F2 displayed and thermostat cannot read floor temperature in underfloor heating mode.

Safety: Turn off power to the thermostat and boiler before working on wiring. Use a non-contact voltage tester to confirm power is off.

• Step 1 – Check sensor wiring at thermostat:
  • Remove the thermostat from the wall plate.
  • Locate the Floor Sensor terminals (often marked S1/S2 or similar).
  • Ensure both sensor wires are firmly clamped and not broken.
  • Tighten terminal screws gently.
• Step 2 – Measure sensor resistance:
  • Disconnect the two floor sensor wires from the thermostat.
  • Use a multimeter set to kΩ range.
  • Measure resistance across the two sensor wires.
  • A typical Protherm NTC floor sensor (10k at 25°C) should read between 5–20 kΩ depending on floor temperature.
  • If the meter shows OL (open circuit), the sensor or cable is broken.
• Step 3 – Inspect cable route:
  • If accessible, check junction boxes or conduit where the sensor cable runs.
  • Look for crushed, cut, or pinched cable sections.
• Step 4 – Replace floor sensor:
  • If the sensor is open circuit, install a new Protherm NTC floor sensor in the existing conduit.
  • Pull out the old sensor carefully using the cable as a draw wire.
  • Feed the new sensor cable through the conduit to the thermostat.
  • Connect to the sensor terminals and secure.
• Step 5 – Reconfigure thermostat:
  • In the installer menu, ensure Floor sensor is enabled and the correct sensor type is selected.
  • Restore power and confirm F2 is cleared and floor temperature is displayed.