Setting the floor temperature sensor: correct depth and placement

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Setting the floor temperature sensor: correct depth and placement

The floor temperature sensor is a small, seemingly inconspicuous part of the entire underfloor heating system – yet its correct placement determines whether your thermostat will regulate accurately or whether the system will overheat and undercool cyclically. Over the years of practice, I have seen dozens of installations where everything else was connected correctly, and yet the system did not work properly – precisely because the sensor was lying at the wrong depth, too close to the pipe, or placed in an unsuitable room zone. This article is therefore devoted exclusively to this topic: where exactly to place the sensor, at what depth, what to avoid, and how to check the entire installation before pouring the screed.

Why the placement of the sensor matters more than you might expect

A thermostat for underfloor heating can operate in two basic modes: either it senses the air temperature in the room (air sensor) or it senses the temperature of the floor itself (floor sensor), or it combines both signals. When we talk about a floor sensor, its task is to provide the thermostat with an accurate overview of the current surface temperature of the floor.

If the sensor is placed too close to the heat source – that is, directly next to the pipe – it will measure a temperature that is much higher than the actual average surface temperature. The thermostat will "believe" that the floor is sufficiently warm and will turn off the heating before the heat has had time to spread evenly. Result: the floor is cold in some places and hot in others, comfort is low, and the system cycles excessively – turning on and off too quickly.

Conversely, if the sensor is placed too far from the heat source or in a place where the floor is naturally cooler (near an external wall, near a door with infiltration), the thermostat will feel that more heating is needed – and the system will run longer than necessary. This can lead to overheating of the floor, excessive wear of the thermostat, and unnecessarily high energy costs.

The topic of when it is advantageous to use a floor sensor instead of an air sensor and what the differences in their behavior are, is discussed in more detail in the article Thermostat with floor sensor vs. air sensor: difference and when to use which in this Knowledge Center.

Where the sensor is physically located: anatomy of the installation

The floor temperature sensor is usually a smaller sensor (NTC thermistor or PT1000) placed in a plastic protective tube – so-called corrugated protective sleeve or rigid plastic tube. This protective sleeve performs two functions: it protects the sensor from mechanical damage during concreting and at the same time allows the sensor to be replaced without having to cut the floor. This is crucial – the sensor is an electronic component with a limited lifespan, and if the protective sleeve is not installed correctly, replacement becomes an expensive reconstruction.

The protective sleeve must be led from the distribution box or from the wall (where the thermostat is located) smoothly and without sharp bends all the way to the sensor location. The sensor itself should be inserted to the end of the protective sleeve – that is, to the position where it actually senses the temperature. Free space in the protective sleeve behind the sensor is undesirable, as air in it forms an insulating layer and slows down the sensor's response.

Correct depth of sensor placement

The depth at which the sensor should be located depends on the thickness of the screed layer and the floor construction. In general:

The sensor should be placed in the middle height position of the screed layer – that is, not at the bottom edge (next to the pipes), nor directly under the finish layer.
For a standard screed layer with a thickness of 60–80 mm and pipes with a diameter of 16–20 mm laid on thermal insulation, the central position of the sensor is approximately 25–35 mm from the bottom of the screed layer, that is, in the height position between the pipes and the finish layer.
The sensor must never directly touch the pipe or touch it. The minimum distance between the axis of the sensor and the axis of the pipe should be at least 10–15 cm in the horizontal plane.
If the sensor is led along the protective sleeve (horizontally), check the position of its tip in relation to the surrounding pipes – it must be exactly between two adjacent pipes, ideally in their center.

In practice, I often come across the situation where electricians or installers install the sensor quickly and without measuring – simply pushing the protective sleeve into the screed layer at the spot where they happen to be standing. The result is unpredictable. I recommend always thoughtfully placing the sensor before concreting, indicating its position relative to the pipes with a meter, and fixing the position of the protective sleeve (e.g., with plastic clamps to the reinforcing mesh or to the system board), so that it does not shift during concreting.

