: 10 bar at 70 °C (most manufacturers)
- Maximum temperature at a standard operating pressure of 6 bar: 95 °C
- Short-term peak temperature: 110 °C
For underfloor heating, where the temperature rarely exceeds 45–50 °C, these parameters offer a large safety margin. For high-temperature radiator systems at 70–75 °C, PEX-AL-PEX is still fully suitable. A more detailed breakdown of pressure parameters and standards can be found in the article How to choose PEX-AL-PEX pipe: diameter, wall thickness and pressure parameters.
Branded pipe or economical version – does it matter?
On the market you'll find products from various price categories. For example, VERME PE-AL-PEX pipe 16 × 2 is a proven, more economical alternative for standard installations where the system parameters are within a comfortable range – i.e. not extremely high temperatures or pressures. It's always key to verify that the pipe is certified according to EN ISO 21003 (for water) or EN ISO 22391, or that the manufacturer declares the maximum pressure and temperature.
In practice I've seen installations where a customer saved a few euros on cheap unbranded pipe, but then couldn't find standard fittings for it – and had to either resize the entire run or buy special adapters. Certified pipe from established manufacturers guarantees dimensional compatibility with common press fittings, which really isn't a minor detail for a system that has to last decades once installed.
Practical scenarios: how I would decide on site
Scenario 1: New family house, combined system
A customer is building a 180 m² house, wants underfloor heating on the ground floor and radiators upstairs. Boiler 18 kW, utility room on the ground floor. Solution: From the boiler, 26 × 3 mm goes to the horizontal manifold. Two branches lead from the manifold – one for the underfloor loops on the ground floor (16 × 2 mm, 7 loops), the other a riser to the upper floor at 20 × 2 mm, which then splits further to 8 radiators at 16 × 2 mm. Result: a hydraulically balanced system without the need for an extremely powerful circulation pump.
Scenario 2: Apartment renovation, new water distribution
A customer is renovating a 3-room apartment and wants new cold and hot water distribution. An 80 l hot water tank in the bathroom, inlet pressure 4 bar. Solution: Main cold water branch from the water meter assembly at 20 × 2 mm to the manifold. Branches to the toilet, sinks, shower, kitchen – 16 × 2 mm. Hot water from the tank – 16 × 2 mm directly to the outlet points (the apartment is small, so stagnation is not an issue). Result: comfortable pressure even at the furthest shower, simple pipe routing under the plaster.
Scenario 3: Cottage with heat source in a boiler room, long run
A holiday cottage, boiler room 15 m from the cottage, routed with external pre-insulated pipe, radiators inside. Total output 12 kW. Solution: Outdoor section 26 × 3 mm (lower pressure loss on the long run), transitioning inside to 20 × 2 mm for the main distribution, 16 × 2 mm to the radiators. Without oversizing the outdoor run, the pressure loss over the 15-metre supply section would be problematic with 16 mm pipe.
Flexibility and space constraints: diameter also affects the work on site
Choosing the diameter isn't just a matter of hydraulics. A larger diameter pipe is heavier, less flexible, and requires a larger minimum bending radius. For work in grooves, behind ceilings, or in the floor, this can be a key factor. A 16 × 2 mm pipe can be bent almost by hand, while a 32 × 3 mm pipe requires a mechanical bender and the work of two people.
Indicative minimum bending radii (without a bender):
- 16 × 2 mm – approx. 80 mm (by hand with a mandrel or spring)
- 20 × 2 mm – approx. 100 mm (by hand, preferably with a bending spring)
- 26 × 3 mm – approx. 130–150 mm (bender recommended)
- 32 × 3 mm – approx. 160–200 mm (mechanical bender required)
Bending techniques and proper procedures are described in detail in the article Minimum bending radius of PEX-AL-PEX pipe and proper forming technique without kinking.
The most common mistakes in choosing diameter – what I've seen in practice
Over the years I've seen recurring mistakes that lead either to poor system function or to unnecessary costs during renovation.
Mistake #1 – Underestimating the cross-section of the main branch. A customer installs a 20 kW boiler, but makes the main branch to the manifold in 16 mm. Result: a noisy installation, boiler overheating, low efficiency. The only solution is to replace the pipework, which once installed means excavation or breaking open a groove.
