Installation of a Floor Heating Thermostat: Step-by-Step Guide

We are still preparing products.

But you can look at other categories.

Why to take installation seriously: the basic philosophy of installation

A floor heating thermostat may look like a simple thing at first glance – you connect two wires and that's it. In practice, it's not that simple. Over the past fifteen years in the heating technology field, I've seen dozens of installations where the customer thought they could do it themselves in an hour. Sometimes they really did – but just as often I returned to the same apartment a year later because the floor was heating at full power regardless of the set temperature, or the program wasn't working, or the sensor was embedded in a place where it measured anything but the actual floor temperature.

This article is therefore about how to install a floor heating thermostat correctly the first time – including those details that most manuals keep silent about. We'll go through the entire process from preparation to final testing, with specific dimensions, values, and situations from real customer cases.

If you're still choosing a specific thermostat model and don't know what to look for before buying, see the article How to choose a thermostat for floor heating: what to watch out for. If you're dealing with switching capacity in relation to the power of the heating mat or cable, you'll also find Switching power of a floor heating thermostat: how to determine it correctly useful.

What you'll need: tools and materials

Before you start doing anything, prepare everything you'll need. Nothing is more nerve-wracking than interrupting work in the middle of installation due to a missing screw or an unsuitable drill.

Tools

Flat and cross screwdriver (smaller, for terminal blocks)
Wire stripper
Multimeter – an absolute necessity, not an optional tool
Drill with a set of drill bits (4–8 mm), or a hammer drill for stone/concrete
Long tube or rod for pushing the cable through the floor (so-called "snake")
Utility knife or box cutter
Water level
Pencil and measuring tape
Electrical tape or shrinkable tubing (as a backup)

Materials

Thermostat suitable for the type of heating (electric mat, electric cable, or water-based floor heating)
Floor temperature sensor – most thermostats are sold with a sensor, but sometimes it needs to be purchased separately
Protective tube for the sensor, ideally corrugated plastic, with a diameter of at least 10–12 mm
Wall mounting box (according to the dimensions of the thermostat, standard 60 mm for single or 80 mm for double)
Power cable (if not already provided): CYKY 3x1.5 mm² minimum, 3x2.5 mm² for more powerful heating systems
Circuit breaker or fuse for the floor heating circuit (if not already installed)

Step 1: Safety first – turn off the power

This sounds obvious, but in practice, people often work under voltage – either out of carelessness or because they think they'll "just quickly check if there's a live wire." Floor heating is typically connected to its own circuit with a 10 A or 16 A circuit breaker. Always turn off the corresponding circuit breaker in the distribution board and, if possible, mark it with tape or a label to prevent someone in the household from accidentally turning it back on.

After turning off the circuit breaker, it is also mandatory – not just a good recommendation – to verify the absence of voltage with a multimeter directly in the box or on the terminals. The live wire must be 0 V. Only then can you touch the wires with your bare hands.

Step 2: Choosing the location for the thermostat and preparing the box

The correct position of the thermostat depends on several factors. The classic installation height is 1.0 to 1.5 meters from the floor – similar to a light switch. Lower than that would be within reach of small children, and higher than that would be uncomfortable for most people to read the display.

It is important to avoid:

direct sunlight (the air sensor would measure inaccurately)
proximity to heat sources (radiator, stove, kitchen extractor)
drafts from windows or doors
places behind curtains, furniture, or in recesses
external walls without sufficient insulation (a cold wall would distort the measurement)

Once you've chosen the location, drill a hole for the box. A standard single-gang box has a diameter of 60 mm and a depth of 45–50 mm. Drilling into stone or drywall? For drywall, a hole saw without a masonry bit is sufficient, while for concrete or brick, a masonry bit and then a chisel for leveling are needed. The box must fit firmly and evenly – using a water level is definitely worth it, otherwise the thermostat will hang crooked on the wall and be noticeable from the entire room.

