Start Stop Sequencing (AHU Mixed Air) - Johnson Controls - Metasys - LIT-12011147 - Software Application - Controller Configuration Tool - 15.0

Controller Tool Help

Johnson Controls
Product name
Controller Configuration Tool
Document type
User Guide
Document number
Revision date

The Mixed Air Start Stop Sequencer module sequences the fans and dampers for a controlled start and controlled stop in a mixed air system. The module accepts several inputs which are use to initiate a start or stop sequence such as Preheat Percent Cmd, Reheat Percent Cmd, Cooling Required, Heating Required, and Occupancy. The module accepts several user adjustable times such as Fan Start Delay, Stabilize delay, Damper Close Delay, Fan Stop Delay. The module accepts inputs for static pressure such as Duct Static Pressure 1, Duct Static Pressure 2, and Duct Static Pressure Setpoint. The module accepts configuration inputs such as First Fan To Start, Supply Fan Type, and Intermittent Fan. The module has inputs for a preheat bonnet switch and a reheat bonnet switch which may be used to provide a heating fan on delay. The module also has supply fan status inputs such as Supply Fan OnOff and Supply Fan Percent Cmd. The module has a state output, a duct static pressure output, and a fan ready indicator output.

The unit initiates a start sequence if any of the following is true:

  • Cooling Required = True

  • Preheat Bonnet Switch = Hot AND Preheat Percent Cmd > 0 AND Heating Required = True

  • Reheat Bonnet Switch = Hot AND Reheat Percent Cmd > 0 AND Heating Required = True

The unit goes through the shutdown sequence if either of the following conditions is met:

  • Cooling Required = False AND Heating Required = False

  • The duct static pressure sensor fails.

The unit goes directly to Off, without passing through the normal shutdown sequence if the Supply Fan Control Status stops responding. This situation occurs when a higher level sequencer, such as the No Air Flow Sequencer or the Emergency Mode Sequencer, takes control of the supply fan.

You can view and modify the modules in this group’s logic. The Finite State Machine cannot be modified. For information on how the FSM determines the state, see FSM (AHU Mixed Air Start Stop Sequencing).