Underfloor Heating Thermostat Not Working
Underfloor heating thermostats fail more often than people expect — both wet and electric UFH systems use zone thermostats that can develop faults. Most thermostat problems are caused by failed floor sensors, incorrect settings, or power supply issues rather than the thermostat itself.
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Most likely cause & what to check
Check the thermostat display is illuminated. If blank, check for a fused spur or isolator switch near the thermostat that may have been turned off. Check the fuse in any fused spur.
Check the thermostat settings — the target temperature should be above the current floor or air temperature. Also check the mode: some thermostats have separate heating and cooling modes — ensure it is set to heating.
Check for a floor sensor error code on the display (E1, E2, F1, F5 depending on brand). These codes mean the floor probe has failed or come loose. The probe is a thin wire sensor installed in the floor screed or under tiles. A loose connection at the thermostat back plate is the most common cause — turn off power and reseat the sensor wire.
If the floor sensor has failed, most thermostats can be switched to air-temperature sensing mode temporarily — check your thermostat manual for instructions. This lets the heating work while you order a replacement sensor.
Replacement floor sensors cost £10–25 and are available for most thermostat brands. Fitting a new sensor into existing clips under the floor is straightforward if the floor is accessible; if the sensor is under tiled screed, the probe channel is usually accessible from the edge.
If the thermostat is working (light on, correct setpoint) but the heating is not responding, the output relay in the thermostat may have failed. Test by using the wiring terminals test mode (if available) or swap the thermostat for a known working model.
For wet UFH systems, a working thermostat that is not activating the manifold actuators points to a wiring fault between the thermostat and the actuator heads — check the wiring connections at both ends.
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