Why is my electric underfloor heating not working?

The most common causes in order of frequency are: failed thermostat, failed floor sensor, programming error on the thermostat, tripped MCB or RCD, and damaged heating mat. Start by checking the thermostat display is showing the correct time and schedule, then check the consumer unit. If the circuit breaker has tripped, reset it and monitor. If the floor remains cold after a full heating cycle (allow 4 to 6 hours), call a qualified electrician to test the mat resistance and thermostat.

How do I test my underfloor heating mat resistance?

After safe isolation of the UFH circuit, disconnect the heating mat leads from the thermostat back-box. Using a multifunction tester set to ohms, measure the resistance between the two mat conductors. Compare the reading against the value stated on the mat label or installation certificate — it should be within 10% of the stated value. Then set the tester to 500V DC insulation resistance and measure between each conductor and the mat screen (earth). A healthy mat reads above 20MΩ. Below 1MΩ indicates damaged insulation requiring further investigation.

Can I replace the thermostat on my underfloor heating myself?

In England and Wales, replacing a like-for-like thermostat in a room other than a bathroom is not notifiable under Part P Building Regulations, provided the heating circuit itself is not modified. However, thermostat replacement involves working on the wiring at the back-box, which requires competence with electrical work. If the UFH circuit is in a bathroom or shower room, replacement is notifiable. We recommend using a qualified electrician to ensure the replacement thermostat is correctly programmed and the sensor is functioning.

How long does electric underfloor heating take to warm up?

Electric UFH under ceramic or porcelain tiles typically takes 30 to 60 minutes to reach set temperature. Under stone it can take 90 minutes. Under timber or vinyl it heats faster — typically 20 to 40 minutes. If the floor feels cool after the scheduled heating period has run for longer than these timescales, the system has a fault. Note that newly installed UFH in freshly laid screed should not be operated at full temperature for the first 4 weeks — a gradual commissioning programme must be followed to allow the screed to cure correctly.

My underfloor heating trips the RCD — what does this mean?

RCD tripping from a UFH circuit indicates current is leaking to earth. This almost always means the heating mat insulation has been damaged — either during floor installation (most common), by a nail or screw penetrating the cable, or by age and thermal cycling. An electrician must perform insulation resistance testing to confirm the mat has failed. A tripping RCD is a genuine electrical fault — do not continue resetting and using the system.

What is the floor sensor and why does it matter?

The floor sensor is an NTC thermistor probe, typically 4mm diameter and 1.5 to 2 metres long, that is laid in the floor screed between the heating cables during installation. It measures the actual floor temperature and feeds this information to the thermostat, preventing the floor from overheating. If the sensor fails (reads open circuit or short circuit), the thermostat typically displays an error code and may disable the heating as a safety measure. Most sensors are replaceable via the thermostat back-box without lifting the floor.

How much does it cost to replace an underfloor heating mat?

Heating mat replacement requires lifting the floor covering and the tile adhesive or screed layer, replacing the mat, re-screeding or re-laying adhesive, and re-laying the floor covering. The total cost for a typical bathroom (4 to 6 square metres) is £500 to £1,000. A larger kitchen or living room (15 to 25 square metres) costs £1,200 to £2,500 or more depending on the floor type. The electrical work element is a small proportion — the major cost is the floor covering and screed work.

FAULT FINDING GUIDE

Electric Underfloor Heating Not Working: Fault Finding Guide

Step-by-step fault finding for electric underfloor heating systems — thermostat faults, floor sensor failure, damaged heating mat diagnosis, insulation resistance testing, TDR fault location, and typical repair and replacement costs for 2026.

