How to choose pipe for underfloor heating: PEX vs. multilayer
How to choose pipe for floor heating: PEX vs. multilayer – complete technical guide
Choosing pipe for floor heating is a decision that will influence your system for decades. Pipe embedded in a concrete screed is not easily replaceable – this is something everyone who has ever participated in the renovation of floor heating in an older family home will realize. This topic therefore deserves much more attention than most people give it. In practice, I often encounter the fact that customers approach the purchase of pipe with the same indifference as when buying an extension cord – this is a mistake that can be avoided.
In this article, I will explain the difference between the two main material groups (PEX and multilayer pipe), show when each type is worth it, and give you concrete numbers you can rely on when designing and selecting. If you are looking for information on specific diameters, read the article What pipe diameter to choose for floor heating, where you will find a detailed comparison of sizes from 16 to 25 mm.
Basic classification of pipe for floor heating
Two main groups of plastic pipes are used on the market for floor heating:
- PEX pipes – made from crosslinked polyethylene (PE-Xa, PE-Xb or PE-Xc according to the crosslinking method)
- Multilayer pipes (PE-RT/Al/PE-RT or PEX/Al/PEX) – composite construction with an aluminum layer in the middle
Both groups are approved for floor heating systems, both can handle normal operating conditions, but each has different strengths and weaknesses. It is not a question of "good vs. bad", but "suitable for my specific case vs. less suitable".
PEX pipe: what it actually is and why it is so widespread
PEX (crosslinked polyethylene) is produced by modifying regular polyethylene using a crosslinking process. The result is a material with significantly better mechanical properties at higher temperatures – where regular PE would start to soften and lose shape, PEX remains rigid and durable. This is a key property for applications such as floor heating.
There are three main crosslinking methods for polyethylene, denoted as PE-Xa, PE-Xb and PE-Xc:
- PE-Xa – crosslinking with peroxide during production, highest degree of crosslinking (70–80 %), best shape memory, possibility of expansion using an expander. This is the premium class of PEX pipe.
- PE-Xb – crosslinking with silane after extrusion, degree of crosslinking 65–70 %, good price-to-performance ratio, the most widespread type on the market.
- PE-Xc – crosslinking with electron beams, degree of 60–70 %, even crosslinking across the cross-section, less common.
For floor heating, PE-Xa is the most suitable – its shape memory means that the pipe returns to its original shape after bending. In practice, this means that when laying loops in more complex spacing, the hoses will remain in place without the need to hold them every now and then or fix them with an extremely dense grid of clips. Products from our portfolio use exactly the PE-Xa technology.
Parameters of PEX pipe for floor heating
When you look at the technical data sheet of PEX pipe, you will notice the pressure class markings PN6 and PN10. These values are not absolute – they refer to a specific temperature. A typical situation looks like this: PN10 applies at a temperature of +60 °C, PN6 applies at a temperature of +90 °C. In other words, the higher the temperature, the lower the maximum operating pressure.
For standard low-temperature floor heating (water temperature 35–45 °C, maximum 55–60 °C), PEX pipe therefore operates with a large safety margin. More on this topic can be found in the article What pressure and temperature (PN6, PN10) suit my floor heating system.
Within this category, several configurations of PE-Xa pipe are available:
- PE-X 17×2 mm pipe in coils of 120 m – suitable for smaller areas, one loop in an apartment or one larger loop in a house
- PE-X 17×2 mm pipe in coils of 240 m – covers a standard family house with multiple loops without unnecessary joints
- PE-X 17×2 mm pipe in coils of 600 m – for larger projects, developer buildings or commercial spaces
- PE-X 20×2 mm pipe in coils of 200 m – larger diameter for higher flow or longer loops
- PE-X 25×2.3 mm pipe in coils of 200 m – for distribution lines or special applications
Multilayer pipe: aluminium as a key element
Multilayer pipe (also referred to as composite, or according to its structure PE-RT/Al/PE-RT or PEX/Al/PEX) has a fundamental structural difference compared to pure PEX: a thin welded aluminium foil is located between the inner and outer polyethylene layers. This aluminium layer typically has a thickness of 0.2–0.4 mm and is applied either longitudinally welded or spirally.
