Common thermostat issues for underfloor heating and how to fix them

We are still preparing products.

But you can look at other categories.

Common faults of thermostats for floor heating and how to fix them

A thermostat for floor heating is a small device that most people don't think about until something stops working. That's when panic sets in – the floor is cold, the apartment is freezing, and nobody knows where to start looking for the problem. From experience, I know that most faults have rather trivial causes that can be resolved without an electrician and without replacing the entire thermostat. This article will therefore systematically cover the most common faults – from the simplest to the more complicated – and for each one I will provide a specific solution.

Before we get into the specific faults, a short note: problems with thermostats for floor heating vary greatly depending on whether it's electric floor heating (resistive mats, cables) or water-based floor heating (hydronic heating). Electricity reacts instantly and faults manifest differently than in water-based systems, where temperature changes slowly. I will point this out where relevant.

Why thermostats for floor heating break down more often than other thermostats

A thermostat for floor heating operates in a specific mode – in most cases it directly switches resistive cables or mats, which means a real load of 6, 10, or even 16 amps. This is a significantly higher load than a standard room thermostat, which only controls a boiler (where a contact of 0.1 A is sufficient). Relay contacts in the thermostat wear out mechanically over years of operation – this is a physical reality, not a manufacturing defect.

Another factor is the sensor. Most thermostats for floor heating work with a floor temperature sensor (NTC sensor), which is built directly into the floor. This sensor is exposed to repeated temperature cycles, moisture from the concrete, and possibly mechanical stress from the floor covering. After 5–10 years of operation, its resistance can shift outside the calibration range, which results in incorrect regulation.

If you're interested in how to choose a thermostat with these factors in mind from the beginning, I recommend the article How to choose a thermostat for floor heating: what to pay attention to in this Knowledge Center.

Fault No. 1: Thermostat does not respond, display is dark or shows an error message

Symptoms and initial diagnosis

The most common scenario from field experience: a customer calls and says "the thermostat is broken", the floor is not working, the display is black or shows a nonsensical error code. The first step is always to check the power supply – it sounds trivial, but in practice it is the cause in at least 30% of cases.

Check whether the circuit breaker in the distribution board is on. Electric floor heating often has its own circuit, which can trip during overload and nobody notices it.
Check whether the thermostat has the correct power supply. There should be 230 V on the N and L terminals (measure with a voltmeter or a test lamp).
If the thermostat is battery-powered (e.g. some wireless versions), replace the batteries with new ones. Degraded batteries cause reboots and display outages.
Check the terminals – sometimes, in thermostats mounted in a box, the wire can come loose from a loose terminal, especially if someone has unscrewed the thermostat for painting.

Error codes on the display

Modern digital thermostats (e.g. Jablotron AC-83 or SALUS ERT50 – 230V) display error codes that greatly simplify diagnosis. The most common error messages and their meanings:

E1, E2, Err, F1 – most often sensor error: broken sensor cable, disconnected sensor plug or sensor NTC sensor failure.
OL or Hi – the sensor reports an excessively high temperature (out of range). It may be a short in the sensor, or the sensor is lying directly on the heating cable without a protective conduit.
Lo or LL – the sensor reports an excessively low temperature (typically below –10 °C when measuring resistance). The sensor is broken or disconnected.
Flashing lock symbol – the thermostat is locked (child lock). The unlocking procedure is described in the manual, usually it involves holding one or two buttons for 3–5 seconds.

With the Jablotron AC-82 it is standard that when the sensor is disconnected, it displays "E" with a blinking indicator – in this case the thermostat functions only as a timer switch according to the last set program, which can confuse the customer (it seems everything is working, but temperature regulation is not functioning).

Failure No. 2: Thermostat is running, but the floor is not heating

Relay is switching, but current is not flowing

This is a tricky failure, because the thermostat looks functional – the display is on, the set temperature is lower than the room temperature and the relay should be closed. Diagnostic procedure:

Measure the voltage on the output terminals of the thermostat (terminals for the heating cable) using a voltmeter. If there is 230 V there, the problem is not in the thermostat, but in the heating element or its supply.
If there is no voltage on the output terminals, even though the relay should be closed, try to manually start a test (most thermostats have a test mode – set the temperature 5 °C higher than the current one). If the relay still does not switch, the problem is in the thermostat – worn relay, damaged control circuit.
If the voltage on the thermostat output is correct, but the cable is not heating, measure the resistance of the heating cable after disconnecting it from the thermostat. Compare the measured value with the value stated on the cable label or in the manual. A deviation greater than 10 % indicates a cable failure.

