.Buildings.Controls.OBC.ASHRAE.G36.VentilationZones.Title24.Setpoints

Information

This sequence sets the zone minimum outdoor air and minimum airflow setpoints, for compliance with the ventilation rate procedure of California Title 24. The implementation is according to Section 5.2.1.4 of ASHRAE Guideline36, May 2020. The calculation is done following the steps below.

1. For every zone that requires mechanical ventilation, the zone minimum outdoor airflows and set points shall be calculated depending on the governing standard or code for outdoor air requirements.

2. According to section 3.1.2 of Guideline 36, the zone minimum airflow setpoint VMin_flow and the zone cooling maximum setpoint VCooMax_flow should be provided by designer.

Zone ventilation set points

According to Section 3.1.1.2.b of Guideline 36,

• The zone minimum outdoor airflow for occupants VOccMin_flow, per California Title 24 prescribed airflow-per-occupant requirements.
• The zone minimum outdoor airflow for building area VAreMin_flow, per California Title 24 prescribed airflow-per-area requirements.

Zone minimum outdoor air setpoints

The zone absolute minimum outdoor airflow setpoint VZonAbsMin_flow is used in terminal-unit sequences and air-handler sequences. The zone design minimum outdoor airflow setpoint VZonDesMin_flow is used in air-handler sequences only.

• VZonAbsMin_flow shall be reset based on the following conditions in order from highest to lowest priority:
  1. Zero if the zone has a window switch and the window is open.
  2. Twenty-five percent of the VAreMin_flow if the zone has an occupancy sensor and is unpopulated.
  3. If the zone has a CO2 sensor, VAreMin_flow.
  4. Otherwise, VZonDesMin_flow.
• VZonDesMin_flow is equal to the following:
  1. Zero if the zone has a window switch and the window is open.
  2. Twenty-five percent of the VAreMin_flow if the zone has an occupancy sensor and is unpopulated.
  3. The larger of VAreMin_flow and VOccMin_flow otherwise.

Occupied minimum airflow

The occupied minimum airflow shall be equal to VMin_flow except as noted in below section.

Occupied minimum airflow modification

1. If the zone has an occupancy sensor, the minimum airflow VOccZonMin_flow shall be equal to 25% of the VAreMin_flow when the room is unpopulated.
2. If the zone has a window switch, the minimum airflow VOccZonMin_flow shall be zero when the window is open.
3. If the zone has a CO2 sensor:
   1. Specify CO2 setpoint ppmCO2Set according to Section 3.1.1.3 of Guideline 36.
   2. During occupied mode, a P-only loop shall maintain CO2 concentration at setpoint; reset from 0% at set point minus 200 PPM and to 100% at setpoint.
   3. Loop is disabled and output set to zero when the zone is not in occupied mode.
   4. For cooling-only VAV terminal units, reheat VAV terminal units, constant-volume series fan-powered terminal units, dual-duct VAV terminal units with mixing control and inlet airflow sensors, dual-duct VAV terminal units with mixing control and a discharge airflow sensor, or dual-duct VAV terminal units with cold-duct minimum control:
      • The CO2 control loop output shall reset the occupied minimum airflow setpoint VOccZonMin_flow from the zone minimum airflow setpoint VMin_flow at 0% loop output up to maximum cooling airflow setpoint VCooMax_flow at 50% loop output. The loop output from 50% to 100% will be used at the system level to reset outdoor air minimum.

      

   5. For parallel fan-powered terminal units:
      • Determin the maximum flow rate for control CO2 level: when the zone state is cooling, the maximum flow rate is equal to the maximum cooling airflow setpoint VCooMax_flow; when the zone state is heating or deadband, the maximum flow rate is equal to VCooMax_flow minus the parallel fan airflow VParFan_flow.
      • The CO2 control loop ouput shall reset the occupied minimum airflow setpoint from the zone minimum airflow setpoint VMin_flow at 0% loop output up to maximum cooling airflow setpoint for CO2 control at 50% loop output. The loop output from 50% to 100% will be used at the system level to reset outdoor air minimum.

      

Minimum outdoor airflow for single zone VAV air handler unit

The minimum outdoor air setpoint shall be reset based on the zone CO2 control-llop signal from VZonAbsMin_flow at 0% signal to VZonDesMin_flow at 100% signal.

If there is no CO2 sensor, the minimum outdoor air setpoint should be VZonDesMin_flow.

Revisions

• March 22, 2022, by Jianjun Hu:
  First implementation.