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12 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

1The thermostat is the most common cause of electric underfloor heating not working — it is also the easiest and cheapest component to replace, typically costing £60 to £150 fitted.
2The floor temperature sensor is a separate NTC (negative temperature coefficient) thermistor probe buried in the floor screed. Sensor failure causes inaccurate temperature readings or complete heating failure, and the sensor can usually be replaced via the thermostat back-box without lifting the floor.
3A damaged heating mat requires insulation resistance testing with a 500V DC test to identify whether the mat has failed. A healthy mat reads above 20MΩ; below 1MΩ indicates a fault requiring mat replacement.
4Heating mat damage during floor installation (tile laying, floor board fixing) is the most common cause of mat failure. Protect the mat by testing resistance before and after each floor covering stage.
5Replacing a damaged heating mat requires lifting the entire floor covering and screed, making it the most expensive UFH repair — typically £500 to £2,000 depending on room size and floor type.

01 · Fault Finding Guide

How Electric Underfloor Heating Works

Electric underfloor heating (UFH) uses a resistance heating cable or mat laid beneath the floor covering to warm the floor surface from below. Unlike wet underfloor heating (which circulates heated water through pipes), electric UFH is entirely electrical and has no moving parts — making it highly reliable when correctly installed and protected.

Heating mat or cable — a resistance cable woven into a fibreglass mesh mat, or a single twin-conductor cable laid in a serpentine pattern. The cable converts electrical energy to heat by resistance. Mats are rated in watts per square metre — typically 100 to 200W/m². A standard bathroom mat rated 150W/m² covering 4m² draws 600W.
Thermostat — controls the heating schedule and set temperature. Most modern thermostats have a digital display, programmable weekly schedule, and dual-sensor control (air temperature and floor temperature). The thermostat is the most frequently replaced UFH component.
Floor sensor — an NTC thermistor probe buried in the floor during installation. Measures the actual floor temperature to prevent overheating and protect floor coverings. The thermostat uses this reading alongside (or instead of) the air temperature sensor.
Dedicated circuit — the UFH system is fed from a dedicated radial circuit at the consumer unit, usually protected by a 6A or 10A MCB and an RCD. Larger systems may be on a 16A or 20A circuit. The thermostat is the switching device — it connects and disconnects the mat as required.

Most domestic electric UFH systems operate at 230V. The heating mat is a Class II (double-insulated) appliance in the floor but must be earthed via a screen conductor. The thermostat back-box contains the circuit connections and sensor wiring.

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02 · Fault Finding Guide

Safe Isolation Before Any Work

Isolate before opening the thermostat back-box

The thermostat back-box contains 230V live conductors connected directly to the heating mat. Always isolate at the consumer unit and prove dead at the back-box terminals before touching any wiring.

For UFH fault finding and thermostat replacement, isolation is straightforward:

1Identify the UFH circuit at the consumer unit — it is usually labelled "Underfloor Heating," "UFH," or the room name. Switch off the MCB.
2Remove the thermostat front plate — the front plate clips or screws off the back-box. The back-box remains fixed to the wall and contains the wiring.
3Prove dead at the back-box terminals — test with a GS38-compliant voltage indicator between L and N, L and E, and N and E. Only proceed when all tests confirm dead.

03 · Fault Finding Guide

Thermostat Faults

The thermostat is the most common cause of electric underfloor heating not working. It is also the easiest and most cost-effective component to replace. Before diagnosing a thermostat fault, check the programming — many apparent faults are simply incorrect schedule settings or a thermostat that has lost its programme after a power cut.

Check the schedule and set temperature — the thermostat must be programmed to a heating period that covers the current time. The set temperature must be above the current floor or air temperature for the heater to activate. Check the current time on the thermostat is correct — many thermostats lose the time after a power interruption and default to 00:00.
Error codes — modern thermostats display error codes on the screen when a fault is detected. Common codes are: E1 or E2 (floor sensor fault), E3 (air sensor fault), Hi (floor overtemperature). Refer to the thermostat manual for the specific code meaning. E1/E2 codes mean the sensor must be tested or replaced.
Relay failure — inside the thermostat, a relay switches the mat circuit on and off. A failed relay (either stuck open or stuck closed) can cause no heat or continuous heat regardless of the set temperature. After safe isolation, the thermostat can be bypassed by connecting the supply live directly to the mat live — if the mat heats, the relay in the thermostat has failed.
Thermostat replacement — replacement thermostats are widely available and most brands follow a standard wiring layout (L, N, E supply; L, N load to mat; sensor terminals). Always record the existing wiring connections with a photograph before disconnecting. The replacement thermostat must be compatible with the mat wattage and the sensor type (NTC 10kΩ or 12kΩ depending on the manufacturer).