Horizontal position of the sensor: between the pipes, not above them

Many people ask whether it matters exactly where in the room plan the sensor is placed. It does – and very much so. The basic rules are as follows:

The sensor must be placed in the active zone of the floor heating – that is, in the area where the pipes are located. If the sensor is in a passive zone (e.g., under fixed furniture, in a corner near the entrance door without pipes), it measures a temperature that is not representative of the entire floor.
Avoid placing it close to external walls, where the floor is naturally cooler due to thermal bridges and lower ambient temperature.
I do not recommend placing the sensor in an area that will be permanently covered by furniture (cabinet, bed, seating unit). The floor beneath them heats up more slowly, and the thermostat will respond to a temperature in an area where no one is present and where we have no interest in heating.
The ideal position is in the center of the room, in the active heating zone, at a central distance between two adjacent pipes.
The distance of the sensor from the wall where the thermostat is mounted should be as short as possible with a straight route – a long cable in the protective sleeve leads to greater heat losses and possible condensation, which affects the accuracy of the measurement.

Sensor in the bathroom: special rules

The bathroom is a specific environment in terms of floor heating. It is a wet area where a sensor with appropriate protection (IP rating) is required, and the bathroom floor is usually small – so the choice of sensor placement is limited. A few practical tips:

Do not place the sensor directly under the shower tray or the bathtub. There are no pipes under the bathtub (it is a dead zone), and under the shower, the floor is exposed to alternating water and air temperatures, which distorts the measurement.
The ideal position in the bathroom is the central part of the floor – for example, in front of the sink or in the center of the space between the sanitary unit and the wall.
In the bathroom, a floor sensor is used almost exclusively – an air sensor makes no sense here, as it would measure a non-representative air temperature due to shower steam and would regulate poorly. Thermostats such as SALUS ERT20 - 230V or SALUS ERT50 - 230V have the option to work exclusively with a floor sensor, which is ideal for the bathroom.
Make sure the protective sleeve is watertight connected – in the bathroom, water can penetrate the sleeve and damage the sensor. Seal the end of the sleeve (on the sensor side) with silicone or sealing compound.

How long should the protective sleeve be and how to route it properly

The protective sleeve runs from the thermostat box (usually at a height of 80–120 cm on the wall) down the wall and then along the floor to the location where the sensor tip should be. Its total length depends on the project, but a few technical rules apply universally:

The protective sleeve must be continuous and without kinks or sharp bends. The bend radius should not be less than five times the diameter of the sleeve – for a standard 8 mm corrugated sleeve, this means a minimum radius of 40 mm.
It must not branch or connect – it must be a single continuous section from the thermostat to the sensor tip.
The length of the sensor cable is usually 2–6 meters (depending on the thermostat model). If a longer route is needed, check with the thermostat manufacturer what is the maximum allowed length of the sensor cable and what cross-section it should have – with too long or thin wires, measurement errors due to resistance can occur.
Always fix the protective sleeve to the base before concreting – plastic clips for the reinforcing mesh are available. A loose sleeve can shift, and the sensor tip will end up in the wrong place.
Seal the end of the sleeve – either with a plug or with silicone – to prevent cement milk from entering during concreting. If the sleeve becomes clogged, the sensor cannot be replaced, and the entire route is ruined.

Protective sleeve: diameter, material and type

Two types of protective sleeves are commonly used on the market:

Corrugated (wavy) plastic sleeve – flexible, easy to shape. Commonly available in a diameter of 16 mm (external) or 20 mm. Advantage: easy to lay, flexible. Disadvantage: larger diameter, takes up more space in the screed layer and with a 60 mm thick screed layer, the space for the pipe may be limited.
Rigid smooth plastic tube (e.g., made of PE or PVC) – smaller external diameter (8–10 mm), smooth inner surface facilitates inserting the sensor. Disadvantage: harder to bend, requires shaping with heat or elbow fittings.

The recommended internal diameter of the protective sleeve should be at least 2–3 mm larger than the diameter of the sensor with the connecting cable – otherwise, the sensor may get stuck when inserted and cannot be removed during replacement.

