Duct pressure control - Johnson Controls - LIT-12011950 - Building automation software - Smart Equipment Controller - 5.0

Smart Equipment Controls Sequence of Operation Technical Bulletin

Johnson Controls
Product name
Smart Equipment Controller
Document type
Technical Bulletin
Document number
Revision date
Product status

This section describes the duct pressure control setpoints and related data, inputs, outputs, and operation.

Setpoints and related data

The setpoints and related data include the following items:

  • Duct pressure setpoint (DctPrs-SP)
  • Duct static pressure (DctPrs)
  • Duct pressure shutdown setpoint (DctShutdownSp)


The input for duct pressure control is 0 to 5 VDC from the duct pressure sensor to terminal DCT PRS.


The outputs include the following items:

  • 24 VAC from the FAN to energize the fan motor or enable VFD
  • 2 to 10 VDC from the VFD terminal.


When the fan is energized, the 2 to 10 VDC output from VFD terminal is used to maintain the supply duct pressure to the duct pressure setpoint. If the duct pressure is above setpoint, the VFD output decreases. If the duct pressure is below setpoint, the VFD output increases.

The rate of change of the VFD output is determined by the deviation from setpoint and length of time away from the setpoint.

If the duct pressure reaches the duct pressure shutdown setpoint, the fan and all other outputs of the unit de-energize.

Note: If the unit is in a heating mode, the control continues to vary the DC output to control duct pressure to the duct static pressure setpoint. As a result, in any VAV heating mode, all VAV boxes must be commanded open far enough to get adequate airflow to support the heating function and to prevent the heat section high temperature limit switches from opening.
Note: When electric heat is active, the supply fan operates at 100%.