When the return air temperature (AI, 10 °C, for example) is lower than the return air temperature setpoint (configurable parameter, 20 °C, for example), after the Unit Start/Stop switch (DI) is closed, the control program triggers Outside Air Damper Control (AO) immediately which links to Return Air Damper Control (AO) too, opens the fan (DO) after 60 seconds, and then opens the valve (AO) gradually from 0 to 10 V (fully open).

The valve (AO) closes from 10 to 0 V gradually when one adjusts the return air temperature setpoint towards 10 °C (the return air temperature exemplified above). It fully closes when the setpoint is lower than 10 °C. In addition, if there is a dead zone in the PID logic, say, 1 °C, the output remains unchanged if the setpoint increment is less than 0.5 °C (half of the dead zone valve).

Filter Alarm: Occurs when the filter differential pressure switch (DI) is closed after the fan is ON for 30 seconds. Accordingly, the alarm LED (DO) is closed and generates an output.

Low Limit Temperature Alarm: Occurs at any time when the low limit temperature protection switch (DI) is open. Accordingly, the alarm output (DO) is closed, the outside air damper is OFF with 0% of output and the valve is immediately adjusted to fully open (10 V).