PEX-AL-PEX Pipe for Underfloor Heating: Spacing, Loop Length and Maximum Temperature

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PEX-AL-PEX Pipe for Underfloor Heating: Spacing, Loop Length and Maximum Temperature

Underfloor heating is now standard in new buildings and is increasingly common in renovations too. A properly designed underfloor heating system depends on three key parameters that are closely interrelated: the spacing of the pipes in the floor, the length of the individual loops, and the operating temperature of the water. If any of these parameters is "off," the result is either an insufficiently heated floor or, conversely, uneven overheating, higher pressure drop, and shortened system lifespan.

In this article, we will focus exclusively on PEX-AL-PEX pipe – a multilayer pipe with an aluminum layer – because its properties (shape memory, low oxygen diffusion, long lifespan) make it particularly suitable for underfloor heating. We will go through specific numerical values, show real-world examples, and explain why each of these parameters matters far more than it might seem at first glance.

Why pipe spacing in the floor matters so much

Pipe spacing (also referred to as "pitch" or "axial distance") directly determines how evenly the floor will heat up. The principle is simple: heat spreads sideways from the pipe through the screed. If the pipes are too far apart, cold strips form between them – this is known as the "zebra effect" on the floor. Conversely, too small a spacing unnecessarily lengthens the overall pipe run, increases the loop's pressure drop, and is neither economically nor technically justified.

In practice, spacing ranges from 10 to 30 cm. The specific value depends on several factors:

Room heat loss – the higher the heat loss (e.g., a corner room, large windows, poor insulation), the smaller the spacing needed to cover the loss at the same water temperature.
Water temperature in the system – with a low-temperature system (35/28 °C), a smaller spacing compensates for it; with a higher supply temperature, spacing can be increased.
Screed thickness and material – anhydrite conducts heat better than cement screed, allowing greater spacing at the same surface temperature.
Floor covering – tiles (ceramic, stone) have low thermal resistance, while wood flooring and thick carpets significantly increase thermal resistance and require smaller spacing.
Pipe diameter – thinner 16 mm pipe has lower output per running meter, so at the same spacing it delivers less heat than 20 mm pipe.

Typical spacing values for various situations

Experience from dozens of projects shows that most living rooms with good insulation (new construction according to current standards) work out to a spacing of 15 cm with 16×2 or 20×2 mm pipe and a supply temperature of around 40 °C. For perimeter zones near large glazed areas, spacing is reduced to 10 cm. Utility rooms, hallways, and storage areas are usually fine with a spacing of 20–25 cm.

An important note from practice: 10 cm spacing is technically and financially demanding. At this pitch, 10 running meters of pipe are used per square meter, which quickly raises overall costs. Therefore, this spacing is really only used in areas with high heat loss, not across the entire floor.

Maximum length of underfloor heating loops

Loop length is one of the parameters where mistakes are made most often in practice. Not because the principle is complicated, but because many installers – and DIY installers alike – underestimate the pressure drop of long loops. A loop that is too long means the circulation pump cannot guarantee sufficient flow, the water cools down too much by the far end of the pipe, and the last meters of the loop practically don't heat at all.

The maximum recommended loop length depends primarily on pipe diameter:

16×2 mm pipe: maximum 80–100 m per loop. In practice, most designers stick to 80 m as a safe limit.
20×2 mm pipe: maximum 100–120 m per loop. The larger internal diameter (16 mm) means lower pressure drop at the same flow rate.
26×3 mm pipe: used more for distribution lines from the boiler to the manifold, not directly for underfloor heating loops. Loop length here is not as strictly limited, since flow velocity and pressure drop behave differently.

Why is 80 m such an important limit for 16×2 mm pipe? The pressure drop of a circular pipe increases quadratically with flow velocity and linearly with length. For loops longer than 100 m with standard circulation pumps (typical pumps with a head of 6 m water column), the loop becomes hydraulically unbalanced compared to shorter loops on the manifold – even when equipped with flow regulators. In the worst case, water "takes the path of least resistance," so shorter loops get overheated and the long one stays underheated.

