Common Faults in Electric Underfloor Heating and How to Fix Them
Common Faults in Electric Underfloor Heating and How to Solve Them
Compared to traditional heating systems, electric underfloor heating is remarkably reliable. It has no moving parts, requires no regular servicing, and if installed correctly, will last 20–30 years without issue. Nevertheless, faults do occur – and when they do, diagnosis tends to be frustrating precisely because the entire system is hidden under the floor. From my own experience, I know that most faults can be identified and resolved without breaking up the floor, as long as you know where to look and how to proceed systematically. This article covers the most common problems, from a non-working thermostat through tripping circuit breakers to actual mechanical cable damage – and offers a concrete diagnostic procedure and solution for each one.
Why Electric Underfloor Heating Fails: Causes at a Glance
Before we get into specific faults, it's important to understand where the system's weak points lie. Electric underfloor heating consists of several components: the heating element (cable, mat, or foil), a thermostat with a temperature sensor, the electrical connection, and protective devices in the distribution board. A fault can occur at any of these points – and statistically speaking, most problems don't come from the heating element itself, but rather from the thermostat, sensor, or electrical connection. The heating cable embedded in the screed is very well mechanically protected; the thermostat, on the other hand, is in daily contact with the user and is exposed to mechanical vibration, dust, and occasional moisture.
The second most common cause is installation errors. A cable bent too sharply, crossed loops, insufficient screed coverage, a loose connection at the terminal block – all of these may not show up immediately but will manifest after several months or years of operation. So if your system is less than two years old and problems are starting to appear, it's worth checking the installation documentation to confirm the installation was carried out according to specifications. We'll look at this in more detail for each specific fault.
Underfloor Heating Not Working at All – Step-by-Step Diagnostic Procedure
This is by far the most common problem I encounter. The customer calls: "I switched on the heating in autumn and the floor is cold." Most people's first reaction is panic – they imagine having to break up the tiles. In reality, that's the very last step, one we reach in only a small number of cases.
Step 1: Check the power supply at the distribution board
Before anything else, open the distribution board and check the breaker for the underfloor heating circuit. Look for a tripped breaker (in the position between on/off) or a tripped RCD (residual current device). The heating circuit should have its own breaker – usually 10 A or 16 A depending on the installation's power – and should be protected by a 30 mA RCD. If the breaker has tripped, switch it back on. If it trips again immediately, this signals a short circuit or earth fault, and you need to continue the diagnostic process. If the breaker holds, move on to the thermostat.
Step 2: Check the thermostat
The thermostat is the most common culprit for faults. Check the following, in this order:
- Does the thermostat have power? Is the display or indicator light on? If not, check whether the live and neutral wires are correctly connected to the thermostat terminals. Sometimes the fault is simply a loose terminal screw.
- Is the thermostat in "child lock" or lockdown mode? Many digital thermostats have a lock function – the display lights up but doesn't respond to buttons. Check the specific thermostat's manual for how to disable this function.
- Is the target temperature set too low? If the current floor temperature is higher than the set temperature, the thermostat correctly won't start heating.
- Is night setback or a schedule active that's currently keeping the heating switched off? For wifi thermostats such as HAKL TH 951wifi or HAKL TH 952wifi, also check the settings in the mobile app.
Step 3: Test the floor temperature sensor
If the thermostat has power but doesn't start heating (or, conversely, runs constantly), the problem may lie in the floor temperature sensor. The sensor is a resistive element (NTC thermistor) whose resistance decreases as temperature rises. At room temperature of around 20–22 °C, the sensor should show a resistance of roughly 10–15 kΩ (this depends on the specific sensor, usually 12 kΩ at 25 °C). Disconnect the sensor from the thermostat and measure the resistance with a multimeter. If the multimeter shows zero (short circuit) or infinity (open circuit), the sensor is damaged and needs replacing. The good news: the sensor sits inside a conduit near the edge of the room and can be replaced without breaking up the floor – simply pull out the old conduit and insert a new one with the sensor.
