.Buildings.Controls.OBC.ASHRAE.G36.AHUs.MultiZone.VAV.Economizers.Subsequences.Limits.SeparateWithDP

Information

Block that outputs the position limits of the return and outdoor air damper for units with a separated minimum outdoor air damper and differential pressure control. It is implemented according to Section 5.16.4 of the ASHRAE Guideline 36, May 2020.

Differential pressure setpoint across the minimum outdoor air damper

• Per Section 3.2.1, designer should provide the design minimum pressure difference across the minimum outdoor air damper, dpDesMinOutDam. The absolute minimum pressure difference (dpAbsMinOutDam) should also be provided if complying with California Title 24 requirements.
• Calculate the outdoor air set point with Buildings.Controls.OBC.ASHRAE.G36.AHUs.MultiZone.VAV.SetPoints.OutdoorAirFlow.ASHRAE62_1.AHU if complying with ASHRAE 62.1 requirements. Otherwise, see the set points in Buildings.Controls.OBC.ASHRAE.G36.AHUs.MultiZone.VAV.SetPoints.OutdoorAirFlow.Title24.AHU if complying with Title 24 requirements.
• The minimum outdoor air differential pressure set point shall be calculated per Section 5.16.4.1 (if complying with ASHRAE 62.1 requirements) or Section 5.16.4.2 (if complying with Title 24 requirements).

Open minimum outdoor air damper

Open minimum outdoor air damper when the supply air fan is proven ON and the system is in occupied mode and the minimum differential pressure set point is greater than zero. Damper shall be closed otherwise.

Return air damper

• Return air damper minimum outdoor air control is enabled when the minimum outdoor air damper is open and the economizer outdoor air damper is less than a projected position limit, which is 5% when supply fan speed is at 100% design speed proportionally up to 80% when the fan is at minimum speed.
• Return air damper minimum outdoor air control is disabled when the minimum outdoor air damper is closed or the economizer outdoor air damper is 10% above the projected position limit as determined above.
• When enabled, the maximum return air damper set point is modulated from 100% to 0% to maintain the differential pressure across the minimum outdoor air damper at set point.

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:
  First implementation.