What pressure and temperature (PN6, PN10) suit my underfloor heating system

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Pressure and temperature in a floor heating system: what PN6 and PN10 mean and why it matters

When you look at the technical specifications of any floor heating pipe, you will encounter the abbreviations PN6 and PN10. Most people ignore them as "numbers for technicians" and buy the first pipe with the correct diameter. This is a mistake that can manifest years later – as a crack in the concrete subfloor, a deformed floor, or a quiet leak that you only discover when moisture starts climbing up the wall of an adjacent room. This article explains what PN6 and PN10 really mean, what pressures and temperatures are typical in a floor heating system in Slovakia, and how to consciously choose a pipe that will last for decades without problems.

What PN means – nominal pressure in short

PN is an abbreviation from the French Pression Nominale, meaning nominal pressure. In practice, it means the maximum continuous operating pressure in bars that a given product can withstand at a reference water temperature of 20 °C. That is:

PN6 = maximum continuous operating pressure 6 bar at a medium temperature of 20 °C
PN10 = maximum continuous operating pressure 10 bar at a medium temperature of 20 °C

This sounds simple, but here comes the first catch where many people make a mistake: these values apply to cold water (20 °C). As soon as you increase the medium temperature, the actual pressure load that the pipe can withstand decreases. Not linearly, but quite significantly. That is why manufacturers of PEX pipes list combined values – for example:

PN6 at a temperature of +90 °C, PN10 at a temperature of +60 °C

This is exactly what you will find in the product parameters of items such as PEX Pipe 17x2 – 120 m or PEX Pipe 20x2 – 200 m. This dual labeling tells you: if the water going through the pipe is as hot as 90 °C, the pipe can safely withstand a pressure of 6 bar. If the water is cooler – 60 °C – the pipe can safely withstand 10 bar. Between these two extreme states, there is a smooth dependency.

What pressures and temperatures are actually in a floor heating system

Now let's look at what actually happens in a floor heating pipe loop in a typical family home. Unlike radiator systems, where the heating water temperature can reach 70–80 °C, floor heating works fundamentally differently.

Typical operating temperatures

A modern floor heating system typically operates with the following parameters:

Supply water temperature (flow temperature): 30–45 °C in normal operation, occasionally up to 50–55 °C in extreme cold or with poor house insulation
Return water temperature (return temperature): 25–38 °C
Temperature difference between supply and return (ΔT): usually 5–10 K
Maximum allowed floor temperature: 29 °C in occupied zones, 35 °C in edge zones (according to EN 1264)

Heat pumps, which are today the most common source of heat for floor heating, typically supply water at 35–45 °C. Condensing boilers with proportional control operate in the range of 35–55 °C. Only old direct gas boilers without proportional control can push water at 60 °C or more into the floor heating loop – but in such a case, a mixing unit must be installed to reduce the temperature before it enters the loops.

Typical operating pressures

The pressure in a floor heating system depends on several factors:

Static pressure (hydrostatic): in a single-story family house, it is practically zero load. Every 10 meters of water column height corresponds to 1 bar. In a typical house with floor heating, the height difference between the manifold and the highest point of the loop is minimal.
Operating pressure (static + dynamic): most systems in family homes operate with a filling pressure of 1.0–1.5 bar, and the operating pressure after heating rises to 1.5–2.5 bar
Pressure during system filling: when filling a cold system, it is recommended to fill it to 1.0–1.2 bar
Maximum safety pressure: the safety valve on the boiler is usually set to 3 bar

This leads to a simple conclusion: standard floor heating in a single-family house operates at a pressure of 1.5–2.5 bar and a water temperature of 35–55 °C. This is far below the PN6 limit even at high temperatures. Both parameters – PN6 and PN10 – easily meet these conditions with a multiple safety margin.

When does the choice between PN6 and PN10 actually matter

If normal operation is so far from the limits, why do we even talk about PN6 vs. PN10? Because the pressure in the system is never just the operating pressure. There are several situations where pressure increases significantly:

Pressure testing before embedding

This is the most important practical case. Before the pipe is embedded in concrete, a pressure test must be carried out. This test is performed at a much higher pressure than the operating pressure. The standard EN 1264-4 and most manufacturers recommend a test pressure of 6 bar (for PN10 pipe even 8–10 bar) for at least 24 hours. Some designers and installers set the test pressure as high as double the operating pressure, plus 1 bar.

