On the display it shows Open and the heating no longer heats.
Explanation of the situation:
Open mode should actually detect when a window is open and then turn off the heating. The unit goes into Open mode when the room temperature drops a few degrees in a short time. After the room temperature stabilises again, the heating should leave Open mode on its own. However, as the temperature sensor of the Climastar heater is very precise (+/- 0.05 °C), it may not detect stabilisation and remain in Open mode.
You can easily deactivate the open mode with the attached instructions.