blockModulation

Outdoor and return air damper position modulation sequence for multi zone VAV AHU
Diagram of ModulationOpen the diagram
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Information

This is a multi zone VAV AHU economizer modulation block. It calculates the outdoor and return air damper positions based on the supply air temperature control loop signal. The implementation is in line with ASHRAE Guidline 36 (G36), PART 5.N.2.c. Damper positions are linearly mapped to the supply air control loop signal. This is a final sequence in the composite multi zone VAV AHU economizer control sequence. Damper position limits, which are the inputs to the sequence, are the outputs of Buildings.Obsolete.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.VAV.Economizers.Subsequences.Limits and Buildings.Obsolete.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.VAV.Economizers.Subsequences.Enable sequences.

When the economizer is enabled, the PI controller modulates the damper positions. Return and outdoor damper are not interlocked. When the economizer is disabled, the damper positions are set to the minimum outdoor air damper position limits.

The control charts below show the input-output structure and an economizer damper modulation sequence assuming a well configured controller. Control diagram:

Image of the multi zone AHU modulation sequence control diagram

Multi zone AHU economizer modulation control chart:

Image of the multi zone AHU modulation sequence expected performance

Parameters

TypeNameDefaultDescription
Commissioning › Controller
RealuMin-0.25Lower limit of controller input when outdoor damper opens (see diagram)
RealuMax+0.25Upper limit of controller input when return damper is closed (see diagram)
RealuOutDamMax(uMin + uMax)/2Maximum loop signal for the OA damper to be fully open
RealuRetDamMin(uMin + uMax)/2Minimum loop signal for the RA damper to be fully open

Connectors

TypeNameDefaultDescription
Buildings.Controls.OBC.CDL.Interfaces.RealInputuTSupSignal for supply air temperature control (T Sup Control Loop Signal in diagram)
Buildings.Controls.OBC.CDL.Interfaces.RealInputuOutDamPosMinMinimum economizer damper position limit as returned by the damper position limits sequence
Buildings.Controls.OBC.CDL.Interfaces.RealInputuOutDamPosMaxMaximum economizer damper position limit as returned by the economizer enable-disable sequence. If the economizer is disabled, this value equals uOutDamPosMin
Buildings.Controls.OBC.CDL.Interfaces.RealInputuRetDamPosMinMinimum return air damper position limit as returned by the economizer enable-disable sequence
Buildings.Controls.OBC.CDL.Interfaces.RealInputuRetDamPosMaxMaximum return air damper position limit as returned by the economizer enable-disable sequence
Buildings.Controls.OBC.CDL.Interfaces.RealOutputyOutDamPosEconomizer damper position
Buildings.Controls.OBC.CDL.Interfaces.RealOutputyRetDamPosReturn air damper position

Revisions

  • April 24, 2021, by Michael Wetter:
    Removed limit on time rate of change on damper signal. The dampers model has already a delay which should make them sufficiently slow. Adding this delay in the controller makes it harder to tune the controls. This change also removed the parameter samplePeriod.
  • October 13, 2017, by Michael Wetter:
    Corrected implementation for when dampers are such positioned that they prevent the mixed air temperature from being below the freezing set point.
  • October 11, 2017, by Michael Wetter:
    Corrected implementation to use control loop signal as input.
  • September 29, 2017, by Michael Wetter:
    Corrected implementation by adding reverse action.
  • June 28, 2017, by Milica Grahovac:
    First implementation.