Concrete examples from practice: what happened when it was done wrong

Example 1: A family house, bathroom 5 m². The electrician installed the sensor directly under the edge of the shower box. When the owners used the shower cabin, cold water evaporated and cooled the floor area directly above the sensor. The thermostat interpreted this as a temperature drop and turned on the heating – even though the rest of the bathroom was sufficiently warm. The floor outside the shower overheated, while the system was "fighting" a local cooling. Solution: we moved the sensor to the central part of the bathroom – the result was immediate – stable regulation, smooth thermostat operation.

Example 2: A living room 30 m², sensor embedded directly into the pipe (the installer put it on the pipe and poured the screed over it). The temperature measured by the sensor was always 5–8 °C higher than the actual floor surface temperature. The thermostat turned off the heating when the floor was still not warm. The owners complained about a cold floor, even though the thermostat indicated the set temperature had been reached. The solution required cutting – the sensor was physically embedded without a protective sleeve. An expensive repair that could have been avoided.

Example 3: Apartment renovation, sensor in a protective sleeve, but the sleeve was run with two sharp bends. When inserting the sensor, it got stuck in the first bend. The owners could not replace the sensor without damaging the floor covering. As a compromise, a shorter sensor was used and partially left outside the planned depth – which again reduced the accuracy of the measurement.

Setting the maximum floor temperature limit on the thermostat

Correct installation of the sensor is only the first step. Equally important is setting the maximum (limit) floor temperature correctly on the thermostat. This is a function that prevents overheating of the floor covering – it is a protection against damage to laminate floors, vinyl tiles, and also solid wood.

Typical recommended maximum surface floor temperatures:

Ceramic tiles: up to 35 °C (practically no material limitation, but hygienically not recommended above this)
Anhydrite screed / vinyl floor (LVT): 27–28 °C surface (vinyl floor manufacturers are very sensitive to this)
Laminate: 26–28 °C surface (a higher limit risks deforming the joints)
Solid wood / wooden parquet: 26 °C (extremely sensitive to drying out)

Thermostats such as Jablotron AC-83 or SALUS ERT50 - 230V have the functionality to set the maximum floor temperature directly in the menu – the sensor thus serves not only for regulating comfort temperature, but also as a safety limit. This is a very practical feature, especially if it is an expensive wooden floor.

Calibration and verification of measurement accuracy

After completing the installation and before commissioning, I recommend always verifying that the sensor is measuring correctly. The procedure is simple:

Let the system run for at least 2–3 hours in a stable state.
Place an infrared thermometer (pyrometer) on the floor surface directly above the location where the sensor is placed.
Compare the measured surface temperature with the temperature displayed by the thermostat.
The deviation should not be more than 1.5–2 °C. A higher deviation indicates a problem: the sensor is too close to the pipe, is placed at the wrong depth, or there is an air bubble in the protective sleeve.

Most modern thermostats (including Jablotron AC-82 or SALUS RT10-230V) allow manual calibration correction of the measured temperature in the range of ±3 °C. This is a compensatory offset – if the thermostat shows a systematic 1.5 °C higher than the actual surface temperature, set the correction to -1.5 °C and the thermostat will regulate more accurately.

Warning: calibration is a temporary solution. If the deviation is greater than 3 °C, the cause must be physically removed – by repositioning the sensor or replacing it. Calibration cannot be relied upon as a permanent solution for incorrect installation.

Replacing the sensor without breaking the floor: how to do it

If the sensor is properly placed in a protective sleeve, its replacement is a matter of a few minutes. Procedure:

Turn off the thermostat power supply (circuit breaker).
Unscrew the thermostat from the wall and disconnect the sensor terminals (usually terminals A and B, or marked differently according to the documentation).
Attach a thin fixing cord (e.g., thin nylon wire) to the end of the old sensor, which you will then use to pull the new sensor through.
Pull out the old sensor from the sleeve – it should come out freely. If it gets stuck, gentle turning while pulling may help.
Using the cord, pull the new sensor through the sleeve to the tip.
Connect the new sensor to the thermostat and check the measurement.

If the sensor cannot be pulled out and the sleeve is blocked – the situation is more complicated. In such a case, it is sometimes possible to leave the old sensor in place and run the new sensor in a milled groove under the baseboard or skirting board to the thermostat. It is not ideal, but in an extreme case it is an acceptable compromise.

