Sequence of Operation for Air Handling Units

Input & Output Signals & SOP for Air Handling Units


  • Supply air temperature sensor
  • Mixed air temperature sensor
  • Return air temperature sensor
  • Outdoor air temperature sensor
  • Humidity sensor
  • CO2 sensor
  • Fan status switches
  • Heating/cooling valve status switches


  • Supply air damper actuator
  • Return air damper actuator
  • Mixing box damper actuator
  • Heating coil control valve actuator
  • Cooling coil control valve actuator
  • Fan motor control relay
  • Alarm and fault signal outputs

Control sequence:

  1. The controller reads the input signals from the various sensors to determine the current conditions in the system.
  2. If the outdoor air temperature is below a certain setpoint and the humidity level is within acceptable limits, the controller opens the economizer damper and allows outside air to be used for cooling instead of mechanical cooling.
  3. The controller adjusts the supply air damper position based on the required airflow rate and static pressure setpoints.
  4. The controller adjusts the return air damper position to maintain neutral pressure in the space being served.
  5. The controller adjusts the mixing box damper position to ensure proper mixing of the outside and return air streams.
  6. If the supply air temperature is below the desired setpoint, the controller modulates the heating coil valve to increase the supply air temperature.
  7. If the supply air temperature is above the desired setpoint, the controller modulates the cooling coil valve to decrease the supply air temperature.
  8. If the humidity level is too high or too low, the controller adjusts the operation of any humidification or dehumidification components to maintain the desired humidity setpoint.
  9. If the CO2 level is too high, the controller increases the ventilation rate to bring in more outside air.
  10. The controller monitors the status of the fans and heating/cooling valves and triggers alarms or fault signals if any issues are detected.

in MEP
Sequence of Operation for Air Handling Units
Administrator December 22, 2023
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