When the return air temperature (for example, AI, 10 °C) is lower than the return air temperature setpoint (for example, configurable parameter, 20 °C), after the Unit Start/Stop switch (DI) closes, the control program immediately triggers Outside Air Damper Control (AO), which is also linked to Return Air Damper Control (AO). Then, the fan (DO) opens after 60 seconds, and the valve (AO) opens gradually 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 10 °C (as exemplified above). The valve fully closes when the setpoint is lower than 10 °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).