.Buildings.Controls.OBC.ASHRAE.G36.AHUs.SingleZone.VAV.Economizers.Subsequences.Enable

Information

This is a single zone VAV AHU economizer enable/disable sequence based on Section 5.18.7 and Section 5.1.17 of ASHRAE Guideline 36, May 2020. Additional conditions included in the sequence are:

• Buildings.Controls.OBC.ASHRAE.G36.Types.FreezeProtectionStages,
• Supply fan status u1SupFan,
• Buildings.Controls.OBC.ASHRAE.G36.Types.ZoneStates.

The economizer is disabled whenever the outdoor air conditions exceed the economizer high limit cutoff setpoint plus the hysteresis low limit parameter (TOutHigLimCutLow or hOutHigLimCutLow) as shown in the figure. And the economizer is enabled whenever the outdoor air conditions are below the economizer high limit cutoff setpoint plus the hysteresis high limit parameter (TOutHigLimCutHig or hOutHigLimCutHig). This sequence allows for all device types listed in ASHRAE 90.1-2013 and Title 24-2013.

In addition, the economizer is disabled without a delay whenever any of the following is true:

• supply fan is off,
• zone state is heating,
• freeze protection stage is not 0.

The following state machine chart illustrates the transitions between enabling and disabling:

Revisions

• August 29, 2023, by Hongxiang Fu:
  Because of the removal of Logical.And3 based on ASHRAE 231P, replaced it with a stack of two Logical.And blocks. This is for #2465.
• August 1, 2020, by Jianjun Hu:
  Updated according to ASHRAE G36, May 2020.
• July 30, 2019, by Kun Zhang:
  Added the option to allow fixed plus differential dry bulb temperature cutoff.
• July 06, 2017, by Milica Grahovac:
  First implementation.