A power cable (230 V, N, PE – live, neutral, protective conductor) and a cable from the heating element (mat, cable, or water circuit) must be brought into the box. In addition, a protective tube for the temperature sensor will run down the wall from the box to the location of the sensor in the floor.

Step 3: Installation of the temperature sensor in the floor

This is the step that most amateur installations get wrong. A floor sensor is not a detail – it is the foundation of the entire system's proper functioning. If you place it in the wrong location, the thermostat will switch on and off at the wrong times, and the floor temperature will behave unpredictably.

Where should the sensor be?

The sensor must be placed between two heating loops or cables, not directly over or under them. The ideal distance from the edge of the room is at least 50 cm, and at least 50 cm from the wall. Typically, the sensor is laid in the center of the room or in a place where the floor is used most often – for example, in front of a sofa or in the center of a bathroom.

Depth of installation and protective tube

The sensor must be placed in a protective tube (corrugated pipe), not freely in concrete or screed. The protective tube serves two purposes: it protects the sensor cable from mechanical damage and – most importantly – allows the sensor to be replaced without breaking the floor. The end of the protective tube is bent upward toward the wall and ends in the thermostat box.

The protective tube with the sensor is embedded in the levelling layer, screed, or adhesive mortar under the tiles. The depth should be the same as the depth of the heating cable or mat – typically 2–4 cm below the surface of the finished floor. The sensor must not be too close to the surface (it would react too quickly and sensitively to surface changes) or too deep (it would react slowly with a large delay).Floor heating cable / mat Sensor Sensor protection tube → wall Thermal insulation (EPS, PIR) – min. 50 mm Ceiling slab / base layer

As a final step in this phase: before sealing the floor, always check the sensor with a multimeter. Most floor sensors are NTC thermistors with a resistance of 10 kΩ at 25 °C. At room temperature, the multimeter should show a value in the range of 8–12 kΩ. If you measure an open circuit (∞) or a short circuit (0 Ω), the sensor is damaged and you must replace it before pouring the floor – not after.

Step 4: Connecting the wires in the thermostat box

This is the heart of the entire installation. Before making any connections, carefully read the wiring diagram of the specific thermostat – each manufacturer may have a different terminal numbering. Never connect wires blindly based on their color if you don't know where they go. Always verify the assignment with a multimeter.

Standard thermostat terminal block for floor heating

Most thermostats for electric floor heating have a terminal block with the following terminals:

L (Live / Phase) – incoming phase 230 V
N (Neutral / Neutral) – incoming neutral
L1 (Load / Load) – output phase to the heating (switched contact)
N1 or N – output neutral to the heating (sometimes connected to the incoming neutral)
PE / ⏚ – protective conductor (earth) – must not be missing!
A / B or SF / SN – terminals for the temperature sensor (polarity does not matter)

Specifically, if you are installing, for example, SALUS ERT50 - 230V, you will find a removable terminal block on the bottom of the thermostat, which is a great advantage when replacing – you can connect the terminal block before mounting the device itself and save you the hassle of maneuvering in a tight box. The same applies to SALUS ERT20 - 230V.

Step-by-step connection procedure

Stripping the wires: cut the outer sheath of the cable so that the individual wires comfortably reach the terminals without tension. Strip each wire by 8–10 mm.
Connecting the protective conductor (green-yellow) first – this is a safety practice, not just a habit.
Connecting the neutral conductor (blue) to the N terminal.
Connecting the phase conductor (brown or black) to the L terminal.
Connecting the output cable to the heating element: phase to L1, neutral to N1 (or according to the diagram).
Connecting the sensor to terminals A/B – polarity usually does not matter, but check the manual.

After each connection, gently pull on each wire to verify that the terminal holds it securely. A loose wire can pull out when closing the box, and then you will spend an hour trying to figure out why the thermostat is not working.

More about typical errors when connecting and schemes for different configurations can be found in the article Thermostat connection for underfloor heating: scheme and most common errors.