What does this aluminium layer bring?
- Oxygen barrier – aluminium is practically impermeable to oxygen. This is important for systems with steel components (boiler, radiators, pumps) – oxygen diffusing through the walls of PEX pipe causes corrosion.
- Stiffness and shape stability – after bending, multilayer pipe remains in position. For the installer, this can be an advantage (no fixation required during laying) as well as a disadvantage (it is more difficult to rewind onto a reel).
- Lower linear expansion – aluminium significantly limits thermal length expansion. PEX has an expansion of ~0.2 mm/m·K, while multilayer pipe has only ~0.025 mm/m·K. For long straight runs, this is a relevant factor.
- Higher stiffness when bending – requires a larger bending radius and greater force, bending tools are almost essential.
Oxygen diffusion: a real problem or a marketing argument?
This is a topic that is still being discussed among professionals. Pure PEX without an oxygen barrier does indeed allow a certain amount of oxygen to pass through – we are talking about values around 0.1 mg O₂/l·d (according to DIN 4726). For closed systems with proper degassing and without open joints (i.e., purely plastic or copper components), this is not a critical value.
Problems arise in systems where PEX pipe is connected to an older boiler with a cast iron or steel heat exchanger, steel radiators, or where the system is only partially degassed. In such cases, oxygen diffusion can accelerate corrosion. The solutions are two-fold: either use pipe with an oxygen barrier (multilayer), or install an oxygen separator into the hydraulic circuit.
Modern condensing boilers and heat pumps in closed systems with aluminium radiators have minimal problems with standard PEX without a barrier, provided the system is properly filled and degassed.
Main practical differences between PEX and multilayer pipe
Instead of long theories, here is an overview in a table that I have compiled based on experience from many projects:
| Property | PEX (PE-Xa) | Multilayer (Al) |
|---|---|---|
| Oxygen barrier | No (standard) / Yes (with EVOH layer) | Yes (aluminium layer) |
| Shape memory | Yes (PE-Xa) – springs back | No – holds shape |
| Flexibility / ease of laying | Excellent, can be bent without tools | Moderate, suitable bending tool |
| Thermal expansion | Higher (~0.2 mm/m·K) | Low (~0.025 mm/m·K) |
| Fittings and bends | Expansion (for PE-Xa) or crimped | Crimped, threaded |
| Price per meter | Low to moderate | Moderate to high |
| Recyclability | Simple (one material) | More complicated (separation of Al) |
| Lifespan | 50+ years | 50+ years |
When to choose PEX and when to choose multilayer pipe
Scenarios for PEX pipe
New construction with a heat pump or condensing boiler and aluminium radiators: A closed system, properly degassed, with low operating temperatures (35–50 °C). PEX without a barrier can handle this application without problems. In addition, laying PEX is faster – the pipe can be easily bent into loops without a bender, and its shape memory allows it to spring back into the plane, which installers appreciate with dense spacing (e.g., 10–15 cm in a bathroom).
Large area with many circuits: For a family home with 200–400 m² of floor heating, PEX is more economically advantageous. Larger reels – for example, PEX 17×2 mm / 600 m – reduce the price per meter and minimize waste. You can cover a larger project without unnecessary leftovers.
Customer with a limited budget: PEX is generally cheaper than equivalent multilayer pipe. If the system is properly designed and an oxygen separator is part of the solution, PEX is a full-featured and certified option.
Scenarios for multilayer pipe
Renovation of an older house with existing steel radiators and a cast iron boiler: Here, the oxygen barrier is really important. An older system, more open, less regularly degassed – oxygen diffusion could cause corrosion over the years. Multilayer pipe eliminates this risk directly in the wall.
Reconstruction with expansion tank of an open system: Although exceptional today, there are older installations. Here, the risk of oxygen ingress is maximal.
Visible or precast piping outside of concrete: Multilayer pipe maintains shape – when running piping in a technical room or in precast elements, it looks neat and requires fewer clamps than PEX.
