Condensation or visible moisture inside the thermostat cover.

Safety first: Moisture can cause short circuits. If you suspect water has entered the device, switch off power to the heating system.

• Dry the area: Gently remove the thermostat from the wall and allow it to dry in a warm, dry room. Do not use direct heat sources like hairdryers.
• Improve ventilation: Reduce humidity in the room with extraction fans or dehumidifiers.
• Consider relocation: Move the thermostat to a less humid location if possible, or improve wall insulation behind it.
• Inspect for corrosion: If you see corrosion on contacts or PCB, the thermostat should be replaced.