Step 4: Measure the resistance of the heating cable
If both the thermostat and the sensor are fine, proceed to measure the cable itself. Disconnect the cable from the thermostat terminal block and measure the resistance between the two conductors. The correct value is given in the cable's documentation or directly on the label – for a 1000 W/230 V cable, this works out to around 53 Ω; for 1500 W, around 35 Ω. A deviation of ±10% is acceptable. If the resistance is zero, this indicates a short circuit (usually mechanical damage). If it's infinite, this indicates a break. In both cases, the problem is directly in the heating element.
At the same time, measure the insulation resistance with a multimeter set to 500 V DC (if you have one) or at least to the highest available resistance range – the insulation resistance between each conductor and earth (the cable's braiding) must be at least 1 MΩ, ideally several MΩ or more. If it's lower, there is damage to the cable's insulation.
Breaker or RCD Keeps Tripping
This is a more serious symptom than simply non-functioning heating. If the breaker holds when the thermostat is off but trips as soon as it's switched on, we're looking for a short circuit or earth fault directly in the heating element or at the terminal block connection.
The first thing I do in this situation: disconnect the heating cable from the thermostat and switch the breaker back on. If it holds, the problem is in the heating element or terminal block (not the thermostat). Then I measure the resistance value directly on the disconnected cable, and especially the insulation resistance. If the insulation resistance has dropped below 1 MΩ – and I've come across cases where it was just a few kΩ – this indicates insulation damage, which, with a closed floor, means either a faulty installation (the cable was bent too sharply or crossed over itself and both loops rubbed against each other during thermal expansion) or mechanical damage caused by a tool during a later renovation.
If the RCD still trips even with the cable disconnected (i.e. only the thermostat remains in the circuit), the fault lies either in the thermostat or in the wiring leading to it. The thermostat can be replaced relatively easily and cheaply – for example, the digital thermostat HAKL TH 900 is a practical, reliable solution that anyone who knows how to wire a socket can install.
Short circuit at the terminal block – an underestimated cause
The terminal block, where the cold tail of the cable is connected to the supply wiring, is a critical point. In practice, I see several issues: insufficiently tightened screws (vibration and thermal movement of the cable loosen the screw over time), oxidation of the contact when aluminium wiring is used, or moisture penetrating the enclosure. The terminal block should be embedded in screed or at least secured and covered. If it isn't, check it physically.
Uneven Floor Heating – Hot Spots and Cold Spots
A customer complains: "The middle of the bathroom floor is hot, but near the wall it's almost cold." Or the opposite: "It's scorching by the entrance door, but nowhere else." These symptoms almost always point to an error in cable layout during installation, not a fault in the cable itself.
The most common causes of uneven heating:
- Uneven loop spacing: If the installer didn't maintain a consistent spacing (usually 80–150 mm depending on output and area), some zones will have a higher cable density and thus a higher output per m², while others will have less. The result is hot spots.
- Cable covered by furniture: Under furniture, heat doesn't dissipate into the room (the room can't absorb it), so the cable overheats, and the thermostat may not detect this if the sensor is located elsewhere. Over time, this can damage the cable's insulation and shorten its lifespan.
- Poor floor sensor placement: The sensor should be positioned between two cable loops, not directly above one. If the sensor sits above the cable, it will measure a higher temperature and the thermostat will switch off heating before the rest of the floor actually warms up.
- Varying screed thickness: If the screed was poured in two stages or settled unevenly, the thickness above the cable will vary and heat transfer to the surface will be uneven. This is sometimes encountered in old buildings or during renovations.
Thermostat Displays an Error or Incorrect Temperature
Modern digital thermostats have a built-in error code system. The most common codes I encounter:
- E1 / Err1 / F1: Broken floor sensor – the multimeter shows infinite resistance at the sensor terminals. Solution: replace the sensor.