Worn relay – the most common mechanical failure

The relay in the thermostat is an electromagnetic switch. Each time it closes and opens under load, an electric arc is created, which gradually erodes the contacts. A standard thermostat for an electric floor switches on average 8–15 times a day. Over 10 years of operation, that is 30,000–55,000 switching operations. The contacts are simply not designed to last forever.

Signs of a worn relay:

The thermostat is working, but the floor heats irregularly – sometimes yes, sometimes no.
During the test, you can hear the "click" of the relay, but the voltage on the output is zero or unstable.
The floor works when the thermostat is tapped several times (contacts shift).

Solution: replace the thermostat. Repairing the relay is technically possible, but economically unprofitable – a new thermostat such as SALUS RT10-230V is cheaper than a service intervention.

Failure No. 3: Thermostat measures wrong temperature – the floor is overheated or constantly cold

Signs of incorrect temperature measurement

The customer complains that they set 25 °C, but the floor is either hot to the touch (45–50 °C) or barely warm (18–20 °C), while the thermostat displays a value close to the set one. This type of failure usually has three causes:

Shifted NTC sensor resistance – the sensor ages and its resistance characteristic changes. The thermostat then measures the temperature systematically distorted.
Wrong sensor position – the sensor has slipped out of the protective channel and is lying directly on the heating cable. In this case, it measures the cable temperature (70–80 °C), not the floor surface temperature.
Incorrect sensor type setting – some thermostats allow switching between air and floor sensors. If an air sensor is set, while a floor sensor is connected (or vice versa), the entire regulation is non-functional from the beginning.

The topic of correct sensor position and depth is discussed in detail in the article Setting the floor temperature sensor: correct depth and position.

How to check the condition of the NTC sensor

The NTC sensor (Negative Temperature Coefficient) is a thermistor – its electrical resistance decreases with increasing temperature. Most thermostats for floor heating use sensors with a nominal value of 10 kΩ at 25 °C (type 10K/B3950) or 12 kΩ at 25 °C. Verification is simple:

Disconnect the sensor cable from the thermostat.
Measure the resistance of the sensor at room temperature (e.g. 20 °C) using a multimeter.
Compare with the tabular value. At 20 °C, a 10K sensor should have approximately 12.5 kΩ, at 25 °C exactly 10 kΩ. A deviation greater than 5–10 % indicates sensor aging.
If the sensor measures infinite resistance (OL on the multimeter) – it is broken. If it measures zero or very low resistance – it is short-circuited.

Replacement NTC sensors are cheap (3–8 €) and standardized. It is important to maintain the same type (10K or 12K) and the same B-coefficient (B3950 or B3977), otherwise the measurement will be inaccurate even after replacement.

Failure No. 4: Thermostat switches too often or the floor oscillates around the set temperature

Hysteresis and its setting

The thermostat does not regulate temperature continuously – it operates as a two-position controller with hysteresis. This means that the floor heating turns on when the temperature drops below the set value minus hysteresis, and turns off when the temperature rises above the set value plus hysteresis. Standard hysteresis is 0.5 °C; some thermostats allow setting 0.3–2.0 °C.

If hysteresis is set too narrowly (e.g., 0.2 °C), the thermostat switches very often – the relay overheats and wears out faster, and the floor "pulses". If hysteresis is too wide (2 °C), the temperature fluctuates and comfort is reduced. For electric floor heating, optimal hysteresis is 0.5–1.0 °C; for water-based systems, it can be 1.5–2.0 °C.

Incorrect position of the air temperature sensor

If the thermostat measures air temperature (not floor), its measurement is also affected by radiation from the heated floor. Result: the thermostat detects "enough heat" and turns off the heating before the floor is properly heated. Then the floor quickly cools down and the cycle repeats. Solution: the thermostat should not be in the direct line of radiation from the floor (do not place it on the floor or low on the wall without insulation from it).

The difference between floor and air temperature sensors, including where each is suitable to use, is covered in the article Floor thermostat with floor sensor vs. air sensor: difference and when to use which.

Failure No. 5: Thermostat lost settings – resets after power failure

This "failure" is not actually a failure – it is a feature of cheaper models that do not have backup memory (EEPROM or battery). After a power outage, the thermostat returns to factory settings (usually 5 °C or turned off state).

The solution is simple: choose a thermostat with permanent memory. For example, SALUS ERT20 – 230V and SALUS ERT50 – 230V retain programs even without a backup battery thanks to non-volatile memory. If a customer repeats that their "program disappeared" after a power failure, it is almost always a matter of the thermostat model, not a malfunction.