Thermostat replacement costs £60 to £150 all-in (thermostat £30 to £100, labour £30 to £50 for a straightforward like-for-like replacement). If the replacement thermostat is in a bathroom, the work is notifiable under Part P and an Minor Electrical Installation Works Certificate must be issued.

04 · Fault Finding Guide

Floor Sensor Issues

The floor temperature sensor is a critical component that is often overlooked during fault finding. Sensor failure is the second most common cause of UFH not working and is frequently misdiagnosed as a mat fault.

Testing the sensor — after safe isolation, disconnect the sensor leads from the thermostat sensor terminals. Measure resistance between the two sensor wires with a multimeter set to ohms. A standard NTC 10kΩ sensor reads approximately 10,000 ohms at 25°C, decreasing as temperature rises (at 40°C it reads approximately 5,800 ohms). An open circuit (OL) means the sensor has failed. A reading close to zero means the sensor is shorted. Both require replacement.
Replacing the sensor — the sensor probe sits in a conduit tube laid between the heating cables during installation. The conduit tube typically runs back to the thermostat back-box, allowing the sensor to be withdrawn and replaced without lifting the floor. Pull the old sensor out carefully (the conduit may be a tight fit in screed), thread the new sensor in, and reconnect at the thermostat. Replacement sensors cost £8 to £25.
Sensor not in conduit — some older installations have the sensor laid directly in the screed without a conduit. If the sensor fails in these installations, it cannot be replaced without lifting the floor. As a workaround, some thermostats can be reconfigured to use air-temperature-only control, removing dependence on the floor sensor until the floor is lifted for another reason.

05 · Fault Finding Guide

Damaged Heating Mat

A damaged heating mat is the most serious UFH fault because it almost always requires lifting the floor to repair or replace. Mat damage occurs most commonly during floor installation — tiles being laid, adhesive being spread, or fixings penetrating the cable.

Common causes of mat damage — trowelling tile adhesive too aggressively, screws or nails penetrating the cable (particularly in timber floor installations), heavy tools or equipment dropped on the floor during installation, or subsequent fixings drilled through the floor into the cable after installation.
Symptoms of a damaged mat — RCD tripping when the heating is activated, low insulation resistance between the mat conductors and earth, resistance reading significantly different from the stated mat resistance, or partial heating (one section of the floor heats but another remains cold).
Locating the damage — a time-domain reflectometer (TDR) can locate a break or fault in the cable by measuring the time taken for a pulse to reflect back from the fault. TDR testing is specialist work, but can pinpoint the fault to within 0.5 metres, minimising the area of floor that must be lifted. Without TDR, the entire mat area must be uncovered to locate the damage.

Prevention is far preferable to cure. Always test mat resistance before and after laying tile adhesive, and again after grouting. Record the initial resistance on the installation certificate so you have a reference value for future testing.

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06 · Fault Finding Guide

Insulation Resistance Testing of the Heating Mat

Insulation resistance (IR) testing is the definitive test for the condition of a UFH heating mat. It should be carried out during installation at each stage, on completion of installation, and when a fault is suspected.