Here is a crucial difference: PN6 pipe can be pressure tested at 6 bar, which is its nominal limit. PN10 pipe allows for testing at a higher pressure with a higher safety margin. Any repair after embedding becomes extremely costly and invasive – this is why pressure tests are carried out before embedding, precisely to detect any potential problems (faulty connections, damaged areas) in time.

Water hammer (hydraulic shock)

Water hammer is a pressure impulse that occurs when a valve is suddenly closed or when a circulation pump starts up. In floor heating systems, water hammer is usually dampened by an expansion tank and slow opening of thermostatic heads, but if the system is poorly designed or the expansion tank is undersized, short-term pressure peaks can reach 3–5 bar.

Power outage and restart

During system restart after a power outage, a rapid start of the pump in a cold system, where the water is denser, can again generate short-term pressure surges.

Multi-zone systems and apartment buildings

In apartment buildings or larger commercial structures, where the floor heating system has multiple floors and long distribution lines, operating pressures are significantly higher. A boiler in the basement supplies the upper floors, the static pressure of the water column increases, and dynamic pressure from circulation pumps is added. Here, PN10 is clearly the right choice.

PEX-a, PEX-b and multilayer pipe – how they differ in behavior under pressure

Not all pipes with the same PN rating are the same. It depends not only on the nominal pressure, but also on the type of material and pipe construction. In practice, for floor heating you mainly encounter:

PEX-a (cross-linked polyethylene by the Engel method)

This is the highest quality type of PEX in terms of cross-linking degree (minimum 70 %). It has a pronounced memory effect – after deformation, the pipe returns to its original shape when heated. PEX-a has the best mechanical properties of all PEX types at higher temperatures, giving it real operating resistance at the level and slightly above the declared PN values. This is the type of pipe you will find on atria.sk – for example, PEX Pipe 17x2 – 240 m or for larger projects PEX Pipe 17x2 – 600 m.

PEX-b and PEX-c

Cheaper variants with a lower cross-linking degree (PEX-b cross-linked with peroxides, PEX-c with electron beam). The declared PN values are the same, but the real long-term resistance at the combination of high temperatures and pressures is slightly lower. The difference becomes apparent especially during long-term operation at the upper limit of the parameters.

Multilayer (PEX-AL-PEX) pipe

It has an aluminum layer inside, which gives it significantly better pressure resistance and thermal stability – typical values are PN10 at 95 °C. The disadvantage is the higher price, worse flexibility, and the need for special fittings. More about this option can be found in the article PEX pipe vs. multilayer pipe: differences, advantages and disadvantages.

Practical scenarios: how to decide for a specific situation

Theory is clear – now let's look at how it looks in everyday practice for different types of buildings and heat sources.

Scenario 1: New single-family house with a heat pump

This is the most common situation in Slovakia today. An air-to-water or ground-to-water heat pump delivers water at a temperature of 35–45 °C. The system operating pressure is 1.5–2.5 bar. A 17 mm or 20 mm pipe for floor heating with the marking PN6 at 90 °C / PN10 at 60 °C provides more than double the safety margin compared to actual operating conditions. A pressure test of 6 bar before concreting does not pose any problem for such a pipe.

Recommendation: PEX pipe 17x2 or 20x2 with the declaration PN6/90°C and PN10/60°C is fully sufficient. For example, PEX Pipe 20x2 – 200 m for larger areas or PEX Pipe 25x2.3 – 200 m for distribution sections with higher flow.

Scenario 2: Reconstruction of an older house with a gas boiler without mixing

This situation is more complicated. An older gas boiler without an equithermal control can let water into the system at a temperature of 70–80 °C. If the investor plans to connect the floor heating directly without a mixing unit (which is bad), the pipe will be exposed to high temperatures. In this case, PN6 at 90 °C is still sufficient from the pressure point of view, but the problem is the long-term thermal aging of the PEX material when repeatedly heated above 60 °C. Moreover, the standard EN 1264 itself limits the temperature of the floor water.

Recommendation: Install a mixing unit (thermostatic or motorized) and limit the supply water temperature to the floor heating loop to a maximum of 55 °C. Then the situation is identical to scenario 1.

Scenario 3: Apartment building, 4–6 floors, central boiler room

In an apartment building with a central boiler room, the situation is fundamentally different. The distribution system from the boiler room to individual apartments operates at a higher pressure – easily 3–5 bar just due to the static pressure of the water column. In addition, the pressure from circulation pumps is added. In each apartment, there must be an apartment-specific control unit, but the pressures entering the floor heating loop can be higher than in a single-family house.