Failure No. 6: Thermostat regulates correctly, but the floor is still cold – problem outside the thermostat

From field experience: about 20–25% of "thermostat failures" are actually faults elsewhere in the system. The thermostat works, but the heating is non-functional for other reasons. Here is a list of things to check:

Broken heating cable or mat: mechanical damage during floor reconstruction, hidden joint, corrosion in wet areas. Test: measuring cable resistance after disconnecting from the thermostat.
Incorrect cable length: too long a cable = lower power = slow heating. Too short = overheating.
Water floor heating: clogged circuit or closed valve: the thermostat opens the actuator (servo valve), but the circuit is not flowing.
Defective actuator (servo motor): in water floor heating, the thermostat controls the actuator, not the water directly. If the actuator is mechanically stuck or has a broken internal mechanism, the circuit remains closed despite the thermostat working normally.
Too thick or thermally insulating floor covering: a carpet with a thick underlay can increase thermal resistance so much that heat is not perceptible on the surface, even though the cable is working normally.

If you are interested in the complete procedure of correct installation including common mistakes made during it, read the articles Installation of a thermostat for floor heating: step-by-step guide and Wiring a thermostat for underfloor heating: diagram and most common errors.

Failure No. 7: Thermostat overheats or smells burnt

This is a serious failure that must be addressed immediately. Overheating of the thermostat can have these causes:

Switching power exceeded: the thermostat is rated, for example, for 10 A (2 300 W), but the heating cable draws 12 A. The relay and terminals overheat, insulation smells or cracks. Solution: immediate shutdown, replacement with a thermostat of higher capacity or use of an external relay.
Loose terminal: if the wire is not securely clamped, the contact has transition resistance. This turns into heat. The terminal blackens, insulation melts. Solution: tightening the terminals, replacing the damaged terminal.
Inappropriate wire cross-section: for a load of 10 A, the supply should have at least 1.5 mm² Cu, for 16 A at least 2.5 mm².

The topic of switching power and dimensioning is covered in a separate article Switching power of a floor heating thermostat: how to determine it correctly.

Failure No. 8: Programmable thermostat ignores the set program

The customer sets up a weekly schedule – heating from 6:00 in the morning, from 17:00 in the evening – but the thermostat seems to ignore the program and heats or does not heat according to its own will. In practice, it is almost always one of these problems:

Incorrectly set clock: after a power outage, the thermostat clock must be reset. If the thermostat does not have a backup battery for the real-time clock (RTC), the time is lost after each outage.
Active manual override (OVERRIDE): most thermostats allow setting the temperature manually, which temporarily overrides the program. The customer forgets about it, and the thermostat runs in manual mode instead of programmed mode.
Locked thermostat: the child lock prevents changing the settings, but the program may be outdated (set last winter with a different schedule).
Confusion between sensors: the program is set to heat up to 25 °C, but the sensor measures air temperature (e.g. 22 °C), while the floor is at 35 °C. The customer thinks the program "does not turn off," because the floor is hot – in reality, the air has not yet reached 25 °C.

A comparison of electronic and programmable thermostats in terms of comfort and error rate can be found in the article Electronic vs. programmable thermostats for floor heating: which one is worth it.

Step-by-step systematic diagnostic procedure

From practice, it follows that the fastest way to find a fault is not random testing, but a systematic approach from the power source towards the load. Here is a proven procedure that works for the vast majority of faults:

Power supply (circuit breaker, voltage at the N and L terminals of the thermostat) – verification with a voltmeter, or a test lamp.
Display status and error codes – reading the manual for the given model.
Condition of the sensor – measuring resistance with a multimeter after disconnecting the sensor from the thermostat.
Testing the thermostat output – measuring voltage at the output terminals when the thermostat should be heating.
Condition of the heating cable – measuring resistance after disconnecting from the thermostat, comparing with the nameplate values.
For a water system: condition of the actuator – verifying the movement of the actuator (servo actuator) when powered with 230 V.

If this procedure does not reveal the fault and the thermostat still does not work properly, it is reasonable to replace it with a new one and test the system with a new device. Thermostats for floor heating are not expensive, and the electrician's service time would cost more than a new device.

