Thermostat with floor sensor vs. air sensor: difference and when to use which
Floor sensor thermostat vs. air sensor: difference and when to use which
When a customer buys a thermostat for floor heating, most of them focus on design, price or whether the thermostat is programmable. Few realize at first that one of the most important questions is: which sensor will this thermostat use to measure and regulate temperature? And this very choice determines whether your floor heating will work efficiently, comfortably and without problems, or whether you will be fighting for years with cold floors, inefficient on/off cycles, or even damaged floor covering.
This topic is devoted precisely to this difference – technically, practically and with examples from real customer projects. I will show you how each type of sensor works, its advantages and limitations, in which situations one or the other is better, and when it is worth reaching for a thermostat that combines both types in one device.
How a thermostat with a floor (floor) sensor works
A floor sensor – technically it is an NTC thermistor (usually with a resistance of 10 kΩ at 25 °C) – is laid directly into the floor during installation, into a protective sleeve (tube), which is embedded into the screed layer. The sensor measures the temperature of the floor material – that is, the actual thermal mass surrounding the heating.
A thermostat with a floor sensor works in a simple way: it compares the measured floor temperature with the set value (e.g. 28 °C) and switches the heating circuit on or off accordingly. It is therefore a regulation of the floor surface temperature, not the air temperature in the room.
In practice: if you set a value of 28 °C on a thermostat with a floor sensor, the system will heat until the screed around the sensor reaches exactly this temperature – regardless of the air temperature in the room. The floor will therefore always reach the set surface temperature.
How a thermostat with an air sensor works
An air sensor is integrated directly into the body of the thermostat or is connected to it via an external cable and measures the temperature of the air in the room. It works the same way as a standard room thermostat – only instead of controlling a conventional radiator, it controls the floor heating system.
A thermostat with an air sensor compares the measured air temperature with your set value (e.g. 21 °C) and heats until the air in the room reaches the desired temperature. In this case, the floor heating performs its original function: heating the room through a heated floor surface.
The advantage of this approach is that the regulation is oriented towards actual comfort – the air temperature we perceive. The disadvantage is that the thermostat "does not know" how warm the floor itself is. If it is a sunny day and the sun heats the room through the window, the air sensor will correctly assess that the temperature of 21 °C is achieved and the heating will turn off. The floor may still be cold, however.
Combination of both sensors: dual regulation mode
More modern thermostats for floor heating allow combining both sensors simultaneously. This is by far the most flexible and for most applications the best way of regulation. The thermostat has a built-in air sensor and an input for an external floor NTC sensor and can operate in several modes:
- Air mode (A): regulates only according to air temperature, ignoring the floor sensor.
- Floor mode (F – Floor): regulates only according to floor temperature, ignoring the air.
- Combination AF: regulates primarily according to air, but the floor sensor serves as protection – when the floor temperature reaches the set upper limit (e.g. 27 °C), the heating is automatically turned off regardless of the air temperature.
- Combination FA: regulates primarily according to the floor, but the air sensor serves as a secondary protection.
You can find such a combination, for example, in thermostats SALUS ERT50 – 230V, which offers all three modes (air, floor, combined), or in Jablotron AC-83, where it is possible to set the regulation priority and floor temperature limits independently of the set air temperature.
When to use a floor sensor exclusively
There are situations where the floor mode (regulation based solely on the surface temperature of the floor) makes a clear more sense than the air mode. Here are the most common ones from practice:
1. Bathroom and tiled areas without other heating
This is a classic situation. In the bathroom, the electric floor heating cable is primarily used for warming the floor for a pleasant barefoot feeling – not for heating the entire room (which is usually heated by a radiator or central heat recovery). In such a case, the air temperature does not concern us – we want the floor to be warm, say 26–28 °C, regardless of how many degrees are in the room.
An air thermostat would be completely unsuitable here: if you took a shower in the morning and the room filled with steam and warmed up to 24 °C, the thermostat would turn off the heating and you would step onto a cold floor.
2. Outdoor areas and heating systems (ramps, sidewalks, winter gardens)
Floor heating outdoors – on terraces, ramps, garage entries – is designed to prevent freezing. Here, an air sensor is almost unnecessary, as the outdoor air is influenced by wind, sun, and other factors. A floor sensor embedded in concrete or the lower layer of the terrace measures the actual temperature of the material and prevents freezing with much higher accuracy.
