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

Information

This is a multi zone VAV AHU economizer enable/disable sequence based on the Section 5.16.7 of the ASHRAE Guideline 36, May 2020. Additional conditions included in the sequence are: freeze protection (freeze protection stage 0-3, see Section 5.16.12), supply fan status (on or off, see Section 5.16.5).

The economizer is disabled whenever the outdoor air conditions exceed the economizer high limit setpoint. This sequence allows for all device types listed in ASHRAE 90.1-2013 and Title 24-2013.

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

• The supply fan is off (u1SupFan = false),
• the freeze protection stage Buildings.Controls.OBC.ASHRAE.G36.Types.FreezeProtectionStages is not stage0.

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

After the disable signal is activated, the following procedure is applied, in order to prevent pressure fluctuations in the HVAC system:

• The return damper gets fully opened (yRetDam_max = uRetDamPhy_max and yRetDam_min = uRetDamPhy_max) for retDamFulOpeTim time period, after which the return damper gets released to its minimum outdoor airflow control position (yRetDam_max = uRetDam_max and yRetDam_min = uRetDam_max).
• The outdoor air damper is closed to its minimum outoor airflow control limit (yOutDam_max = uOutDam_min) after a disDel time delay.

Revisions

• October 13, 2017, by Michael Wetter:
  Added freeze protection that tracks mixed air temperature.
• August 3, 2017, by Michael Wetter:
  Removed unrequired input into block and2 as this input was always true if and2.u2 = true.
• June 27, 2017, by Milica Grahovac:
  First implementation.