Step 5: Thermostat installation into the box and primary setup

After connecting all the wires, carefully push the thermostat into the box. Arrange the wires inside the box so that no sharp bends occur and the sensor cable is not compressed by the body of the thermostat. For thermostats with a removable front part (e.g., Jablotron AC-83), first fix the rear mounting plate with screws into the box and then attach the front part.

For thermostats without a removable front part (e.g., SALUS RT10-230V – mechanical type), the thermostat is compact, mounted on the box in one piece and fixed with screws.

Basic setup before first switching on

Before switching on the circuit breaker, set the following parameters:

Type of sensor: Most digital thermostats allow you to choose whether you will control the temperature according to the floor sensor, the air sensor in the thermostat, or a combination of both. For underfloor heating, typically the floor sensor or combined mode is selected.
Floor temperature limit: If you have a wooden or laminate floor, set the maximum floor temperature to 27–29 °C. For tiles or vinyl, the limit can be higher, up to 35 °C. Exceeding the limit can damage the floor covering.
Type of heating element: Some thermostats distinguish between cable and mat for accurate power calculation.

Step 6: Switching on and first testing

Now switch on the circuit breaker. The thermostat should light up. Set the desired temperature a few degrees above the current floor temperature (e.g., if the floor is 20 °C, set it to 24 °C). The thermostat should switch the load – either audibly with a relay click or (with a triac output) silently. Check this with a multimeter on the output L1/N1 – you must measure 230 V.

Then set the desired temperature lower than the current floor temperature. The thermostat should disconnect the load – the output L1/N1 should show 0 V. If this behavior is confirmed, the basic function of the thermostat is working properly.

Then let the system run for at least 30–60 minutes and observe whether the floor actually heats up. For a newly poured floor or after the first start of the season, it may take up to 2–3 hours for the screed to warm through its entire thickness and for the floor to feel warm to the touch.

Step 7: Programming – setting the weekly schedule

If you have selected a programmable thermostat, this is the step where most people spend the most time – not because it is difficult, but because they don’t take the time to read the manual. The basic principles of programming are the same for most devices:

Setting the current time and day: Without this, the program does not work correctly. Most thermostats lose time after a power failure – models with battery backup (e.g., internal lithium) retain the time.
Defining temperature profiles: Typically, 4–6 temperature bands per day are available (morning wake-up, leaving for work, returning home, night setback). For each band, you set the start time and the desired temperature.
Setting workdays vs. weekend: Most modern thermostats allow different programs for workdays (Mon–Fri) and weekend (Sat–Sun), or each day separately.

Practical example: A customer in a family house used a thermostat set to a constant temperature of 23 °C for 24 hours. After switching to a programmable Jablotron AC-82 with a program (wake-up 6:00 – 22 °C, leaving 8:00 – 18 °C, returning 15:30 – 22 °C, night 22:00 – 19 °C), the electricity consumption for underfloor heating decreased by more than 30 % without any noticeable difference in comfort. This is a real result, not a marketing statistic.

More about the differences between types of thermostats and their energy-saving potential can be found in the article Electronic vs. programmable thermostats for underfloor heating: which one is worth it.

Step 8: Calibration and fine-tuning

After a few days of operation, it is time for calibration. If you have a thermostat with a floor sensor, compare the measured temperature of the sensor with a thermometer placed directly on the floor (an infrared thermometer is ideal). A deviation of ±1 °C is normal; a larger deviation may indicate a problem with the sensor placement or a faulty sensor.

Most better thermostats allow manual correction (offset) of the displayed temperature – typically in the range of ±3 to ±5 °C. If the thermostat shows 22 °C, but the actual floor temperature is 24 °C, set the offset to -2 °C and the thermostat will switch correctly.

At the same time, check whether the floor heats evenly – walk barefoot over the entire area and look for places where the floor is noticeably colder. A local cold spot may indicate a broken heating cable or an air pocket under the tile that hinders heat transfer.

