Suppose a summer scenario when the current return air temperature (AI) is 30 °C and the return air temperature setpoint is the default value (18 °C). After detecting that the Unit Start/Stop switch (DI) is closed, the program triggers Outside Air Damper Output (DO) immediately, 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 30 °C (the return air temperature exemplified above). It fully closes when the setpoint is higher than 30 °C.

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).