Combination of floor sensor and air sensor: how to set it up

Some thermostats allow to operate in so-called combined mode – air sensor (built into the thermostat) regulates the temperature according to the air temperature in the room, but the floor sensor serves as a safety limit. This is an ideal solution for living rooms with more expensive floor covering: the comfort of regulation is determined by the air temperature, but the floor is never heated above the set limit.

For a bathroom or hallway, where floor temperature is a priority rather than air temperature, the thermostat is set to use only the floor sensor. In this case, it is even more important that the sensor is properly placed – the entire regulation depends entirely on its measurement.

A more detailed comparison of these two strategies can be found in the article Floor sensor thermostat vs. air sensor: difference and when to use which and also in Electronic vs. programmable thermostats for underfloor heating: which one is worth it.

Most frequently asked questions (FAQ)

At what depth should the sensor be placed in the screed layer?

The sensor should be placed at the middle height of the screed layer – with a standard screed thickness of 60–80 mm and pipe diameter of 16–20 mm, this is approximately 25–35 mm from the bottom of the screed layer. The sensor must not be placed directly on the pipe or just under the walking surface. In the horizontal plane, it must always be placed between two adjacent pipes, not directly above them.

Can I place the sensor under a spot where a cabinet will be placed?

No, this is one of the most common mistakes. Under solid furniture, the floor is thermally insulated and heats up more slowly. The thermostat would measure the temperature in a zone where there is no air circulation, and the floor in the living area would be regulated incorrectly. Always place the sensor in a free, passable area of the room – ideally in the center of the active heating zone.

Do I need to replace the sensor if the thermostat breaks down?

Not necessarily – it depends on whether the new thermostat uses the same type of sensor (NTC or PT1000) and the same resistance at the reference temperature. Always check the compatibility of the sensor before purchasing a new thermostat. If the sensor is compatible, it is sufficient to connect the existing sensor to the new thermostat. If not, the sensor must be replaced – which is a quick task if the protective sleeve is properly installed.

What if the protective sleeve cracked during the concrete pouring and concrete got into it?

This is a serious problem – a clogged protective sleeve cannot be cleaned. If you notice it during the concrete pouring or shortly after (while the concrete is still not hardened), you can try to rinse it with water. If the concrete is already hardened, the sleeve is ruined. The solution is to run a new sensor along an alternative route – in a groove under the skirting board, or under the laminate/carpet threshold – which is a compromise, but still functional.

What is the correct distance of the sensor from the underfloor heating pipe?

The minimum horizontal distance between the axis of the sensor and the axis of the nearest pipe should be 10–15 cm. The sensor must be exactly between two adjacent pipes, at their central distance. With a typical pipe spacing of 15 cm (axis to axis), this means the sensor lies 7.5 cm from each pipe – this is the minimum, but acceptable. With a spacing of 20 cm (axis to axis), the sensor is 10 cm from each pipe, which is even better.

Can the sensor be too long and what to do with the excess cable?

Never coil the excess sensor cable into a loop or trap it in the screed – induced heat from the pipes could affect the sensor. The excess cable should be led to the thermostat box and folded there. If the cable is too long and does not fit into the box, use an intermediate junction box in the box and cut off the excess. Never shorten the cable on the sensor side – only on the thermostat side, and only if you have experience with soldering and insulating connections.

Conclusion: a properly placed sensor will save you trouble for years to come

The floor temperature sensor is a cheap component worth a few euros – but if it is improperly placed, it causes problems and repair costs in the hundreds to thousands of euros. The correct depth, correct horizontal position between the pipes, a quality protective sleeve without sharp bends, and proper sealing before concrete pouring – these are four pillars of trouble-free and long-term operation of the entire underfloor heating system.

If you are unsure about choosing a thermostat that will work reliably with your sensor, take a look at other articles in this Knowledge Center, especially How to choose a thermostat for underfloor heating: what to pay attention to or SALUS thermostat vs. Jablotron for underfloor heating: model comparison. And if you are dealing with the entire installation from scratch, I recommend reading Installation of a thermostat for underfloor heating: step-by-step guide – the entire process including the correct installation of the sensor is described there in the context of other installation steps.

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.