Based on the supply air temperature value and setpoint, the PID calculation drives the valve to generate outputs proportionally or counter-proportionally after the fan is ON, depending on whether "Summer" or "Winter" is selected for "Winter_Summer Mode Select". When there is a low limit temperature alarm, the logic stops the fan immediately and fully opens the heating valve until the alarm is gone; when there is a supply air temperature sensor fault, it stops the valve from generating outputs.