Electric Underfloor Heating Not Working: Fault-Finding Guide

Summary

Electric underfloor heating (UFH) faults fall into a small number of repeatable categories: no power reaching the thermostat, a tripped RCD/RCBO, a failed floor or air sensor, a broken heating element (mat or loose cable), or a faulty thermostat unit itself. The mat or cable element is buried under tile, screed or floor covering and cannot be visually inspected without lifting the floor — which means diagnosis has to be done electrically, from the thermostat and the consumer unit, before any floor covering is disturbed. Getting the diagnosis right before opening anything up saves a genuinely expensive mistake: lifting good tile to "find" a fault that was actually a blown fuse.

The most common real-world cause of a "dead" UFH system is not a broken element at all — it's a tripped RCD/RCBO (often caused by moisture ingress at a connection, not the element itself) or a failed floor probe sensor, both of which are diagnosable and fixable without touching the floor covering. Genuine element (mat/cable) failures are much less common, usually caused by physical damage during installation (a screw, nail, or tile spacer piercing the cable) or, less often, a manufacturing fault. This guide walks through a structured fault-finding sequence, with an ASCII decision tree, before covering each fault category in more detail.

For pricing a UFH repair or replacement, see electric underfloor heating pricing guide. For water-fed (wet) UFH systems, which fault differently, see wet underfloor heating.

Key Facts

• Thermostat blank/no display — check supply first: breaker/RCD at consumer unit, then the thermostat's own internal fuse (many models have a small fuse behind the faceplate — isolate before opening)
• Thermostat display on, no heat, no error — most likely a stuck or failed relay inside the thermostat, or the heating circuit has an open circuit (broken element/connection)
• Thermostat displays an error code (E1, E2, SEN, or similar, code varies by manufacturer) — almost always a sensor fault (floor probe or air sensor), not the heating element itself; check the specific manufacturer's error code table
• RCD/RCBO tripping repeatedly on reset — strongly suggests an earth fault, most commonly at the element (moisture ingress or physical damage) rather than the thermostat — do not keep resetting and re-tripping; this indicates a genuine fault requiring investigation, not a nuisance trip
• Floor probe (NTC thermistor) resistance — typically 10k ohm or 12k ohm at 25°C depending on manufacturer; check the specific kit's datasheet, as this varies by brand and model
• Heating element (mat/cable) resistance — should match the kit's rated resistance (calculated from Ohm's law: R = V² ÷ P, e.g. a 750W 230V mat should read roughly 70.5 ohms) within a small tolerance; a reading of infinite resistance (open circuit) indicates a break in the element, a reading of zero or very low resistance can indicate a short
• Insulation resistance test — a qualified electrician tests the element to earth with an insulation resistance tester; a low reading indicates moisture ingress or physical damage to the cable insulation
• Cold tail / connection point — the factory-made joint between the heating cable/mat and the cold lead into the thermostat is a common failure point, particularly if disturbed or poorly protected during floor laying
• Physical damage during installation — screws, tile spacers, self-levelling compound trowels, or subsequent fixings (e.g. drilling for a toilet pan or skirting) piercing the element is the most common cause of genuine element failure
• Underfloor heating installation certificate/schematic — a competent installer should have left a record of the loop resistance at commissioning and a layout drawing showing cable/mat position — this is the single most useful document for fault-finding and should always be requested before diagnosis begins
• Zone valve / relay box faults (multi-zone systems) — where several thermostats feed a central relay box, a fault can sit in the relay box rather than any individual room's element or thermostat
• Timer/programming issues mistaken for faults — always rule out a programming error (frost protection mode, holiday mode, incorrect schedule) before assuming a hardware fault, especially on WiFi/smart thermostats
• Part P notification — any fault-finding or repair work involving the fixed wiring, the thermostat's mains connection, or the circuit at the consumer unit is notifiable electrical work in a dwelling and must be carried out or certified by a Part P registered electrician, or notified to Building Control

Quick Reference Table

Detailed Guidance

Fault-finding decision tree

START: UFH not heating
|
|-- Thermostat display blank?
|     |-- YES --> Check consumer unit: breaker tripped / RCD tripped?
|     |             |-- YES --> Reset ONCE. Trips again? --> STOP. Earth fault
|     |             |             suspected. Call electrician for insulation
|     |             |             resistance test. Do NOT keep resetting.
|     |             |-- Stays on --> Check thermostat's own internal fuse
|     |             |                 (isolate supply before opening faceplate)
|     |             |-- NO (breaker fine) --> Check thermostat wiring/supply
|     |                                        connection - electrician required
|     |-- NO (display lit) --> Continue below
|
|-- Error code showing on display?
|     |-- YES --> Check manufacturer's error code table
|     |             |-- Sensor-related code --> Test floor probe resistance
|     |             |    at thermostat terminals against rated value
|     |             |             |-- Out of range --> Faulty probe.
|     |             |             |    Replace if accessible spare probe fitted;
|     |             |             |    otherwise electrician assessment needed
|     |             |             |-- In range --> Wiring/connection fault -
|     |             |                  electrician required
|     |-- NO error, but no heat --> Continue below
|
|-- Thermostat calling for heat (relay audibly clicks / heat symbol shown)?
|     |-- NO click / no heat symbol --> Thermostat relay likely failed -
|     |     replace thermostat unit (like-for-like, by electrician if
|     |     hardwired)
|     |-- YES, relay clicks, still no heat --> Element circuit fault likely
|                    |
|                    v
|-- ELECTRICIAN REQUIRED FROM THIS POINT:
|     Isolate circuit. Measure element (mat/cable) loop resistance
|     at the cold tail connection.
|             |-- Infinite / open circuit --> Break in element - locate
|             |    via thermal imaging or continuity mapping; repair or
|             |    replace section
|             |-- Reads correctly vs rated resistance --> Element intact.
|             |    Check insulation resistance to earth
|             |             |-- Low reading --> Moisture ingress or damaged
|             |             |    insulation - element likely needs replacing
|             |             |-- Reads healthy --> Fault is in wiring between
|             |                  element and thermostat, or thermostat itself
|             |-- Reads near zero / short --> Short circuit in element -
|                  element damaged, replacement of affected area required

