CH / CHL means child lock.
This is to prevent small children from changing the temperature or touching the buttons on the heater. It can be activated and deactivated in the Mill app or directly on the heater.
Press the gear button to enter the menu.
Use the + and - buttons to navigate through the menu until you see CHL in the display.
Press the gear button to activate "Child Lock". CHL will blink twice on the display, and child lock will be activated.
To deactivate child lock, quickly press the gear button 5 times. CHL will blink on the screen and the child lock will be deactivated.
Activate/deactivate child lock in the app (for Generation 3M and Generation 4):
Select the heater you want to activate/deactivate child lock on.
Scroll down to Child Lock. Here you can turn child lock on or off.
If the switch is green, child lock is activated.
If the switch is grey, child lock is deactivated.