.Buildings.Obsolete.Controls.OBC.ASHRAE.G36_PR1.AHUs.MultiZone.VAV.SetPoints.OutdoorAirFlow.Zone

Information

This sequence conducts calculations of the minimum outdoor airflow rate at the zone level. It gives outputs below.

1. The population component of the breathing zone outdoor airflow, VDesPopBreZon_flow.
2. The area component of the breathing zone outdoor airflow, VDesAreBreZon_flow.
3. Design zone primary outdoor air fraction, yDesPriOutAirFra.
4. Uncorrected outdoor airflow rate, VUncOutAir_flow. This flow rate is calculated based on: occupancy (if there is occupancy sensor) or design occupancy (if there is no occupancy sensor), and air distribution effectiveness (warm or cool air).
5. Primary outdoor air fraction, yPriOutAirFra.
6. Primary airflow rate, VPriAir_flow.

The calculation is done using the steps below.

1. Compute the required breathing zone outdoor airflow using the following components.

   • The area component of the breathing zone outdoor airflow, VDesAreBreZon_flow.
   • The population component of the breathing zone outdoor airflow, VDesPopBreZon_flow.

   The number of occupant in the zone can be retrieved directly from occupancy sensor (nOcc) if the sensor exists, or using the default occupant density (occDen) and multiplying it with zone area (AFlo). The occupant density can be found from Table 6.2.2.1 in ASHRAE Standard 62.1-2013. For design purpose, use the design zone population (desZonPop) to determine the minimum requirement at the ventilation-design condition.

2. Compute the zone air-distribution effectiveness. Table 6.2.2.2 in ASHRAE 62.1-2013 lists some typical values for setting the effectiveness. Depending on difference between zone space temperature TZon and discharge air temperature (after the reheat coil) TDis, Warm-air effectiveness zonDisEffHea or Cool-air effectiveness zonDisEffCoo should be applied.

3. Compute the required zone outdoor airflow zonOutAirRate. If the zone is in any mode other than occupied mode (uReqOutAir=false) or if the zone has window switches and the window is open (uWin=true), set zonOutAirRate = 0. Otherwise, the required zone outdoor airflow zonOutAirRate shall be calculated as follows:

   • If the zone is populated (nOcc > 0), or if there is no occupancy sensor (have_occSen = false):
     • If the discharge air temperature at the terminal unit is less than or equal to the zone space temperature, set zonOutAirRate = (breZonAre+breZonPop)/disEffCoo.
     • If the discharge air temperature at the terminal unit is greater than zone space temperature, set zonOutAirRate = (breZonAre+breZonPop)/disEffHea.
   • If the zone has an occupancy sensor and is unpopulated (nOcc=0):
     • If the discharge air temperature at the terminal unit is less than or equal to the zone space temperature, set zonOutAirRate = breZonAre/disEffCoo.
     • If the discharge air temperature at the terminal unit is greater than zone space temperature, set zonOutAirRate = breZonAre/disEffHea.

4. Compute the outdoor air fraction for the zone yPriOutAirFra as follows. Set the zone outdoor air fraction to

       yPriOutAirFra = zonOutAirRate/VPriAir_flow
   

   where, VPriAir_flow is the maximum between the measured discharge air flow rate from the zone VAV box VDis_flow and 0.1% of AHU level design uncorrected minimum outdoor airflow rate VUncOut_flow_nominal. For design purpose, the design zone outdoor air fraction yDesPriOutAirFra is

       yDesPriOutAirFra = desZonOutAirRate/minZonPriFlo
   

   where minZonPriFlo is the minimum expected zone primary flow rate and desZonOutAirRate is the required design zone outdoor airflow rate.

References

ANSI/ASHRAE Standard 62.1-2013, Ventilation for Acceptable Indoor Air Quality.

Stanke, D., 2010. Dynamic Reset for Multiple-Zone Systems. ASHRAE Journal, March 2010.

Revisions

• December 23, 2020, by Michael Wetter:
  Changed the min attribute of desZonPop to zero, and used the expression as a default value. This was done because the expression involves the zone floor area, which is, if obtained from Spawn, a non-literal value.
• March 13, 2020, by Jianjun Hu:
  Separated from original sequence of finding the system minimum outdoor air setpoint.
  This is for #1829.
• February 27, 2020, by Jianjun Hu:
  Applied hysteresis for checking ventilation efficiency.
  This is for #1787.
• January 30, 2020, by Michael Wetter:
  Removed the use of fill when assigning the unit attribute.
  This is for #1728.
• January 12, 2019, by Michael Wetter:
  Added missing each.
• October 28, 2017, by Michael Wetter:
  Corrected bug in guarding against division by zero.
• September 27, 2017, by Michael Wetter:
  Changed handling of guard against zero division, as the flow rate can be zero at the instant when the fan switches on.
• July 6, 2017, by Jianjun Hu:
  Replaced cooCtrlSig input with TZon and TDis inputs to check if cool or warm air distribution effectiveness should be applied. Applied hysteresis to avoid rapid change.
• July 5, 2017, by Michael Wetter:
  Revised implementation.
• May 12, 2017, by Jianjun Hu:
  First implementation.