Practical example: a 150 m² family house

Imagine a family house with a total floor area of 150 m². The investor wants to cover the entire area with underfloor heating using PEX-AL-PEX 16 × 2 pipe and a spacing of 15 cm. That works out to 6.67 m of pipe per m². In total, this comes to 150 × 6.67 = approx. 1,000 m of pipe.

With a maximum loop length of 80 m, we need at least 13 loops. In practice, rooms are divided so that one loop covers one functional zone (living room, kitchen, bedroom, etc.) and its length does not exceed the limit. A bathroom with an area of 6 m² and a spacing of 10 cm needs only 60 m of pipe – that's one loop. A large 30 m² living room with a spacing of 15 cm needs 200 m – that's at least three separate loops.

Trying to "squeeze" the whole living room into a single 200 m loop is a classic mistake that shows up in the first heating season: the far part of the floor is noticeably colder, the customer complains, and fixing it requires either replacing the manifold or installing an additional booster pump. Both options are more expensive than getting the design right from the start.

Balancing loop lengths at the manifold

Ideally, all loops would be the same length – hydraulic balancing would then be simple. In reality, loop lengths differ because rooms have different areas. The solution is regulating valves on the manifold (flow meters), which allow throttling to adjust the flow of each loop according to its hydraulic resistance. Shorter loops are throttled, longer loops get full flow. This hydraulic balancing is a mandatory part of every correctly designed underfloor heating system.

Maximum water temperature for underfloor heating with PEX-AL-PEX

PEX-AL-PEX pipe is technically capable of working at much higher temperatures than the typical operating temperatures of underfloor heating. According to standards and manufacturers' technical documentation, the continuous operating temperature is 95 °C at 6 bar pressure, and short-term (emergency situations, pump failure) even up to 110 °C. This poses no problem for the pipe itself.

The limiting factor in underfloor heating, however, is not the pipe but the floor surface temperature. The European standard EN 1264-2 sets the maximum floor surface temperature:

Living spaces: maximum 29 °C floor surface
Bathrooms and hygiene areas: maximum 33 °C
Perimeter zones near exterior walls and under windows: maximum 35 °C

These floor surface limits directly determine the maximum water temperature in the system. If we want to achieve 29 °C on the floor surface with a 65 mm screed, a 15 cm spacing, and tile flooring, the supply water temperature typically must not exceed 45 °C at a typical supply/return temperature difference (delta T) of 5–8 °C. In other words, the system operates at 40/32 °C or 45/37 °C parameters, which is significantly less than the 95 °C the pipe is certified for.

Why the floor surface limit matters so much

The reason is both hygienic and physiological. Feet are very sensitive to temperature. A floor surface above 29 °C causes long-term discomfort, swelling of the feet, and health issues. It is somewhat paradoxical that although the heating system could technically handle much higher temperatures, user comfort is only achievable with a low-temperature setting. This is precisely why underfloor heating is almost always a low-temperature system, and why it works so well in combination with heat pumps, which are most efficient at low output temperatures.

In new buildings with good insulation (room heat loss of 30–40 W/m²), a supply temperature of 35–40 °C is fully sufficient. In renovations of older houses with poorer insulation (heat loss of 60–80 W/m²), a supply temperature of 50–55 °C may be needed, which can however be problematic in terms of surface comfort. In such cases, a combination with radiators for "top-up heating" is recommended, with underfloor heating providing the base thermal comfort.

Choosing the right PEX-AL-PEX pipe diameter for underfloor heating

Two dimensions are mainly used in practice for underfloor heating:

PEX-AL-PEX 16 × 2 mm pipe is by far the most commonly used diameter for underfloor loops in houses and apartments. The internal diameter of 12 mm ensures good hydraulic properties for lengths up to 80 m, the pipe bends well and takes up little space in the screed. At a 15 cm spacing and an 80-meter loop, it covers an area of approximately 12 m².

PEX-AL-PEX 20 × 2 mm pipe is used for larger loops or when higher output is needed (lower pressure drop at the same flow rate). The 16 mm internal diameter allows longer loops (up to 120 m) and higher flow rates. The disadvantage is greater stiffness – it needs a larger bending radius and takes up more space in the screed.

The 26 × 3 mm and 32 × 3 mm diameters are not commonly used for the underfloor loops themselves – they are used as main distribution lines from the boiler room or heat source to the distribution stations on individual floors or apartments.

