It’s difficult to give sensible advice without some more information regarding your project.
Basic principals are that when a widget attached to a virtual pin is changed in the app then it triggers the corresponding `BLYNK_WRITE(vPin) callback function only once. You should assign the incoming value from the widget to a global variable.
You then need to use a BlynkTimer to take sensor readings, and compare the results to your defined temperature set point in auto mode.
In manual mode you need to override this automatic logic processing, and the easiest way to do this is via an if statement to check if you are in auto or manual mode.
This is covered briefly here…
Pete.