blockLimits

Single zone VAV AHU minimum outdoor air control - damper position limits
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Information

This block implements the single zone VAV AHU minimum outdoor air control with a single common damper for minimum outdoor air and economizer functions based on outdoor airflow setpoint (VOutMinSet_flow) and supply fan speed (uSupFanSpe), designed in line with ASHRAE Guidline 36, PART 5.P.4.d.

The controller is enabled when the supply fan is proven on (uSupFan=true), the AHU operation mode uOpeMod is Occupied, and Freeze protection stage uFreProSta is 1 or smaller. Otherwise the damper position limits are set to their corresponding maximum and minimum physical or at commissioning fixed limits, as illustrated below:

Image of damper position limits state machine chart

If limit modulation is enabled, the outdoor air damper position yOutDamPosMin is computed as follows:

  1. Calculate outdoor air damper position yDam_VOutMin_curSpe which ensures minimum outdoor airflow rate VOutMin_flow at current supply fan speed uSupFanSpe as a linear interpolation between the following values set at commissioning:
    • minimum damper position at minimum fan speed for minimum outdoor airflow yDam_VOutMin_minSpe and
    • minimum damper position at maximum fan speed for minimum outdoor airflow yDam_VOutMin_maxSpe.
  2. Calculate outdoor air damper position yDam_VOutDes_curSpe which ensures design outdoor airflow rate VOutDes_flow at current supply fan speed uSupFanSpe, as a linear interpolation between the following values set at commissioning:
    • minimum damper position at minimum fan speed for design outdoor airflow yDam_VOutDes_minSpe and
    • minimum damper position at maximum fan speed for design outdoor airflow yDam_VOutDes_maxSpe.
  3. Calculate outdoor air damper position yOutDamPosMin which ensures outdoor airflow setpoint VOutMinSet_flow at current supply fan speed uSupFanSpe as a linear interpolation between yDam_VOutMin_curSpe and yDam_VOutDes_curSpe, proportional to ratios of VOutMinSet_flow to VOutDes_flow and VOutMin_flow.

The chart below illustrates the OA damper position limit calculation:

Image of damper position limits control chart

Parameters

TypeNameDefaultDescription
Commissioning › Damper position limits
RealyFanMin0.1Minimum supply fan operation speed
RealyFanMax1Maximum supply fan operation speed
RealyDam_VOutMin_minSpe0.4OA damper position to supply minimum outdoor airflow at minimum fan speed
RealyDam_VOutMin_maxSpe0.3OA damper position to supply minimum outdoor airflow at maximum fan speed
RealyDam_VOutDes_minSpe0.9OA damper position to supply design outdoor airflow at minimum fan speed
RealyDam_VOutDes_maxSpe0.8OA damper position to supply design outdoor airflow at maximum fan speed
RealVOutMin_flowCalculated minimum outdoor airflow rate
RealVOutDes_flowCalculated design outdoor airflow rate
Commissioning › Physical damper position limits
RealoutDamPhyPosMax1Physically fixed maximum position of the outdoor air damper
RealoutDamPhyPosMin0Physically fixed minimum position of the outdoor air damper

Connectors

TypeNameDefaultDescription
Buildings.Controls.OBC.CDL.Interfaces.RealInputuSupFanSpeSupply fan speed
Buildings.Controls.OBC.CDL.Interfaces.RealInputVOutMinSet_flowMinimum outdoor airflow setpoint
Buildings.Controls.OBC.CDL.Interfaces.IntegerInputuOpeModAHU operation mode status signal
Buildings.Controls.OBC.CDL.Interfaces.IntegerInputuFreProStaFreeze protection status signal
Buildings.Controls.OBC.CDL.Interfaces.BooleanInputuSupFanSupply fan status signal
Buildings.Controls.OBC.CDL.Interfaces.RealOutputyOutDamPosMinMinimum outdoor air damper position limit
Buildings.Controls.OBC.CDL.Interfaces.RealOutputyOutDamPosMaxMaximum outdoor air damper position limit

Revisions

  • August 3, 2026, by Michael Wetter:
    Removed duplicate annotation. This is for IBPSA, issue 2158.
  • March 13, 2020, by Jianjun Hu:
    Replaced multiAnd block with and3 block to avoid vector related implementation.
    This is for #1709.
  • July 06, 2017, by Milica Grahovac:
    Refactored implementation.
  • April 15, 2017, by Jianjun Hu:
    First implementation.