Also worth mentioning is the VERME PE-AL-PEX 16 × 2 pipe, which is a suitable alternative for underfloor heating at the same dimensions, with manufacturing under quality guarantee ensuring stable mechanical properties over long-term operation.

Pipe-laying patterns: meander vs. spiral (snail pattern)

The way the pipe is laid in the floor affects how evenly the temperature is distributed across the entire area. There are two basic patterns:

Meander pattern (serpentine): The pipe is laid back and forth in parallel runs. The advantage is simplicity of installation. The disadvantage is that the supply (hot water) is always on one side of the room and the return (cooler water) on the other. The result is a temperature gradient – one half of the room is warmer than the other. This pattern is suitable for long, narrow rooms or perimeter zones.

Spiral pattern (snail): The pipe is laid in a nested spiral, with supply and return alternating next to each other. The result is a very even temperature distribution across the whole area, since the warm and cool sides compensate for each other. This pattern is technically more demanding to lay (it requires more turns and more space for the minimum bending radius), but for living spaces the comfort result is superior to the meander pattern. Most designers specify it for living rooms, bedrooms, and kitchens.

Protective sleeve and dilatation: what must not be forgotten

Thanks to its aluminum layer, PEX-AL-PEX pipe has a significantly lower thermal expansion than pure plastic pipe (linear expansion coefficient of approx. 0.026 mm/m·K compared to 0.15–0.17 mm/m·K for pure PEX), but it still needs to be protected when crossing expansion joints in the screed. At every point where the pipe crosses a floor expansion joint, it must be wrapped in a protective corrugated sleeve for at least 30 cm on each side of the joint (i.e., at least 60 cm total). Without this protection, the cyclical movement of the screed (heating – cooling) could mechanically damage the pipe at the contact point.

A protective sleeve is likewise used where the pipe passes through a wall (penetration), where the edges of the wall could mechanically damage the pipe surface as the structure moves. This is a detail that is very often overlooked in DIY installations – and its absence does not show up immediately, but only after several years of operation.

For more detailed information on proper bending and shaping technique, see the article Minimum bending radius of PEX-AL-PEX pipe and correct shaping technique without kinking, which also describes specific bending radii for individual diameters.

Practical step-by-step loop design procedure

To provide real practical value, let's summarize the loop design procedure as it would be carried out by an experienced technician:

1. Heat loss calculation: Before any loop planning, you need to know how much heating output each room requires. Heat loss depends on the area, ceiling height, insulation quality of walls, roof and windows, and the climate zone. Without this step, any design is just guesswork.

2. Determining the system supply temperature: Based on heat losses and the chosen floor covering, the supply water temperature is determined. For heat sources with variable output temperature (heat pump, condensing boiler), the system is designed for the local design outdoor temperature.

3. Spacing calculation: Using nomograms or software (e.g., from pipe manufacturers), the required pipe spacing is determined for each room. The result is a table: room – area – heat loss – spacing – pipe length.

4. Dividing into loops: Each room gets as many loops as required by the total pipe length (maximum 80/100 m per loop depending on diameter). We try to design the loops so their lengths are as similar as possible – hydraulic balancing is then easier.

5. Choosing the laying pattern: For each room, either a meander or spiral pattern is chosen. Meander for small, narrow spaces, spiral for large living spaces.

6. Selecting and sizing the manifold: The manifold is selected according to the number of loops. Each loop must have a regulating valve (for hydraulic balancing) and a flow meter (for setting and monitoring flow). Correct flow setting is critical for comfort – more in the article Installing PEX-AL-PEX pipe: bending, crimping, and proper joints step by step.

7. Pressure test before pouring: Before pouring the screed, the system must undergo a water pressure test (at least 6 bar, 24 hours). This is a legal requirement and also the only chance to detect any leak before the screed hides it for several decades.

Pipe consumption: calculation for typical projects

A practical question for every project is: how many meters of pipe should I order? The basic formula is:

Pipe consumption [m] = (room area [m²] / spacing [m]) + (allowance for supply and return to the manifold)

Example: Kitchen 10 m², spacing 15 cm (0.15 m): 10 / 0.15 = 66.7 m for the loop itself. Allowance for supply + return to the manifold (depends on distance): e.g. 2 × 4 m = 8 m. Total consumption: approx. 75 m. One roll of PEX-AL-PEX 16 × 2 is commonly supplied in 100 m coils – the kitchen will be covered from one coil with a margin.