Long straight runs: For distribution piping from the boiler to manifolds, where pipe is run in larger diameters (20 or 25 mm) over long sections, some designers appreciate the lower thermal expansion of multilayer piping. On the other hand, PEX requires expansion loops for long runs.
Dimensions and diameters: what to choose for floor heating
For floor heating circuits themselves (loops in concrete), the most commonly used today is an outer diameter of 17 mm (PE-Xa 17×2 mm – inner diameter 13 mm) or 20 mm (PE-Xa 20×2 mm – inner diameter 16 mm). Sizes 16 mm and 18 mm are also available from various manufacturers, but 17 mm is by far the most widespread standard in Slovak and Czech installations.
Basic rule from practice:
- 17×2 mm – circuits up to approx. 80–100 m, standard rooms, spacing 15–25 cm
- 20×2 mm – longer circuits up to 120 m, larger rooms or higher required heat output, or distribution piping
- 25×2.3 mm – larger distribution piping, connections between manifold and main circuit, low pressure loss on long runs
If you are solving the calculation of required length, I recommend the article How to calculate the required pipe length for floor heating – there you will find formulas and practical examples for various spacings and room areas.
Installation and laying: where the real decision is made
Experience from practice shows that installation errors cause more problems than the material itself. A few key rules:
Bend radius
PEX 17×2 mm has a minimum recommended cold bend radius of about 5–8 times the outer diameter – so with an outer diameter of 17 mm, at least 85–136 mm. With PE-Xa (shape memory), bending is easier and the risk of breakage is lower than with PE-Xb. Multilayer 16×2 mm has a higher recommended radius (greater resistance to bending), typically 5 times – but for maintaining shape, it is appreciated in elbows.
Loop spacing
Standard spacing is 10, 15, 20 or 25 cm. In bathrooms or entrance areas, denser spacing (10–15 cm) is used due to higher heat demand and lower floor temperature. In well-insulated living rooms, 20–25 cm is sufficient. A detailed laying procedure and fixing using clips or pipe channels is described in the article Installation of pipe for floor heating: spacing, laying and fixing.
Connections and fittings in concrete
Golden rule: no connection must remain in the concrete screed. Each loop must run from the manifold back to the manifold in one uninterrupted piece. This applies equally to PEX and multilayer pipe. Even high-quality connections are practically unreparable in concrete. Therefore, it is important to correctly estimate the length of each loop before laying – calculate with a reserve of 10–15 % extra.
Pressure test before concreting
Before concreting, a pressure test is mandatory. The system is filled with water and pressurized to 1.5–2 times the operating pressure (minimum 6 bar, ideally under pressure for 24 hours). During this time, you monitor whether the pressure drops. This is your last chance to detect leaks before concreting. More about tests in the article Common faults and leaks in floor heating pipe: causes and solutions.
Lifespan and long-term reliability
Both materials – PEX and multilayer pipe – will last 50 years or more without problems when installed correctly and within operating parameters. This figure is not a marketing claim, it is a value confirmed by long-term tests according to standards ISO 15874 and ISO 15875. When you install floor heating in a new building today, the pipe should last practically the entire life of the building.
Factors that shorten lifespan:
- Long-term exceedance of maximum temperature (90 °C at PN6) – e.g., regulation failure, missing thermostatic valve on the manifold
- Scale and corrosion from poor quality heating water (pH outside the range 6.5–9.0)
- Mechanical damage during construction or renovation (forgetting the position of pipes, when drilling or cutting)
- UV radiation – PEX degrades with prolonged exposure to direct sunlight (on the warehouse or when running outside)
About preventive care and how to extend the system's lifespan, you will learn more in the article Maintenance and descaling of floor heating pipe: how to extend lifespan.
Costs and economics of choice
Let's look at real numbers. For a family house with 150 m² of floor area at a spacing of 20 cm, you will need approximately 750–900 m of pipe (including distribution to manifolds and reserve). The price difference between PEX and equivalent multilayer pipe is on average 15–35 % in favor of PEX – depending on the specific manufacturer and dimension.
