blockReturnFanDirectPressure

Information

Setpoint for return fan discharge pressure and relief air damper for a multi zone VAV AHU according to Section 5.16.10 of ASHRAE Guideline G36, May 2020.

Note that this sequence assumes that the AHU units with return fan having the return fan with direct building pressure control have the minimum outdoor air damper.

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. Relief damper is only enabled when the associated supply and return fans are proven on (u1SupFan=true) and the minimum outdoor air damper is open (to be controlled in a separate sequence). The relief 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 relief damper is enabled, a control loop modulates the relief 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 yRelDam = 0 (closed) to yRelDam = 1 (open).
2. From 0.5 to 1, the building pressure control loop resets the return fan discharge static pressure setpoint from p_rel_RetFan_min to p_rel_RetFan_max. The p_rel_RetFan_min and p_rel_RetFan_max are specified in Section 3.2.1.4.

Parameters

Connectors

Components

Revisions

• October 16, 2017, by Michael Wetter:
  Revised implementation, normalized control input and updated documentation.
• October 16, 2017, by Jianjun Hu:
  First implementation.