Specific situations and practical solutions

Renovation without floor demolition: replacing an old thermostat

If you are replacing an old thermostat with a new one, the floor sensor is already installed. The first question is: is it an NTC 10 kΩ or another type? Most modern thermostats support NTC 10 kΩ at 25 °C, but older devices may have NTC 15 kΩ or PT1000. Measure the sensor resistance with a multimeter – if it significantly differs from 10 kΩ, you either need to choose a thermostat that supports that sensor type (some models have a configurable sensor type), or replace the sensor (which is possible if it was installed in a protective sheath).

Another problem during replacement is when the old thermostat had a larger box size (e.g., 80 mm double box) and the new one is designed for a 60 mm box. The solution is a reducer frame – most manufacturers sell it as an accessory.

Floor heating as supplementary vs. primary

A thermostat connected to floor heating that serves only as supplementary (e.g., in a bathroom, where the main heating element is a panel radiator) is handled differently than a thermostat for primary heating of the entire house. For supplementary heating, a simple thermostat with floor protection and maximum temperature setting is sufficient – for example, SALUS RT10-230V. For primary heating, a program is essential, and it is worth investing in a model with a weekly schedule and the ability to set temperature deviations.

Hydronic floor heating: differences in wiring

In hydronic floor heating, the thermostat does not switch the heating element directly, but a valve actuator (servomotor) on the circuit. The servomotor usually has a 24 V or 230 V power supply and must be adapted to the thermostat output. Most thermostats for floor heating are primarily designed for electric heating, but models with 230 V / max. 5 A output can switch the servomotor directly. In more complex systems with multiple zones, zone controllers are used and the thermostat is only a control element for the central distributor.

Switching capacity problem

One of the most common mistakes is using a thermostat with a lower switching capacity than the actual power of the heating mat. For example, a standard bathroom mat with an area of 2 m² has a power of 300–400 W. Most thermostats can handle 3,500 W (16 A), which is more than enough. The problem arises with large areas – for example, an entire living room of 40 m² with a power of 80 W/m² = 3,200 W. Some cheaper thermostats have an output of only 8 A (1,840 W) and cannot handle such a load long-term – the relay or triac overheats and the thermostat fails. Always check the mat power vs. the thermostat switching capacity, more on this topic can be found in the article Switching capacity of a thermostat for floor heating: how to determine it correctly.

Legislation and safety regulations

In Slovakia, electrical installations must comply with the standard STN 33 2000. The installation of the thermostat itself (replacing an existing device) is not subject to certification – a manually skilled person can do it. However, introducing a new electrical circuit (a new circuit breaker for floor heating) must be carried out and inspected by a person with the appropriate electrical installation qualification and a revision report must be issued. This inspection is mandatory for new construction and for major reconstruction of electrical installations.

If you are unsure where the boundary of your competence lies, a simple rule: if you are replacing an existing thermostat and the wiring is the same, you can do it yourself. If you are laying new cables or adding a new circuit breaker, call an electrician.

Most common installation errors and how to avoid them

After years in the field, I have compiled a list of errors that repeat regardless of whether it is an experienced DIYer or a first-time installer:

Sensor placed directly over the heating cable: The sensor thinks the floor is hot, although the floor surface is still cold. Result: the thermostat turns off too early, the floor is warm but not hot enough.
Missing or bent protective sheath for the sensor: When replacing the sensor (which will happen sooner or later), floor demolition is necessary. The sheath must be straight, without sharp bends.
Protective conductor (PE) not connected: A thermostat without grounding is not safe. Even if it works electrically, in case of a fault, there is a risk of electric shock.
Incorrect maximum floor temperature setting: A wooden floor heated to 35 °C will deform – this is not a warranty claim, it is an installation error.
Thermostat in an unsuitable location (draft, sunlight): The air sensor in the thermostat measures incorrectly and the system is unable to maintain a stable temperature.
Sensor not tested before pouring the floor: After pouring, you cannot reach the sensor. If you did not notice that the sensor is broken at that time, you will have to demolish the floor.

