F.20

Glow-worm Underfloor Heating Sensor Fault

Thermostat shows F.20 and floor temperature control is disabled

Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Failed floor probe NTC sensor, Damaged sensor cable in screed, Incorrect sensor type configured, Sensor not connected to thermostat terminals

How to Fix / Troubleshooting

Safety: Isolate power to any underfloor heating manifold controls before working on wiring. The floor sensor circuit is low voltage.

Diagnosis:

Locate the floor sensor entry point (usually in conduit from the wall box to the floor). If accessible, gently pull the sensor cable to check for continuity.
With power off, disconnect the sensor from the thermostat and measure its resistance. Compare to the manufacturer’s NTC chart for the approximate floor temperature.
Check that the thermostat is configured to use a floor sensor (if installed) and that the correct sensor type is selected.

Remedy:

If the sensor is open‑circuit or shorted, replace it. If it is buried in screed without conduit, a new sensor may need to be installed in a new conduit run.
After replacement, reconfigure the thermostat and verify that F.20 clears and that floor temperature is displayed correctly.

Repair Difficulty

Hard 4/5

Required Part

Glow-worm underfloor heating floor sensor

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