- E2 / Err2 / F2: Short-circuited floor sensor – the multimeter shows zero or very low resistance. Again, replace the sensor.
- E3 / Err3: Air sensor (if the thermostat has one) out of range – usually at temperatures above 40 °C or below 0 °C, which can occur if the thermostat is installed in direct sunlight or in a draught.
Specifically, on the HAKL TH 900, a sensor error appears as "- - -" on the display instead of a numerical temperature value. In such a case, the thermostat may switch to an emergency mode with a fixed output value (for example, switching on for 30 minutes and off for 30 minutes), which can lead to interrupted heating. You can find the exact procedure in the manual, but the basic diagnostic step is always the same – measuring the sensor's resistance.
With wifi thermostats such as HAKL TH 952wifi, error states are also displayed in the mobile app along with a description of the fault. The advantage is that you can see the system's status remotely and don't have to wait until you get home to discover the heating hasn't been running all day.
Thermostat always shows the same temperature
If the displayed value stays at, say, 23.5 °C regardless of whether the floor is cold or warm, the sensor is most likely short-circuited or has poor contact at the terminal. Check the physical wiring of the sensor and measure its resistance. This situation is dangerous in terms of floor overheating, because the thermostat "thinks" the temperature is always sufficient and won't turn on the heating – or, conversely, due to a misunderstanding of the control logic, the heating may run continuously.
Mechanical Cable Damage – When and Why It Occurs
This is the most dramatic fault, because repairing it usually requires opening at least part of the floor. But even here there are various scenarios of differing severity.
Damage during later renovation
The most common cause of mechanical damage in practice. Someone was drilling into the floor to install a toilet, shower enclosure, or partition wall – and drilled through the cable. If you know where the cable was installed (from the installation documentation or photo documentation that the installer should have provided), you can estimate where the damage might have occurred. The exact break point can be located using the TDR method (time-domain reflectometry) – a device that sends a pulse into the cable and measures where it reflects back from. Most electrical measurement companies can do this for a fee. The result will give you the fault's distance in metres from the end of the cable, accurate to ±20–30 cm, which significantly reduces the area that needs to be broken up.
Damage from thermal overload (overheating)
The cable is designed to operate at surface temperatures up to 65–90 °C (depending on the manufacturer and cable type). If the cable was permanently covered by a carpet, thick mat, or heavy furniture, and the thermostat's sensor wasn't located in that zone, the cable may have overheated over a long period. This shows up as browned or flaking insulation (visible once the floor is opened), reduced insulation resistance, and eventually a short circuit. Prevention involves correctly positioning the sensor and never placing thick thermal insulators on the heated floor area.
Damage during installation itself
This usually shows up shortly after the first switch-on – within a few weeks. Causes: the cable was bent too sharply (the minimum bending radius is usually 5–6 times the cable diameter, i.e. about 30–40 mm), the cable was mechanically stressed while pouring the screed (for example, someone stepped on it), or the loops shifted and crossed during pouring. The result is a localised concentration of heat that eventually breaks down the insulation. This is why photo documentation before pouring the screed is so important – you can read more about this in the article Installing a Heating Cable: Spacing, Embedding, and Connection.
Underfloor Heating Runs Continuously Without Switching Off
The opposite extreme: the thermostat never switches off the heating, the floor is overheated, and the electricity bill is unbearable. This problem usually has three causes:
- Relay fault in the thermostat: The relay can get stuck in the "on" position. Verification: disconnect the live wire from the thermostat's output terminal. If the heating stops, the relay is damaged and the thermostat needs replacing. Replacement is simple and inexpensive.
- Poor sensor placement: If the sensor is embedded too deep in a thick layer of screed, or is surrounded by thermal insulation (e.g. a foam underlay beneath laminate), it heats up more slowly than the floor surface, and the thermostat "thinks" the floor isn't warm enough yet.
