Bluetooth Application
The Bluetooth application is written using MIT App Inventor.
As far as I know, App Inventor only targets Android devices, I do not own Apple devices!
The .AIA file is saved in the repository path /AppInventor.
This file can be imported into App Inventor and re-configured if desired.
NOTE: It is big and large - I don't particularly like the AI environment, but it is far easier to tame than Android Studio!
App Inventor is located here http://ai2.appinventor.mit.edu/
Note the Bluetooth pairing is first setup in the usual Android settings, the passcode being 1234.
Once paired, the application presents a list of the paired devices to choose from - pick Diesel Heater.
Below are some typical screenshots, progressing from the initial application opened state, Bluetooth connected, Heater started (basic control), Advanced control, Shutdown, Timers, Heater tuning.