Recommendation: In such projects, PN10 at 60 °C is a mandatory minimum, ideally with multi-layer PEX-AL-PEX pipe, which has better dimensional stability and lower thermal expansion. The design documentation must be prepared by a qualified person.

Scenario 4: Industrial hall or commercial space

Larger spaces, larger pumps, longer distribution lines – the pressures are higher. The heat source can be a boiler with a higher output or an air heating unit. Floor heating in such halls operates with larger pipe diameters (25 mm and more) and higher pressures. Here, PN10 is the standard and the project must include a hydraulic calculation.

Wall thickness of the pipe and its relation to pressure class

The nominal pressure PN is not only related to the type of material, but also directly to the wall thickness of the pipe. For PEX pipe, the thicker the wall, the higher the allowable pressure at a given temperature. The wall thickness dimensioning is based on the ratio of the outer diameter to the wall thickness (SDR – Standard Dimension Ratio):

SDR = outer diameter / wall thickness

Lower SDR = relatively thicker wall = higher pressure resistance. For floor heating typically:

Pipe 17x2 mm → SDR = 17/2 = 8.5
Pipe 20x2 mm → SDR = 20/2 = 10
Pipe 25x2.3 mm → SDR = 25/2.3 ≈ 10.9

Pipe with a smaller SDR (e.g. 17x2 compared to 20x2) has a relatively thicker wall in relation to the diameter, which gives it better pressure properties. This is one of the reasons why 17 mm pipe with a wall thickness of 2 mm has proven itself as the golden standard for floor heating – it combines good flow, low pressure losses and decent pressure resistance.

More about the relationship between diameter and hydraulic losses can be found in the article What pipe diameter to choose for floor heating (16, 17, 18, 20, 25 mm), where flow velocities and hydraulic resistances of individual dimensions are also analyzed in detail.

Pipe aging and long-term pressure resistance

Another factor that most people ignore: the pipe does not age uniformly. Long-term pressure and temperature loading causes so-called creep – a slow plastic deformation of the material. For PEX pipe, the lifespan at correct operating parameters is guaranteed by manufacturers for 50 years. The condition is that the temperature and pressure do not exceed the declared limits and the water is not aggressive (pH 6.5–9.5, low chloride content).

Problems arise in the following cases:

Repeatedly exceeding the maximum temperature (e.g. a poorly adjusted boiler, lack of control)
Long-term operation at the upper limit of pressure and temperature simultaneously
Mechanical damage to the pipe during installation (bending with too small a radius)
Oxygen diffusion through the pipe wall into the system – therefore always use pipe with an EVOH oxygen barrier for heating systems

The topic of damages and their causes is discussed in detail in the article Common faults and leaks in floor heating pipes: causes and solutions and How to prevent overheating and pipe damage in floor heating.

How does a pressure test proceed and what should you know before embedding

The pressure test is a key moment in every floor heating installation, and it is precisely here that the PN parameters become practically relevant. The standard pressure test procedure:

Filling the system with water and purging all circuits at the manifold
Pressure filling to the test pressure – for PN6 pipe maximum 6 bar, for PN10 maximum 10 bar (recommended practice: 2× operating pressure + 1 bar, minimum 6 bar)
Visual inspection of all connections, fittings and transitions
Monitoring the pressure for 24 hours – the drop must not exceed the values according to the standard (EN 14336 allows a maximum of 0.2 bar drop per hour in the first phase, 0.02 bar per hour in the stabilized phase)
Pressure test report – signed by the plumber and the investor, archived

Important: during the pressure test, the system must be cold (or at most at a temperature of 20–30 °C), because hot water would reduce the effective PN value. The test report is later proof of correct installation, for example in warranty disputes or insurance claims.

Expansion tank and safety valve – components protecting the pipe from pressure overload

The floor heating system is not an isolated pipe – it is part of the entire hydraulic circuit. Two devices directly protect the pipe from exceeding the maximum pressure:

Expansion tank: absorbs the increase in water volume when heated (water expands by about 2.5 % when heated from 10 °C to 70 °C). An undersized expansion tank will cause the pressure in the system to rise disproportionately with each heating – and the safety valve will increasingly frequently discharge. A properly sized expansion tank keeps the pressure stable within a range of 0.5 bar between the cold and hot state of the system.

Safety valve: is set to the maximum allowable pressure of the system (usually 3 bar for a family house). If the pressure reaches this value, the valve discharges and reduces the pressure. Frequent discharging of the safety valve is a sign of a problem (undersized expansion tank, control error) and is not a normal operating situation.