3. Floor as the primary heating surface in rooms with high thermal load
Kitchens near work islands, areas with large glazing (winter gardens), or rooms where the air temperature fluctuates due to opening doors and ventilation – here it is better to regulate according to the floor, since the air temperature is too unstable a reference.
4. Protection of temperature-sensitive floor coverings
Solid wood floors, laminate, and certain types of vinyl have a maximum allowable surface temperature specified by the manufacturer – usually 27–29 °C. If you were to regulate only by air, it could happen that during long heating periods (e.g., after returning from a vacation, when the apartment is cold), the floor overheats to 35–40 °C, causing wood deformation, delamination of floating planks, or dimensional changes. A floor sensor here serves as precise thermal protection for the material.
When to use an air sensor
An air sensor, on the other hand, is ideal in situations where the main goal is to achieve a certain air temperature in the room and floor heating is the only or dominant heat source.
1. Floor heating as the main (only) heat source
In well-insulated new buildings with a low-energy standard, floor heating is increasingly used as a single heat source – without radiators. Here, an air sensor is the right choice: we want 21 °C in the living room and we don't care whether the floor measures 22 or 25 °C. Heating is turned on when it is colder and turned off when the air temperature is reached.
It is important, however, to combine an air thermostat with a maximum floor temperature limit – which brings us back to the need for the AF combination.
2. Bedroom with comfortable air temperature regulation
In the bedroom, we want 18 °C during sleep and 20 °C during the day. Regulation according to air makes sense here – the floor temperature is only a means to achieve the goal, not the goal itself. A programmable thermostat with an air sensor can save energy more efficiently here, as it takes into account real thermal gains (sunlight, people, appliances).
3. Large rooms with diverse load
In an open layout (living room + dining room + kitchen), where the stove is used during the day, people are cooking, and the sun is shining, an air thermostat has an advantage: it turns off the heating when it is actually warm in the room, even if the floor does not reach the set temperature. Result? Lower energy consumption, as the thermostat takes into account the entire thermal balance of the room.
Technical parameters of sensors: NTC, resistance, cable length
If you are buying a thermostat with floor sensor support, it is important to know that not all floor sensors are the same. The most important parameter is the nominal resistance of the NTC thermistor. Most standard thermostats for floor heating use sensors with a resistance of 10 kΩ at 25 °C (marked as NTC 10k). Some older or special thermostats may require 12 kΩ, 15 kΩ or even 47 kΩ.
If you connect a sensor with the wrong resistance to the thermostat, the thermostat will measure an incorrect temperature – sometimes with a deviation of 5–10 °C, which would result in incorrect switching or continuous heating. Therefore, always check the technical documentation of the thermostat and compare it with the sensor that is delivered with the thermostat.
Another important parameter is the length of the sensor cable. Standard sensors are delivered with a 3–4 m cable, which is sufficient for most installations where the thermostat is on the wall and the sensor tube with the sensor is in the floor below it. If a longer cable is needed, it is possible to extend it – but be careful: the sensor cable has a non-zero resistance and with a very long extension (more than 5–8 m) a measurement error may occur. For long runs, use a cable with a thicker cross-section (at least 0.5 mm²) or consult the extension with the thermostat manufacturer.
The sensor tube (tube) should be placed between the heating loops – not directly on the cables and not too close to the edge of the floor. The correct distance from the wall is at least 50 cm, ideally 80–100 cm. Details on the placement of the sensor can be found in the topic Setting up the floor temperature sensor: correct depth and placement in this Knowledge Center.
Concrete examples from practice: three orders, three different solutions
Order A: Bathroom in a panel apartment, electric cable, tile
The customer had an old bimetal thermostat in the bathroom that measured air. He complained that the floor was cold in the morning, although the thermostat showed 22 °C. The reason was simple: heat from central heating (radiator in the wall) kept the air temperature at 22 °C, so the electric cable in the floor almost never turned on. Solution: replacement with SALUS RT10-230V with a floor sensor set to 27 °C. The result was immediate – the cable switches according to the actual tile temperature, warm floor in the morning is guaranteed.