Other typical problems after installation, their symptoms and solutions can also be found in the article Common thermostat faults for floor heating and how to eliminate them.

Frequently asked questions (FAQ)

Can I replace a floor heating thermostat myself, or do I need to call an electrician?

Replacing an existing thermostat (same wiring, same circuit) can be done by a manually skilled person – technically it is no more complicated than replacing a switch. The key requirement is to turn off the circuit breakers and verify with a multimeter that the terminals are de-energized. However, if you are setting up a new circuit or changing circuit breakers in the distribution board, this requires a qualified electrician and a revision report.

What is the correct position of the sensor in the floor – in the center of the room, or elsewhere?

The sensor should be placed in the part of the floor where you walk most often, but at least 50 cm away from the walls and edges. It must be between two heating loops, not over or under them. Never place it near a door (draft), near a window (cold air), or in a corner, where air circulation is not representative of the entire room. A more detailed guide can be found in the article Setting up the floor temperature sensor: correct depth and placement.

The thermostat does not switch anything on after turning it on – the output has no voltage. What to check?

First, check if the set temperature is higher than the current sensor temperature (the thermostat only switches on when it wants to heat). Then check if the thermostat "sees" the sensor – most devices display an error code (e.g., "E1" or "FL") when the sensor is broken. Measure the sensor resistance directly at the thermostat terminals – it should be in the range of 8–15 kΩ at room temperature. If the value is out of range, the sensor is either broken, poorly connected, or incompatible. See also the article Common thermostat faults for floor heating and how to eliminate them.

Must a thermostat for electric floor heating necessarily have a protective conductor (ground)?

Yes, if the thermostat has a three-pole terminal block with a PE terminal (and most modern models have it), the protective conductor must be connected. It is a safety feature – in the event of insulation failure, it ensures that the circuit breaker disconnects before dangerous voltage appears on the thermostat. Omitting the PE conductor is a violation of safety regulations even if the thermostat "works" without it.

How long does it take for floor heating to heat up for the first time and for the sensor to report the correct temperature?

With a standard cement screed of 40–50 mm thickness, it may take 2–4 hours for the temperature to stabilize across the entire layer. With a thicker screed or if the heating has been off for a long time, it may take 6–8 hours. Therefore, a thermostat with a floor sensor may switch for longer cycles on the first day and then stabilize to shorter cycles. If you do not feel any heat after 4 hours, it is time to check the wiring diagram and the condition of the heating cable.

What is the difference between a thermostat with an air sensor and a floor sensor during installation?

A thermostat with only an air sensor (no external sensor in the floor) is easier to install – it is enough to connect the power and output, no sensor in the floor is needed. The disadvantage is lower control accuracy and no floor protection against overheating. A thermostat with a floor sensor or a combined sensor requires the sensor to be placed in the floor, but offers more precise control and floor surface protection. More on this topic can be found in the article Floor heating thermostat with floor sensor vs. air sensor: difference and when to use which.

Conclusion: investing in proper installation pays off

Installing a thermostat for underfloor heating may seem like a routine task at first glance, but in reality, it involves several steps where mistakes can easily be made. The most important guidelines to remember: never work under voltage, always measure the sensor resistance before pouring the floor, set the maximum floor temperature limit according to the floor covering, and choose a thermostat with switching capacity suitable for the actual load.

If you follow the procedure described in this article, your thermostat will reliably serve you for years without complications. And if something still goes wrong – the article Common thermostat faults for underfloor heating and how to eliminate them will help you systematically and quickly diagnose the problem.

The full range of thermostats for underfloor heating, including models suitable for various types of installations, can be found at atria.sk in the category Thermostats for underfloor heating.

Do you have a question on this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.