If both of these components are correctly dimensioned and functional, a standard PEX pipe with parameters PN6/90°C or PN10/60°C will never come close to its limits in the operation of a family house.

Summary: which pipe for which system

Based on all the above, the following practical conclusions can be formulated:

Family house, heat pump or condensing boiler with mixing unit: PEX pipe PN6/90°C and PN10/60°C is fully sufficient. Pipe 17x2 or 20x2 is the standard choice.
Older boiler with higher temperature, without mixing: first solve the mixing unit, then the pipe choice is the same as above.
Apartment building, commercial building: PN10 is the minimum. Multilayer pipe PEX-AL-PEX is more advantageous for distribution sections with higher temperature and pressure.
Industrial buildings, special applications: a hydraulic calculation and project from a qualified person is required.

For most new builds and renovations in Slovakia, the correct answer is: PEX pipe with a dual parameter PN6 at +90 °C / PN10 at +60 °C is more than sufficient and provides a multiple safety margin against the real operating conditions of floor heating.

Most frequently asked questions (FAQ)

What happens if I use pipe with a lower PN value than needed in the floor heating system?

The pipe will not burst immediately under pressure overload – polymer pipes first gradually deform, swell in weak areas (e.g. at connections) and only after a longer time will a rupture or leak occur. Since the pipe is embedded in concrete, such a failure is extremely costly to repair. Therefore, the safety margin in the pipe parameters is deliberately large – and that is why it is always better to have pipe "with a reserve" in floor heating.

Can I use pipe marked only as PN10 without temperature indication for floor heating?

It is not suitable. The marking PN10 without temperature specification applies at 20 °C. At higher temperatures (which is common in heating), the actual pressure resistance is lower. Always request pipe with a combined declaration – for example PN6 at 90 °C and PN10 at 60 °C – which precisely defines the parameters at real operating temperatures.

Do I have to reach exactly 6 bar during the pressure test, or is less sufficient?

The test pressure should be at least 1.5 times the operating pressure, but in practice, 6 bar is recommended as a standard for floor heating, as it is also the nominal limit of PN6 and serves as a verification of the safety margin. A lower test pressure may not reveal all potential weak points – for example, a poorly tightened fitting or a microcrack in the pipe from installation.

How can I find out what pressure is currently in my floor heating system?

A pressure gauge is installed on the boiler or on the manifold. A cold system should show a filling pressure of 1.0–1.2 bar; a hot system in operation 1.5–2.5 bar. If the pressure gauge shows more than 2.5–3 bar in the hot state and the safety valve discharges, the problem is most often in an undersized expansion tank – not in the pipe. If the pressure in the cold system drops below 0.5 bar, there is a leak somewhere.

Is there a difference in PN values between pipe 17x2 and 20x2?

Not in the declared parameters – both have the same pressure class PN6/90°C and PN10/60°C. The difference is in the SDR ratio (ratio of diameter to wall thickness) – pipe 17x2 has SDR 8.5, thus relatively thick wall, which gives it slightly better pressure resistance in absolute numbers. Pipe 20x2 has SDR 10 and its strength is a higher flow at lower pressure losses. Therefore, 17x2 is used for the actual circuits in the underfloor space, 20x2 for distribution lines to the manifolds in floor heating.

Can PEX pipe for floor heating be used for cold water distribution?

Technically yes – PEX is certified for cold and hot potable water. But pipe purchased in coils for floor heating may not have a certificate for potable water (ACS, WRAS or DVGW). For potable water distribution, always request pipe with explicit certification for potable water. For closed heating circuits (with treated water) this condition does not apply.

Conclusion

The parameters PN6 and PN10 must be understood in the context of the medium temperature – not in isolation. For the vast majority of floor heating systems in family houses in Slovakia, standard PEX pipe with a declaration PN6 at +90 °C / PN10 at +60 °C is a safe, reliable and proven choice with a multiple reserve against real operating conditions. More important than the selection of the PN class itself is the correct installation, pressure testing before embedding, correct sizing of the expansion tank and setting of the heat source. If these steps are carried out correctly, the pipe will last you the entire lifetime of the building without problems.

For further decisions on sizing, circuit lengths and spacing, we recommend reading the articles How to calculate the pipe length needed for floor heating, Installation of pipe for floor heating: spacing, laying and fixing and How to choose pipe for floor heating: PEX vs. multilayer.

Do you have a question about this topic?

Can't decide or dealing with a specific situation in your household? Write to us – we are happy to help.