Ruling out programming and non-hardware causes first

Before any testing, confirm the thermostat isn't in frost protection, holiday, or "off" schedule mode, and that any smart/WiFi thermostat isn't simply disconnected from the app/network while still functioning locally. On multi-zone systems, confirm the fault is isolated to one zone and not a shared relay box or shared supply feeding several rooms — a fault that affects several rooms simultaneously points upstream to a shared component, not to an individual room's element.

Testing the floor sensor (probe)

The floor probe is usually a two-core NTC (negative temperature coefficient) thermistor embedded in the floor buildup, terminating at the thermostat. With the thermostat isolated, measure resistance across the probe's two terminals — this should be in the range specified by the manufacturer (commonly 10k or 12k ohm at 25°C, dropping as temperature rises). A reading of infinite resistance means an open circuit (broken probe wire, most likely at a nick or crush point), a reading of zero suggests a short. Many manufacturers supply UFH kits with a spare accessible floor probe run separately for exactly this reason — check whether one is present and unused before assuming a full floor lift is needed.

Testing the heating element

This step requires isolating the circuit and should only be carried out by a qualified electrician. The element's rated resistance can be calculated from its rated wattage and voltage (R = V² ÷ P) and compared against the kit's datasheet or the installer's commissioning record. A significant deviation from the expected resistance, an open circuit, or a failed insulation resistance test (which checks for a breakdown in the cable's insulation to earth, commonly caused by moisture or physical damage) all point to element failure rather than a control-side fault.

When the floor has to be lifted

If testing confirms the element itself has failed (not the sensor, thermostat, or supply), locating the exact point of damage before lifting flooring saves significant cost. Thermal imaging (with the element powered briefly under controlled conditions) or a time-domain reflectometer can pinpoint a break or short within the cable run, narrowing the area of floor that needs to be opened rather than lifting the whole area.

Frequently Asked Questions

Can I test the floor sensor myself, or does it need an electrician?

Testing resistance at the thermostat terminals with the unit isolated is within reach of a competent, careful person using a multimeter, provided the thermostat itself is safely isolated from the mains supply first. However, any fault found to be upstream of the sensor — in the mains wiring, at the consumer unit, or requiring the thermostat to be replaced — is notifiable electrical work under Part P and should be carried out by a registered electrician, both for safety and to keep the property's electrical certification valid.

My RCD keeps tripping when I reset it — is that dangerous to keep doing?

Yes, repeatedly resetting an RCD that keeps tripping is not safe practice. An RCD trips because it has detected current leaking to earth — repeatedly forcing it back on without investigating why risks an ongoing fault (commonly moisture ingress into a damaged element) getting worse, and in rare cases increases fire or shock risk. Reset once to confirm the trip is repeatable, then stop and get an electrician to test the circuit's insulation resistance before any further attempts.

The floor is warm in some patches but cold in others — is that a sensor fault?

Uneven heating is more often a sign of how the element was laid than a fault developing later — cable runs laid with wider spacing in some areas, mat overlaps, or gaps around fixed furniture will always produce cold patches, independent of any subsequent fault. If the pattern has changed since installation (previously even, now patchy), that points to localised element damage rather than a design issue, and warrants the resistance and insulation testing described above.

Is it worth repairing a damaged element or should I just replace the whole system?

For a small, precisely located area of damage (for example, a single screw puncture found via thermal imaging), a localised repair splicing in a new section of cable is possible and significantly cheaper than a full re-lay, though it does carry some risk of a weaker long-term joint compared to the original factory-made connections. For older systems, systems with multiple failure points, or where the exact fault location can't be pinpointed without lifting a large area of floor, full replacement is usually the more cost-effective and lower-risk option — factor this into the quote alongside the condition of the floor covering itself.

Regulations & Standards

• BS 7671 — Requirements for Electrical Installations (IET Wiring Regulations); governs circuit protection (RCD/RCBO), earthing, and testing requirements for UFH circuits, particularly Section 701 for bathroom installations

• Building Regulations Part P — electrical safety in dwellings; UFH circuit installation, alteration, and most fault-finding/repair work involving fixed wiring is notifiable

• The Electricity at Work Regulations 1989 — statutory duty on anyone carrying out electrical fault-finding or repair work to ensure it is done safely

• BS EN 60335-2 — Household and similar electrical appliances — safety; relevant to thermostat units and control equipment

• Electrical Safety First — Underfloor Heating Safety Guidance — consumer and trade guidance on electric heating system safety

• NICEIC — Part P Notifiable Work Guidance — guidance on what electrical work is notifiable under Part P

• HSE — Electricity at Work Regulations 1989 — statutory duties for electrical fault-finding and repair

• electric underfloor heating pricing guide — pricing for electric UFH installation and repair, including mat/cable and thermostat costs

• wet underfloor heating — fault diagnosis for water-fed (wet) UFH systems, a different fault category entirely

• rcd tripping — general RCD tripping diagnosis, relevant background for the earth fault branch of this guide

• no power to circuit — general no-power fault diagnosis relevant to the supply-side branch of this guide