Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
Exhaust air is modeled with Fan:ZoneExhaust which has an optional field for "Flow Fraction Schedule Name" to control the flow rate.
By "air change rate" I am guessing that you mean the air flow from a variable-volume (VAV) air loop. If it is providing 100% outdoor-air, then use the airterminal unit field for "Design Specification Outdoor Air Object Name" to control the minimum damper position. In the DesignSpecification:OutdoorAir object, specify the ACH and schedule. If it is not 100% outdoor air, then use the air terminal "Minimum Air Flow Fraction Schedule Name" to control the minimum flow rate.
If you want the air loop to supply all of the makeup air for the hoods, then you have two options:
1. Define both schedules, run a simulation and output Output:Variable,*,Schedule Value, timestep;. Then use a spreadsheet to compare the exhaust flow rate and minimum ACH and create a custom ACH schedule that tracks the greater of the two. Then save this schedule to a csv file and use Schedule:Fileto import it and apply it to the terminal unit
2. Use an EMS program to check both values and override the ACH schedule.
