.Buildings.Controls.OBC.ASHRAE.G36.AHUs.SingleZone.VAV.SetPoints.ReliefFanGroup

Information

Sequence for controlling relief fans and their dampers for AHUs using actuated relief dampers with relief fan(s). It is developed based on Section 5.16.9 of ASHRAE Guideline 36, May 2020.

1. All operating relief fans that serve a common or shared air volume shall be grouped and controlled as if they were one system, running at the same speed and using the same control loop, even if they are associated with different AHUs.
2. A relief fan shall be enabled when its associated supply fan is proven ON (u1SupFan=true), and shall be disabled otherwise.
3. Building static pressure (dpBui) shall be time averaged with a sliding 5-minute window and 15 second sampling rate (to dampen fluctuations). The average value shall be that displayed and used for control.
4. A P-only control loop maintains the building pressure at a set point (dpBuiSet) of 12 Pa (0.05 in. of water) with an output ranging from 0% to 100%. The loop is disabled and output set to zero when all fans in the relief system group are disabled.
5. Fan speed signal to all operating fans in the relief system group shall be the same and shall be equal to the PID signal but no less than the minimum speed. Except for Stage 0, discharge dampers of all relief fans shall be open only when fan is commanded ON.
   1. Stage 0 (barometric relief). When relief system is enabled, and the control loop output is above 5%, open the motorized dampers to all relief fans serving the relief system group that are enabled; close the dampers when the loop output drops to 0% for 5 minutes.
   2. Stage Up. When control loop is above minimum speed (relFanSpe_min) plus 15%, start stage-up timer. Each time the timer reaches 7 minutes, start the next relief fan (and open the associated damper) in the relief system group, per staging order, and reset the timer to 0. The timer is reset to 0 and frozen if control loop is below minimum speed plus 15%. Note, when staging from Stage 0 (no relief fans) to Stage 1 (one relief fan), the discharge dampers of all nonoperating relief fans must be closed.
   3. Stage Down. When PID loop is below minimum speed (relFanSpe_min), start stage-down timer. Each time the timer reaches 5 minutes, shut off lag fan per staging order and reset the timer to 0. The timer is reset to 0 and frozen if PID loop rises above minimum speed or all fans are OFF. If all fans are OFF, go to Stage 0 (all dampers open and all fans OFF).
6. For fans in a Level 2 alarm and status is OFF, discharge damper shall be closed when is above Stage 0.

Revisions

• March 5, 2024, by Michael Wetter:
  Corrected wrong use of displayUnit.
• July 15, 2021, by Jianjun Hu:
  First implementation.