- Target temperature set too high: The set target temperature is higher than the system can achieve (for example, 35 °C on a cable under thick laminate, where the actual output is lower). The thermostat runs continuously but never reaches the target. The solution is to set a realistic maximum floor temperature – most thermostats allow you to set an upper limit (for example, 28–30 °C for wood flooring, 35–40 °C for tiles).
Issues Specific to Wifi Thermostats
Wifi thermostats such as the HAKL TH 951wifi offer the convenience of remote control, but also bring new categories of faults that don't exist with classic thermostats.
Thermostat disconnected from the wifi network
The most common "fault" with wifi thermostats isn't technical, but network-related. If you changed your wifi password, replaced your router, or your IP address changed, the thermostat can't connect. Solution: reset the thermostat's wifi settings and re-pair it according to the manual. With dual-band routers (2.4 GHz and 5 GHz), remember that most cheaper IoT devices, including thermostats, only work on 2.4 GHz – connect them exclusively to that network.
App doesn't show current data
If the app shows "device offline" but the thermostat is physically working (the floor is heating, the display is lit), this is a problem with communication to the cloud server. Try: restarting the app, restarting the thermostat (a brief power-off using the breaker), checking that the wifi router has internet access. If the problem persists longer, contact the manufacturer's technical support – cloud issues sometimes occur on the server side, not the device.
Special Cases: Underfloor Heating Under Different Floor Coverings
The type of floor covering significantly affects both diagnostics and the nature of faults. For more details on the specifics of different coverings, see the article Electric Underfloor Heating Under Different Floor Types: Tiles, Vinyl, Laminate; here I'll just summarise the key connections to faults:
- Under tiles (ceramic, stone): The most stable situation. The cable sits in screed with good thermal stability. Faults are rarer, but there's a risk of drilling during installation of fixtures.
- Under laminate / wood: Here, the maximum temperature is critical – exceeding 27–28 °C over a prolonged period damages the flooring (delamination, deformation). The thermostat must have a floor temperature limit set! Laminate floors also flex under load and can put pressure on the cables beneath them, which, with a thin mat installation, can lead to mechanical wear.
- Under vinyl (LVT, SPC): Vinyl is sensitive to temperature, with a maximum recommended value of 28 °C. A bigger issue, however, can be the adhesive under the vinyl – you must use a flooring adhesive type resistant to thermal cycling, otherwise the floor lifts and creates air pockets that reduce heat transfer.
Preventive Maintenance: How to Avoid Faults
Electric underfloor heating doesn't require servicing in the traditional sense, but a few preventive steps can significantly extend its lifespan and prevent faults:
- Test the system once a year: At the start of the heating season, switch the thermostat to manual mode with a high set temperature and verify that the floor actually heats up evenly. Catch problems early, not in the middle of winter.
- Check thermostat settings after a power outage: Many thermostats lose their programme settings during a long power outage. After power is restored, check that the schedule and time match reality.
- Keep your installation documentation: A photo of the cable layout before the screed is poured is invaluable for any later work on the floor. Never allow the heating cable to be embedded without this documentation.
- Don't use thick thermal insulating rugs on the heated area: Rugs thicker than 10 mm significantly reduce efficiency and can cause the cable to overheat in that zone.
- Update the firmware of wifi thermostats: Manufacturers release updates that fix bugs and improve security. Regularly check for available updates in the app.
When to Call an Electrician and When to Fix It Yourself
A clear boundary: anything involving the electrical connection at the distribution board, or replacing breakers or RCDs, requires a qualified electrician. On the other hand, replacing the thermostat, replacing the floor sensor, or resetting settings can be handled by a competent layperson with a multimeter and a wiring guide.
For work within the heated floor (fault location, breaking up the floor, repairing a connection), I always recommend calling in someone with a TDR device and experience – unnecessarily breaking up a large area is expensive and impractical. A good technician can narrow the search down to an area of 30 × 30 cm.