A reserve of 10–15% is reasonable. The pipe is relatively cheap, but a shortage on site can delay work by several days. It is better to order one extra coil and return the surplus (if the supplier allows returns) or use it for other purposes, rather than run short halfway through the project.

Maximum temperature for different types of floor coverings

The floor covering directly affects the maximum achievable floor surface temperature and thus the overall heating output of the system. This is an area where customers are most often mistaken – they want underfloor heating under wood, but are then disappointed with the output.

The following approximate limits apply to each type of floor covering:

Ceramic tiles, stone, concrete: The best heat conductor, with minimal thermal resistance (R ≈ 0.01–0.03 m²K/W). The system works most efficiently, achieving the highest output at the same water temperature. The maximum surface temperature of 29 °C (or 33 °C in bathrooms) is not exceeded even at a higher supply temperature.
Vinyl tiles (LVT, SPC): Slightly higher thermal resistance, but still suitable for underfloor heating. Manufacturers state a maximum floor thermal resistance of 0.15 m²K/W, which most vinyl flooring meets.
Wood flooring (solid, engineered): Thermal resistance depends on thickness and wood type. A 14 mm oak floor has R ≈ 0.09 m²K/W, which is still acceptable. Thicker solid wood (22 mm) can have R up to 0.15 m²K/W. Most wood flooring manufacturers condition the warranty on maintaining a maximum surface temperature of 27 °C and a maximum wood relative humidity of 7–9%. Underfloor heating under wood flooring is feasible, but requires careful product selection and precise system setup.
Carpets: Thermal resistance of 0.10–0.20 m²K/W. Underfloor heating under carpet is only possible with low R values. Thick, woolen carpets practically eliminate the output of underfloor heating – the heat doesn't reach the surface and ends up in the screed.
Floating floors (laminate): Thermal resistance depends on the product. Special "laminate for underfloor heating" has R ≤ 0.10 m²K/W. Standard laminate on a thick foam underlay can have R up to 0.25 m²K/W, which is unsuitable for underfloor heating.

Most common mistakes in sizing and installation

After years of practice, it is clear that most underfloor heating problems stem from the same typical mistakes. Knowing them will protect you from costly repairs:

Loops that are too long: Loops of 120–150 m for 16 mm pipe lead to insufficient flow, uneven temperatures, and permanent balancing issues.
Skipping the pressure test: A leak discovered only after the screed has been poured is a nightmare for both installer and customer.
Forgetting the perimeter expansion strip: An expansion foam strip around the perimeter of each room compensates for the thermal expansion of the screed. Without it, the screed cracks during the first heating.
Rushing the first heat-up: The screed must cure for at least 21 days, and the first heat-up must be gradual (starting at 25 °C and increasing by 5 °C each day). Sudden heating to full output leads to screed cracking.
Incorrect choice of floor covering: Carpets and thick wood flooring over underfloor heating – a frequent customer disappointment that keeps recurring.
Absence of hydraulic balancing: Without setting the flow meter on each loop, the system will never work properly, no matter how good the pipe quality is.

More on typical mistakes can be found in the article Common mistakes when installing PEX-AL-PEX pipe and how to avoid them, where these situations are discussed in much more detail, including correction procedures.

Combining underfloor heating with radiators

In older houses with a poorer thermal envelope, it often happens that underfloor heating alone cannot cover the heat losses – especially during extreme frosts. In such cases, the solution is a combined system: underfloor heating for basic thermal comfort and radiators (or floor convectors) for "supplementary" heating at low outdoor temperatures.

Combining the two requires a mixing station (a three-way or four-way valve with a pump) that lowers the boiler water temperature (e.g., 70 °C) to a temperature suitable for the underfloor loop (e.g., 40 °C). Radiators are connected directly to the primary circuit with a higher temperature, while the underfloor system has its own secondary circuit with temperature control.