Order B: Family house, water floor heating, living room 45 m²
New construction with a heat pump and floor heating throughout the ground floor. The investor wanted a comfortable regulation with a program – different temperatures during the week and during the weekend. Here, an air thermostat with a program makes sense. The built-in Jablotron AC-82 was set to a combined mode AF: air is the primary reference (target temperature 21 °C), the floor sensor works as a protection with a limit of 29 °C (to prevent the anhydrite and floor covering from overheating). In practice, the limit of 29 °C was never reached, because in a low-temperature system (35/28 °C), the floor usually moves between 22–26 °C.
Order C: Winter garden with stone tiles and a long response time
A winter garden is a problematic room: large glazed area, rapid heat losses, rapid heat gains from the sun. The customer had an air thermostat and complained about high consumption. The problem was that the floor heating in the stone tiles had a huge thermal inertia – it takes 3–4 hours to heat up and another few hours to cool down. The air thermostat turned off the heating when the air reached 20 °C, but the floor then continued to emit heat for hours and the space overheated to 24–25 °C. Solution: switching to a floor mode with a setpoint of 22 °C and manual adjustment – the customer understood that with such a thermal reserve, it is necessary to regulate the heat source (floor), not the consequence (air).
How to set it up in practice: tips and values
For standard electric floor heating with tiles in the bathroom or hallway, we recommend floor mode with the following settings:
- Maximum floor temperature: 28–30 °C (for tiles and concrete)
- Maximum temperature for wooden floors: 26–27 °C (comply with the manufacturer's requirements for the floor)
- Minimum floor temperature (anti-frost protection): 5–8 °C
For water floor heating in the living room or bedroom with combined regulation (AF), we recommend:
- Target air temperature: 20–22 °C (according to preferences)
- Limit floor temperature (protection): 27–29 °C
- Thermostat hysteresis: 0.5–1 °C (larger hysteresis = fewer switches, but larger temperature fluctuations)
Some thermostats, such as SALUS ERT20 – 230V, offer simple setting without complicated menus – ideal for customers who want clear control without the need to study an extensive manual. This model works with a built-in air sensor and an input for an external floor sensor, with the floor limit set by a simple rotary knob.
More about the comparison of specific models can be found in the topic Thermostat SALUS vs. Jablotron for floor heating: model comparison, where individual thermostats are analyzed also from the perspective of sensor modes.
Most common mistakes when choosing a sensor
From practice, every service technician knows that there are several recurring mistakes that customers make when choosing or setting up a thermostat sensor:
- Air thermostat in the bathroom with existing central heating: Typical case: the thermostat reacts to the air temperature, which is heated by the radiator or convectors, and the electric cable almost never turns on. Result – cold floor all year round and confused customer.
- Floor sensor placed too close to the heating cable: The sensor measures the temperature of the cable itself, not the surrounding screed. The thermostat turns off the heating too early, the floor is still relatively cold. The correct distance of the sensor from the cable is at least 5–7 cm, ideally in the middle between two loops.
- Mixing NTC sensors with different resistances: The customer buys a replacement sensor from a local store without checking the resistance. The thermostat measures incorrectly and the customer thinks it is broken.
- No floor limit protection with an air sensor: During long heating of the floor after cooling down (e.g. after a holiday), the floor may reach a dangerous temperature without the air thermostat detecting it. Always set a floor limit if your thermostat supports a combined mode.
- Ignoring the thermal inertia of the system: The customer sets the hysteresis too low (0.2 °C) and the thermostat switches every few minutes. Water floor heating has a huge thermal inertia – the thermostat should be set to a hysteresis of at least 0.5–1 °C to avoid overloading the actuator and switching equipment.
For more detailed solutions to these situations, see the topic Common thermostat faults for floor heating and how to eliminate them.
Overview: which type of sensor for which application
| Type of room / application | Recommended sensor mode | Reason |
|---|---|---|
| Bathroom (electric cable) | Floor (F) | Other heating heats the air; we want a warm floor |
| Hallway, entrance | Floor (F) | Supplementary comfort, unstable air temperature |
| Living room – main heating | Combination AF | Air comfort is primary, floor protection is secondary |
| Bedroom | Air (A) or AF | Regulation according to comfortable air temperature |
| Winter garden / large glazed area | Floor (F) | Air fluctuates, large thermal inertia of the floor |
| Terrace / ramp (anti-frost protection) | Floor (F) | Outdoor air is unreliable, need to measure material temperature |
| New build – low-energy house | Combination AF | Optimal regulation with floor protection |
| Wooden floor (solid, laminate) | Floor sensor is mandatory | Material protection – limit max. 27 °C |
Most frequently asked questions (FAQ)
Can I use a thermostat with only an air sensor in the bathroom, where I also have a radiator?