If you're considering replacing the heating element over a larger area, take a look at the range of HAKL TCX heating cables, which are a popular choice precisely for their reliability and availability of spare parts. For new installations, it's also worth reading the articles Installing a Heating Mat Under Tiles: Step-by-Step Procedure and What Output Do I Need for Electric Underfloor Heating in My Space?, so that you choose the correct sizing when carrying out a renovation.
Frequently Asked Questions (FAQ)
Can I replace the floor sensor myself without opening up the floor?
Yes, in the vast majority of cases. The floor sensor sits inside a plastic conduit (tube) that runs from the thermostat to a point beneath the floor. The conduit exits near the wall – simply pull the thin sensor cable out of the conduit and insert a new one. The whole operation takes 5–15 minutes, requires no demolition, and is within anyone's abilities. Choose a new sensor with the same type of resistive element (NTC) and the same resistance characteristic value as the original – you can find this in the thermostat's documentation.
How do I find out exactly where the cable is broken in the floor?
The most accurate method is TDR measurement (time-domain reflectometry), performed by electrical diagnostics companies with the appropriate equipment. This method can locate the break with an accuracy of ±20–30 cm, which reduces the area to be broken up to a minimum. A cheaper but less precise option is thermographic measurement – if the cable is partially working, an infrared camera will show the warm and cold sections. With a complete break, the cable doesn't heat at all, so thermography won't help.
The floor heats up slowly – is this a fault?
Not necessarily. Slow warm-up is normal behaviour with a thick surface covering (thick ceramic, stone) or on first start-up after a longer break, when the screed is cold throughout. Electric underfloor heating typically has a warm-up time of 20–60 minutes; with stone or thick tiles, even 90 minutes. However, if the warm-up time has increased compared to previous seasons without any change in settings, it's worth checking the thermostat and sensor – there may be a control issue.
The thermostat switches correctly, but the floor is never warm to the touch. Why?
The most common cause: the sensor is placed too close to the cable or in an unsuitable location (near a wall with cold massive construction). The thermostat reports that the set temperature has been reached before the entire area actually heats up. Solution: increase the set target temperature by 2–3 °C and observe the result. If this doesn't help, move the sensor to a central position between two cable loops. For more on choosing a suitable thermostat and correct settings, see the article How to Choose a Thermostat for Electric Underfloor Heating: Manual, Digital, or Wifi?
Is it normal to hear sounds from the wall when the underfloor heating switches?
A soft click when switching on and off is normal – it's the sound of the mechanical relay in the thermostat. If you hear loud cracking, bubbling, or repeated rapid clicking (chattering), this is a problem. Rapid clicking suggests the thermostat is oscillating around the set temperature with too small a hysteresis – most thermostats allow you to set the temperature hysteresis (the difference between switch-on and switch-off temperature) to at least 1 °C, ideally 1.5–2 °C. Loud cracking may indicate arcing in the relay contacts – this is a reason to replace the thermostat.
How long does a heating cable last, and when is it time to replace it?
Quality heating cables have a lifespan of 25–30+ years with correct installation and operation. What tends to fail sooner are thermostats (typically 10–15 years) and sensors (5–15 years, depending on the type). If the cable shows correct resistance values and good insulation when measured, there's no reason to replace it based on age alone. Conversely, if the insulation resistance has dropped below 1 MΩ, replacement is justified – even if the cable still "somehow" works, it represents a safety risk.
Conclusion: A Systematic Approach Saves Time and Money
When a customer calls me reporting "the underfloor heating isn't working," the first question I ask is: "When did you last see it working?" This single point determines whether we're looking for a recent change (a new router, a power outage, a renovation) or gradual degradation (an ageing sensor, a loose terminal). A systematic approach – from the distribution board through the thermostat and sensor to the cable – saves hours of unnecessary searching and, in most cases, identifies the problem before we ever need to take a hammer to the floor.
Remember: statistically, more than 70% of reported electric underfloor heating faults are resolved without any intervention in the floor at all. The thermostat, sensor, terminal block, settings – these are your first four stops. The cable under the screed is generally the strongest link in the entire system.
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