blockReturnFanDirectPressure

Information

Setpoint for return fan discharge pressure and exhaust air damper for a multi zone VAV AHU according to ASHRAE guideline G36, PART 5.N.10 (return fan control with direct building pressure).

1. Return fan operates whenever associated supply fan is proven on and is off otherwise.

2. Return fan is controlled to maintain return fan discharge static pressure at setpoint dpBuiSet.

3. Exhaust damper is only enabled when the associated supply and return fans are proven on (uFan=true) and the minimum outdoor air damper is open (to be controlled in a separate sequence). The exhaust dampers is closed when the fan is disabled.

4. The building static pressure is time averaged with a sliding 5-minute window to dampen fluctuations. The averaged value shall be displayed and is used for control.

5. When the exhaust damper is enabled, a control loop modulates the exhaust damper in sequence with the return fan static pressure setpoint as shown in the figure below to maintain the building pressure equal to dpBuiSet, which is by default 12 Pa (0.05 inches).

The output signal of the building pressure control is as follows:

1. From 0 to 0.5, the building pressure control loop modulates the exhaust dampers from yExhDam = 0 (closed) to yExhDam = 1 (open).
2. From 0.5 to 1, the building pressure control loop resets the return fan discharge static pressure setpoint from dpDisMin to dpDisMax. The dpDisMin and dpDisMax are specified in Section G36 PART 3.2A.3.b.

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Revisions

• October 15, 2020, by Michael Wetter:
  Moved normalization of control error to PID controller.
  This is for #2182.
• October 16, 2017, by Michael Wetter:
  Revised implementation, normalized control input and updated documentation.
• October 16, 2017, by Jianjun Hu:
  First implementation.