Summertime, the current return air temperature (AI) is 30°C, and the return air temperature setpoint is the default value of 20°C. After the control program detects that the Unit Start/Stop switch (DI) is closed, it immediately triggers Outside Air Damper Control (AO), which is linked to Return Air Damper Control (AO). Then, it opens the fan (DO) after 60 seconds and gradually opens the valve (AO) from 0 to 10 V (fully open).
The valve (AO) gradually closes from 10 to 0 V when the return air temperature setpoint is adjusted towards 30 °C (as exemplified above). The valve fully closes when the setpoint is higher than 30 °C. Additionally, if the PID logic has a dead zone of 1 °C, the output remains unchanged if the setpoint increment is less than 0.5 °C (half the size of the dead zone).
Filter Alarm: Occurs when the filter differential pressure switch (DI) closes after the fan is ON for 30 seconds. Consequently, the alarm LED (DO) closes and generates an output.
Low Limit Temperature Alarm: Occurs at any time when the low limit temperature protection switch (DI) is open. Consequently, the alarm output (DO) closes, the outside air damper is OFF with 0 % output and the valve immediately adjusts to fully open (10 V).