A key sign appears on the display of my heater and I cannot use the buttons anymore. What is wrong?

The key sign indicates that the Child Safety Lock is active. When during a period of 4 minutes there is no activity and no buttons are pressed, the heater enters the Child Lock safety mode which means that no button can be pressed anymore.

To "unlock" the display buttons, you have to simultaneously press the "+" and "-" buttons. You then will here a beep and the radiator is unlocked again.

Now the "Key" symbol is gone and the display lights up. Now you can use the buttons on display again