Heating

[Ebehan26]

Can anyone tell me how to turn on the heating.

[Dewgle]

OP is using a mod called

Device Energy Management

The answer to OP's question is in the description of the mod:

Originally posted by Alligator, author of the DEM mod:

12. Internal temperature:
The inside temperature in the submarine depends on the outside temperature.
You can find the current display of the internal temperature in the map view by the wind symbol.
If it is very hot, the temperature inside rises sharply.
If it is very cold and the temperature in the submarine falls below 12 °, the sailors can get sick. You will be made aware of this with your own text message.
You then have time to switch on the heating (also newly implemented).
This heats the boat to a constant 19 ° C. The heating requires electricity and generates noise. You will find the switch for the heater as a part of the already existing light switch context use radial menu.

If you cannot see this option then you did not subscribe to DEM's dependency, the Custom Actions mod which can also be found on the workshop.