.Buildings.Controls.OBC.ASHRAE.G36.TerminalUnits.ParallelFanVVF.Subsequences.DamperValves

Information

This sequence sets the parallel fan flow rate setpoint, damper and valve position for variable-volume parallel fan-powered terminal unit. The implementation is according to Section 5.8.5 of ASHRAE Guideline 36, May 2020. The calculation is done following the steps below.

1. When the zone state is cooling (uCoo > 0), then the cooling loop output uCoo shall be mapped to the primary airflow setpoint from the minimum VActMin_flow to the cooling maximum VActCooMax_flow airflow setpoints. The heating coil is disabled (yVal=0).
   • If supply air temperature TSup from the AHU is greater than room temperature TZon, cooling supply airflow setpoint shall be no higher than the minimum.
2. When the zone state is Deadband (uCoo=0 and uHea=0), then the primary airflow setpoint shall be the minimum airflow setpoint VActMin_flow. The heating coil is disabled (yVal=0).
3. When the zone state is Heating (uHea > 0),
   • As the heating-loop output uHea increases from 0% to 50%, it shall reset the discharge temperature THeaDisSet from the current AHU setpoint temperature TSupSet to a maximum of dTDisZonSetMax above space temperature setpoint.
   • The primary airflow setpoint shall be the minimum flow VActMin_flow.
   • The heating coil shall be modulated to maintain the discharge temperature at setpoint.
4. The VAV damper shall be modulated by a control loop to maintain the measured primary airflow at the setpoint.
5. Fan control
   • When the zone stats is cooling (uCoo > 0), in occupied mode only, the parallel fan starts when primary airflow (VPri_flow) drops below the the minimum outdoor airflow setpoint VOAMin_flow (if using California Title 24, it should be the zone absolute minimum outdoor airflow rate) minus half of the minium fan rate minRat and shuts off when primary airflow rises above the VOAMin_flow (or the zone absolute minimum outdoor airflow rate). Fan airflow rate set point is equal to the minimum outdoor airflow rate minus the current primary airflow set point VPri_flow_Set.
   • When the zone state is Deadband (uCoo=0 and uHea=0), the parallel fan starts when primary airflow (VPri_flow) is below the minimum outdoor airflow VOAMin_flow (or if using California Title 24, the zone absolute minimum outdoor airflow rate). The fan flow rate setpoint is equal to the minimum outdoor airflow rate minus the current primary airflow setpoint VPri_flow_Set.
   • When the zone state is Heating (uHea > 0), fan shall run. As the heating-loop output uHea increases from 50% to 100%, it shall reset the fan airflow setpoint from the setpoint required in deadband proportionally up to the maximum heating-fan airflow setpoint maxRat.

The sequences of controlling damper and the parallel fan for the unit are described in the following figure below.

As specified in Section 5.8.7 of ASHRAE Guideline 36, the airflow setpoint could be override by providing software switches that interlock to a system-level point to:

1. when oveFloSet equals to 1, force the zone airflow setpoint VPri_flow_Set to zero,
2. when oveFloSet equals to 2, force the zone airflow setpoint VPri_flow_Set to zone cooling maximum airflow rate VCooMax_flow,
3. when oveFloSet equals to 3, force the zone airflow setpoint VPri_flow_Set to zone minimum airflow setpoint VMin_flow.

Revisions

• August 24, 2023, by Jianjun Hu:
  Added AHU supply fan status for damper position reset.
  This is for issue 3257.
• January 12, 2023, by Jianjun Hu:
  Removed the parameter have_preIndDam to exclude the option of using pressure independant damper.
  This is for issue 3139.
• August 1, 2020, by Jianjun Hu:
  First implementation.