Technically yes, but it makes no practical sense. The radiator will maintain the air temperature at a sufficient level and the thermostat for the electric floor cable will almost never activate. The floor will remain cold. For bathrooms with supplementary heating, always use floor mode or combination AF, where the primary reference is floor temperature.
What happens if the floor sensor breaks or is damaged?
Most quality thermostats will display an error code (e.g. E1, Err or a blinking icon) in the event of a sensor failure and will either completely switch off the heating or switch to an emergency mode with a fixed switching value. The floor sensor is usually housed in a protective sleeve and can be replaced without breaking the floor – simply pull out the old sensor through the cable and slide in the new one, provided the sleeve is pass-through. Therefore, during installation it is crucial to properly place the sleeve with one end closed and the other open (towards the wall).
What is the difference between the temperature set on the thermostat and the actual floor temperature?
It depends on the location of the sensor. The sensor measures the temperature at a specific point – in the screed or near the heating cable. The surface temperature of the floor covering (tiles, wood) can be 1–4 °C lower than the temperature measured by the sensor in the screed, depending on the thickness of the screed and the covering. For practical setting: if you want a floor surface temperature of 26 °C, set the thermostat to 27–28 °C and after the first week of operation check the surface temperature with a cheap contact thermometer and adjust if necessary.
Are some thermostats with floor sensors suitable for outdoor use?
Thermostats for floor heating are typically designed for indoor mounting in a standard box (AT 60 mm). For outdoor mounting (garage, covered areas), it is necessary to ensure that the thermostat is protected from moisture and condensation – either by placing it indoors with an extended sensor cable going outside, or by using special regulators with a higher IP rating. Standard room thermostats have an IP20 rating, which is not sufficient for outdoor environments.
Can one sensor be used for multiple thermostats in the same room?
It is not technically possible with standard NTC thermistors – one sensor is directly connected to one thermostat and measures resistance, which would be affected by a second thermostat connected in parallel. Each thermostat must have its own sensor. If you have a large room divided into multiple heating circuits with several thermostats, each circuit needs its own sensor in the corresponding part of the floor.
Does the type of sensor affect electricity consumption?
Yes, indirectly. An air sensor reacts to the overall thermal balance of the room, including solar gains, heat from people and appliances – the result is potentially lower consumption in rooms with variable thermal gains, because heating can be turned off sooner. A floor sensor reacts only to floor temperature – regardless of what is happening in the room. In a bathroom with a radiator, the electric cable may run even when the room is warm. Therefore, in such cases it is important to set a reasonable time program – for example, turning off the cable from 22:00 to 5:00 when the bathroom is not in use.
Conclusion: the right choice of sensor is the basis of reliable floor heating
The choice between floor and air sensors is not a matter of price or fashion – it is a technical decision that significantly affects the functionality, comfort and lifespan of your floor heating. A simple rule: if you want a warm floor, regulate the floor. If you want a warm room and have floor heating as the main heat source, regulate the air – and protect the floor by limiting surface temperature.
A combination of both sensors in one thermostat is now available even in the mid-price range and for most common applications it is the most sensible choice. Thermostats such as SALUS ERT50 or Jablotron AC-83 allow you to set exactly the mode that corresponds to your specific situation – without compromising comfort and protection.
Before choosing a thermostat, always ask yourself three basic questions: What is the primary goal of heating (warm floor or warm room)? Is there another heat source in the room? What type of floor covering do you have? Answers to these questions will clearly guide you to the right type of sensor. If you are unsure, the topic How to choose a thermostat for floor heating: what to pay attention to in this Knowledge Centre will help you go through the entire selection process from the beginning.
Do you have a question about this topic?
Can't decide or are you dealing with a specific situation in your home? Write to us – we'll be happy to help.