Mistake #2 – Oversizing underfloor heating loops. Some installers put 20 mm pipe in the floor "for extra margin". Result: a thicker screed, worse bending radius, more expensive fittings, and no hydraulic advantage – since the circulation pump still has to overcome the same pressure loss at a lower flow velocity, which paradoxically worsens heating uniformity.
Mistake #3 – Ignoring the difference between output and number of radiators. A customer has a branch with 5 radiators and thinks 16 mm pipe will be enough for them. If these are powerful towel radiators or large panel radiators, the total output of the branch could be 6–8 kW, which is too much for a 16 mm line. This shows up as the last radiator on the branch being noticeably cooler than the first.
Other installation mistakes are discussed in the article Common mistakes when installing PEX-AL-PEX pipe and how to avoid them.
Frequently Asked Questions (FAQ)
Can I use 16 × 2 mm pipe for the main supply to an underfloor heating manifold?
It depends on the system's output and the length of the run. For small areas (up to about 60–80 m² of underfloor heating) and a short distance from the boiler to the manifold (up to 5–8 m), 16 × 2 mm may be sufficient, but it's on the edge. In most cases I recommend at least 20 × 2 mm for the supply to the manifold, so you have a safety margin and don't run into noise or unbalanced loops.
What PEX-AL-PEX diameter do you recommend for an apartment renovation – both heating and water?
For most apartment renovations, the answer is simple: 16 × 2 mm for water branches and radiator branches, and 20 × 2 mm for the main cold water distribution and for any heating risers serving multiple radiators. There's no reason to go to larger diameters unless the inlet pressure is weak or the runs are very long.
Is there a difference between 16 × 2 mm pipe from different manufacturers in terms of dimensional interchangeability?
The 16 mm outer diameter is standardised, and fittings from different manufacturers (Prandelli, Giacomini, Viega, and others) are dimensionally compatible with one another. However, you need to pay attention to the type of press fitting – some are pressed with a special jaw tool of the TH type, others with the M-Press system. The pressing technique must match the fitting system, not the pipe itself. Always check with your installer which pressing system they use before buying.
Can I combine different PEX-AL-PEX diameters in one system – for example 20 mm supply and 16 mm loops?
Not only can you – it's actually the recommended and common approach. Reducing fittings (reducers, reducing tees) are used for transitions between diameters. Every modern heating system is essentially a hierarchy of diameters: from the main line down to the fine loops. This is both an economically and hydraulically sound solution.
Can 16 × 2 mm PEX-AL-PEX pipe be used for hot water circulation?
Yes, 16 × 2 mm is suitable and commonly used for hot water circulation piping. The circulation flow rate is small (usually around 50–100 l/hour), so a bore of 12 mm (the inner diameter of 16 × 2 mm pipe) is more than sufficient. What matters is correctly designing the circulation branch and properly setting the circulation pump.
What's the difference between PE-AL-PEX and PEX-AL-PEX – does it affect diameter selection?
The designations PE-AL-PEX and PEX-AL-PEX are used for the same type of pipe – multilayer with an aluminium layer. PEX stands for crosslinked polyethylene, while PE can refer to either non-crosslinked or crosslinked material. In practice, sellers use these names interchangeably. Diameter selection is not affected by this designation – the same hydraulic and thermal parameters determine the choice.
Conclusion: three simple rules that won't let you down
When you're faced with choosing a PEX-AL-PEX pipe diameter and don't want to do complicated calculations, three proven rules from practice apply:
Rule one: For loops and branches (the final section to a fixture), go with 16 × 2 mm – this applies to underfloor loops, radiator branches, and water branches to outlet points alike.
Rule two: For branches supplying multiple fixtures or long horizontal distribution runs in a house, use 20 × 2 mm, or 26 × 3 mm if the system is larger or the distances longer.
Rule three: Never underestimate the main line from the boiler to the manifold – it's better to go one size larger, because this line is very difficult to change once installed, and the pressure loss on it affects the entire system.
If you're unsure, you'll find the full range of available sizes on the PEX-AL-PEX pipe category page, where you can compare specific products and their parameters. For larger projects, we recommend consulting a designer or having a hydraulic calculation done – especially when combining underfloor heating with radiators or when the building is large with long distribution runs.
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