implements #16400
This is not exactly as the ticket wants it, displaying an empty field in case "Room Ventilation" was configured previously means that one could leave it like that or even select an empty field in the drop down. A bit more effort would be to add the empty field dynamically only for the above case, but that'd be slightly more effort.
I went for a slightly different approach,when "Room ventilation/10" was selected, I will reconfigure the dropdown to "Apartment Ventilation" and enable the save button.