This solution is standard and hydraulically commonly implemented. It is important that both branches have their own circulation pumps and that the safety valves and expansion tanks are correctly set for both circuits. Details of hydraulic connection are the topic of a separate article, but in terms of piping the rule is: larger diameter pipe (e.g. PEX-AL-PEX 26 × 3 or 32 × 3) is used for primary distribution from the boiler to the mixing station, while 16 × 2 or 20 × 2 remains for the secondary underfloor loops.

Frequently Asked Questions (FAQ)

What pipe spacing should I choose for a well-insulated new building?

For a new building meeting current thermal protection standards (U-values of walls, roof, and windows according to STN 73 0540), a spacing of 15 cm at a diameter of 16 × 2 mm and a supply temperature of 40 °C is the standard solution for most living rooms. For large glazed areas and corner rooms, the spacing in the perimeter zone (width 50–80 cm near the exterior wall) is reduced to 10 cm to compensate for higher heat losses. In utility rooms, hallways, and basements, 20–25 cm is sufficient.

Can I extend a loop beyond 100 m if I install a stronger pump?

Technically yes – a stronger pump can overcome the higher pressure drop of a long loop. The problem, however, is hydraulic balancing of the whole system. If you have a manifold with 8–10 loops of different lengths and one loop is significantly longer, a strong pump will overheat the shorter loops while the long loop remains underheated. A better solution is always to split a long loop into two shorter ones. The maximum lengths of 80 m for 16×2 mm and 100–120 m for 20×2 mm are designed so that the system works with standard pumps and standard manifolds.

What is the actual maximum operating temperature of PEX-AL-PEX pipe in underfloor heating?

The PEX-AL-PEX pipe itself is certified for a continuous operating temperature of 95 °C at 6 bar. In underfloor heating, however, the limiting factor is the floor surface temperature, which according to EN 1264-2 must not exceed 29 °C in living spaces (33 °C in bathrooms, 35 °C in perimeter zones). In practice, this means a maximum supply water temperature of 45–55 °C under normal settings. Most systems operate at 35–45 °C, which is well below the pipe's technical limits. The pipe is nowhere near the limits of its parameters when used for underfloor heating.

Is there a difference between PEX-AL-PEX and PE-AL-PEX? Can I mix them within one system?

In practice, both names are used for the same type of multilayer pipe. The difference is only in manufacturers' nomenclature: PE-AL-PEX emphasizes the polyethylene layers (PE), while PEX-AL-PEX emphasizes cross-linked polyethylene (PEX-a or PEX-b). What matters is that both layers are made of cross-linked polyethylene – this ensures the temperature and pressure parameters needed for heating. Products from different manufacturers are compatible with each other via standardized press or screw fittings of the same diameter. Mixing diameters within a system is of course possible (e.g. 20 mm distribution to the manifold, 16 mm underfloor loops).

Do I need special pipe for underfloor heating, or is standard PEX-AL-PEX for water sufficient?

For underfloor heating, the same PEX-AL-PEX pipe used for water distribution is standardly used – no special type is required. It is important that the pipe be certified for both drinking water and heating (a combination most manufacturers meet), that it has sufficient thermal resistance (min. 95 °C at 6 bar), and low oxygen diffusion (the aluminum layer ensures this). Products such as PEX-AL-PEX 16 × 2 or 20 × 2 from the atria.sk category meet these requirements as standard.

How many loops do I need for a 130 m² family house?

At an average spacing of 15 cm and a diameter of 16 × 2 mm, pipe consumption works out to approx. 6.7 m per m². For 130 m², that's about 870 m of pipe. With a maximum loop length of 80 m, we need at least 11 loops. In practice, rooms are divided according to function and area – a bathroom is usually 1 loop, a bedroom 1–2, a living room with kitchen 3–4. A total of 10–14 loops for a 130 m² house is a common project outcome. We recommend consulting the design with a designer – a correctly sized system is not something to estimate "by eye."

Conclusion: field-tested principles for a reliable underfloor heating system

Underfloor heating with PEX-AL-PEX pipe, when properly designed, is a system that reliably serves for decades with minimal maintenance. The key to success lies in three principles we have covered in detail in this article: pipe spacing must match the room's heat loss and the chosen system settings, loop length must not exceed the hydraulically feasible

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