.Buildings.Obsolete.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.VAV.Economizers.Subsequences.Enable

Information

This is a multi zone VAV AHU economizer enable/disable sequence based on ASHRAE G36 PART 5.N.7 and PART 5.A.17. Additional conditions included in the sequence are: freeze protection (freeze protection stage 0-3, see PART 5.N.12), supply fan status (on or off, see PART 5.N.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 (uSupFan = false),
• the freeze protection stage Buildings.Obsolete.Controls.OBC.ASHRAE.G36_PR1.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, per PART 5.N.7.d, in order to prevent pressure fluctuations in the HVAC system:

• The return damper gets fully opened (yRetDamPosMax = uRetDamPhyPosMax and yRetDamPosMin = uRetDamPhyPosMax) for retDamFulOpeTim time period, after which the return damper gets released to its minimum outdoor airflow control position (yRetDamPosMax = uRetDamPosMax and yRetDamPosMin = uRetDamPosMax).
• The outdoor air damper is closed to its minimum outoor airflow control limit (yOutDamPosMax = uOutDamPosMin) after a disDel time delay.

This sequence also has an overwrite of the damper positions to track a minimum mixed air temperature of TFreSet, which is by default set to 4°C (39.2 F). This is implemented using a proportional controller with a default deadband of 1 K, which can be adjusted using the parameter kPFrePro.

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.