Prevention of failures – what prolongs the life of the thermostat

Most failures can be prevented by choosing and using the thermostat correctly. Here are specific recommendations from practice:

Do not exceed 80 % of the nominal switching power: if the nominal power of the thermostat is 3 500 W, do not use it for a load higher than 2 800 W. The relay will operate in a comfortable zone and the lifespan will be significantly extended.
Always install the sensor in a protective conduit: this way it will always measure the floor temperature, not the cable, and will be replaceable without breaking the floor.
Set a reasonable hysteresis: 0.5–1.0 °C is optimal for electric floor heating.
Mount the thermostat on the wall at a height of 120–150 cm, away from direct sunlight, drafts and heat sources (TV, hob).
Check the terminals once a year: tightening the terminals will prevent overheating and fire.
Do not install the thermostat in the bathroom without a suitable IP rating for the given zone. Humidity damages the electronics and contacts.

When to replace the thermostat and when to have it repaired

The question of repair vs. replacement is always economic. A thermostat for floor heating costs from about 20 € (simple analog) to 80–120 € (programmable with WiFi). An electrician's service call costs 50–100 € per hour. From this it follows a simple rule: if the fault is in the thermostat itself (relay, control circuit, display), replacement is always more economical than repair. If the fault is in the sensor (NTC), it is worth replacing only the sensor (2–8 €) and leaving the thermostat.

An exception are expensive programmable thermostats or special devices with integration into a smart home system, where professional repair may make sense if available.

Most frequently asked questions (FAQ)

Thermostat displays E1 or E2 – what does it mean and what to do?

Codes E1 and E2 usually indicate a temperature sensor error – it is either disconnected, broken, or out of the measurement range. First step: check if the sensor plug is securely connected to the thermostat. If it is connected, measure the resistance of the sensor with a multimeter after disconnecting it. At room temperature of 20 °C, the NTC 10K sensor should have about 12–13 kΩ. If it measures OL (break) or 0 Ω (short), the sensor is damaged and must be replaced. Replacement sensors are available for a few euros and are compatible with most thermostats.

Why did my set program disappear after a power outage?

Simpler thermostats do not have backup memory for the program and real-time clock (RTC). After a power failure, they return to factory settings. If this is a problem for you, choose a model with non-volatile memory (EEPROM), e.g. from the SALUS ERT or Jablotron AC-83 series. These thermostats retain the program even after several days of power outage.

Can I connect a longer sensor cable to the thermostat?

Yes, most thermostats allow this – the sensor is connected using a two-core cable and its length can be extended with a standard two-core cable (e.g. CYSY 2×0.5 mm²) up to 5–6 meters without measurable deviation. At greater distances, the ohmic resistance of the extension may start to distort the measurement, but for normal use (extending from 1.5 m to 3–4 m) it is not a problem. It is important to connect the sensor without insulation gaps and to protect the connection from moisture.

The thermostat is heating even though I have set a lower temperature – is this a fault?

This may be due to several things. Most commonly, it is an active manual override (OVERRIDE or BOOST mode), where you temporarily set a higher temperature and forgot to cancel it – try restoring factory settings or switching to program mode. Another possibility: welded (stuck) relay contacts – the relay remains permanently closed. This is a thermostat fault and it needs to be replaced. A third possibility: the sensor is measuring an incorrectly low temperature (shifted resistance, poor placement) and the thermostat believes it is still cold.

Is it normal for the floor heating thermostat to make clicking sounds?

Yes, with thermostats using a mechanical relay, clicking sounds when switching on and off are normal and expected. A problem would be if the clicking were too frequent (more than once every few minutes) – this would indicate an overly narrow hysteresis band or an unstable sensor. If the thermostat emits squeaking, crackling, or prolonged sounds, it is a sign of a problem with the relay contacts and it should be inspected.

Can I connect two heating circuits to one thermostat?

This depends on the total power consumption. If the combined power of both circuits does not exceed the rated switching capacity of the thermostat (and remains below 80% of this value), it is technically possible. In practice, however, this complicates regulation – both circuits are switched on and off simultaneously, without the possibility of individual control. For larger areas or different rooms, it is better to use a separate thermostat for each circuit.

Conclusion

Problems with floor heating thermostats are in most cases diagnosable and solvable without the help of a professional service technician – a voltmeter, multimeter, and patience with a systematic approach from the power source toward the load are sufficient. The most common causes are very trivial: a tripped circuit breaker, a disconnected sensor, a worn relay, or a forgotten manual program override.

If you are deciding on a new thermostat after a problem with the old one, I recommend looking at the selection in the category thermostats for floor heating – you will find there proven models from SALUS and Jablotron with various levels of functionality. When choosing, consider the switching capacity (not just the price), type of sensor, and whether you care about programmability. The article SALUS vs. Jablotron thermostats for floor heating: model comparison in this Knowledge Center will help you make an informed decision.

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.