On such a project, this can mean a difference of several hundred euros. For most customers, the question is clear: if the system does not require an oxygen barrier, PEX is more economical. If an oxygen barrier is needed, you can consider either multilayer pipe, or PEX + EVOH barrier (a special version of PEX with an ethylene-vinyl alcohol layer), or PEX + oxygen separator in the system.
For larger projects where pipe is purchased in 600 m bundles, the economic advantage of PEX is even more noticeable. For example, PE-Xa 17×2 mm / 600 m provides a low price per meter and covers a larger project with one bundle without unnecessary joints and leftovers.
Most frequently asked questions (FAQ)
Is PEX pipe without an oxygen barrier suitable for a system with a condensing boiler?
Yes, in the vast majority of cases. Modern condensing boilers have aluminum or stainless steel heat exchangers, which are less sensitive to oxygen corrosion than older cast iron boilers. In addition, a closed system with an automatic air vent and proper filling has minimal oxygen diffusion. Problems occur only in the combination of PEX without a barrier + open expansion tank + steel boiler – such a combination practically does not occur in new constructions today.
Can I combine PEX and multi-layer pipe in one system?
Technically yes – for example, distribution lines from the technical room to the manifolds can be made of multi-layer pipe (rigid, holds shape), and the floor loops themselves from PEX (more flexible, cheaper). It is important to use the correct fittings for each type of pipe and not to make combined joints without special transition fittings.
What is the difference between PE-Xa and PE-Xb for floor heating?
Both meet the standards for floor heating. PE-Xa has a higher degree of cross-linking, better shape memory, and is suitable for expansion (Uponor/Wirsbo) fittings. PE-Xb is more common on the market, slightly cheaper, and connected with crimping or screw-on fittings. For standard floor loops, the difference in practice is minimal – the decision is based on price, availability, and the type of fittings your installer prefers.
How many meters of pipe do I need for 1 m² of floor area?
A simple calculation: with a spacing of 15 cm = approx. 6.7 m/m², with a spacing of 20 cm = 5 m/m², with a spacing of 25 cm = 4 m/m². You also need to add the routes from the manifold to the loops (so-called supply and return branches). For an accurate calculation including supply routes and reserves, read the article How to calculate the length of pipe needed for floor heating.
What is the minimum thickness of the concrete screed above the pipe?
The recommended thickness of the screed above the top of the pipe is at least 45–65 mm (so the total thickness of the screed from the insulation including the pipe is usually 65–85 mm). A thinner layer can cause cracking of the screed or so-called striping (visible heat stripes on the floor surface). For anhydrite screeds, the parameters may slightly differ according to the supplier of the mix.
What to do if I discover a leak after the floor is poured?
First step: localization using a thermal camera (heating the system to a higher temperature and monitoring the thermal spot) or a pressure test with gas (nitrogen) and leak detection using an acoustic device. The repair must be discussed with a specialist – in some cases the pipe is cut out, removed, and repaired with fittings; in others, the repair is not feasible without a complete replacement of the loop. More in the article Common faults and leaks in floor heating pipe: causes and solutions.
Conclusion: Which pipe do we really recommend
After years of practice and dozens of projects, our recommendation is quite clear: for new constructions and renovations with modern heat sources (heat pumps, condensing boilers) in closed systems, go for PEX (PE-Xa) pipe. It is more flexible, easier to lay, economically advantageous, and with proper installation it will last just as long as multi-layer pipe.
For renovations of older systems with steel components or in cases where oxygen diffusion is a real risk, multi-layer pipe with an aluminum layer is a safer choice, although at the cost of higher expenses and more complex installation.
When choosing a specific pipe dimension, rely on the hydraulic calculation of the loops – do not be tempted by the phrase "the larger the diameter, the better". An oversized diameter with low flow leads to worse balancing and lower water speed (risk of sedimentation). The optimal flow speed in floor loops is 0.3–0.6 m/s. If you need help with the calculation, see our article What pipe diameter to choose for floor heating.
A good floor heating design is based on three pillars: the right material, the right dimensions, and flawless installation. The pipe itself is only one of them – but it is the pillar you cannot go back to after pouring the concrete.
Do you have a question about this topic?
Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.