Test voltage — 500V DC is the standard test voltage for UFH heating cables. Some manufacturers specify 1000V DC for their cables — check the installation instructions. Do not exceed the specified test voltage as this can damage the cable insulation.
Test points — disconnect the mat cold tail from the thermostat back-box. Apply the IR test between: (1) the two mat conductors (live and neutral of the twin conductor); (2) each mat conductor and the mat screen/earth. All readings should be above 20MΩ for a new installation. A reading below 1MΩ indicates a serious fault.
Resistance check — in addition to the IR test, measure the mat resistance (ohms) between the two conductors with the tester set to the appropriate resistance range. Compare this against the value on the mat label or installation certificate. The reading should be within ±10% of the stated value. A significantly lower resistance can indicate a partial short; a higher resistance or open circuit indicates a break in the conductor.
Record the results — record IR test results on the Electrical Installation Certificate or the manufacturer's installation record form. These records are essential if the floor is damaged in future and a fault must be assessed against the original installation data.

07 · Fault Finding Guide

Repair & Replacement Costs — 2026 Prices

UFH repair costs vary enormously depending on whether the fault is in the thermostat, sensor, or the mat itself. Here are typical UK costs for 2026:

Thermostat replacement — £60 to £150 all-in. Thermostat £30 to £100, labour £30 to £50. Most cost-effective first repair — always start here.
Floor sensor replacement — £40 to £100 all-in. Sensor £8 to £25, labour £30 to £75. Requires the sensor conduit to be accessible from the thermostat back-box.
Fault finding and IR testing — £60 to £120 for a diagnosis visit including mat resistance and IR testing. This confirms whether the mat has failed before committing to the cost of lifting the floor.
Heating mat replacement — bathroom (4 to 6m²) — £500 to £1,000 all-in, including electrical work, lifting and re-laying tiles, and screed repair. The mat itself costs £80 to £200. The majority of cost is floor covering work.
Heating mat replacement — kitchen or living room (15 to 25m²) — £1,200 to £2,500 or more. Under engineered wood or LVT rather than tiles, the flooring replacement cost is higher.

If the floor is due to be replaced for other reasons (worn tiles, updated décor), combining UFH mat replacement with the floor refurbishment significantly reduces the total cost — the floor lifting work is shared between the two projects.

08 · Fault Finding Guide

For Electricians: UFH Work and Certification

Electric underfloor heating installation is notifiable work under Part P of the Building Regulations in England and Wales when installed in a bathroom, shower room, kitchen, or as a new circuit in any other location. Thermostat replacement in a non-bathroom room is generally not notifiable, but always check the specific installation.

The key record for any UFH installation is the mat resistance and insulation resistance at each stage of installation. This data, recorded on the Electrical Installation Certificate, provides the baseline for all future fault finding and is invaluable if the customer reports a fault months or years later.

Record Test Results On Site

Use the Elec-Mate Electrical Installation Certificate app to record mat resistance values, IR test results, and circuit details during installation. The PDF is generated on site and sent to the customer immediately — providing the critical baseline data for future fault finding.

Quote the Floor Replacement Scope

When diagnosing a mat fault, use the Elec-Mate quoting app to produce a detailed quote covering the electrical work element. Present it alongside quotes from your preferred tiling contractor so the customer gets a complete picture of the total project cost.

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Frequently Asked Questions — Underfloor Heating Faults

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Electric Underfloor Heating Not Working: Fault Finding Guide

Key Takeaways

How Electric Underfloor Heating Works

Get the BS 7671 A4:2026 Cheat Sheet — free

Safe Isolation Before Any Work

Thermostat Faults

Floor Sensor Issues

Damaged Heating Mat

Try Elec-Mate free for 7 days

Insulation Resistance Testing of the Heating Mat

Repair & Replacement Costs — 2026 Prices

For Electricians: UFH Work and Certification

Record Test Results On Site

Quote the Floor Replacement Scope

Complete UFH installation certificates on your phone

Frequently Asked Questions — Underfloor Heating Faults

Why is my electric underfloor heating not working?

How do I test my underfloor heating mat resistance?

Can I replace the thermostat on my underfloor heating myself?

How long does electric underfloor heating take to warm up?

My underfloor heating trips the RCD — what does this mean?

What is the floor sensor and why does it matter?

How much does it cost to replace an underfloor heating mat?

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