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The Controller:OutdoorAir references a Controller:MechanicalVentilation object which is always on which references a DesignSpecification:OutdoorAir with OA/person (but this zone has no people) and OA/zone = 9 m3/s. (I should have noticed the Mechanical Ventilation Controller name in the screen shot above.)

In general, the outdoor air flow rate will be the greater of the Controller:Outdoorair Minimum Outdoor Airflow Rate (times Minimum Outdoor Air Schedule) OR the flow requested by Controller:MechanicalVentilation (when it's schedule is active). The schedule in DesignSpecification:OutdoorAir is also applied to any values coming from there which feed into the Controller:MechanicalVentilation. Then add in the economizer and other limits for final OA flow rate. See list of limit checks in order in the Input Output Reference in the introduction to Controller:OutdoorAir.

From this input file, the relevant inputs are:

Controller:OutdoorAir,
Outdoor Air Controller,  !- Name
<snip>
0,                       !- Minimum Outdoor Air Flow Rate {m3/s}
Autosize,                !- Maximum Outdoor Air Flow Rate {m3/s}
<snip>
Always Off Discrete,     !- Minimum Outdoor Air Schedule Name
Always Off Discrete,     !- Minimum Fraction of Outdoor Air Schedule Name
Always On Discrete,      !- Maximum Fraction of Outdoor Air Schedule Name
Controller Mechanical Ventilation 1,  !- Mechanical Ventilation Controller Name

Controller:MechanicalVentilation,
Controller Mechanical Ventilation 1,  !- Name
Always On Discrete,      !- Availability Schedule Name
No,                      !- Demand Controlled Ventilation
ZoneSum,                 !- System Outdoor Air Method
1,                       !- Zone Maximum Outdoor Air Fraction {dimensionless}
Main Zone,               !- Zone 1 Name
OA_Design_Zone,          !- Design Specification Outdoor Air Object Name 1
DataCenter Design Air Dist;  !- Design Specification Zone Air Distribution Object Name 1

DesignSpecification:OutdoorAir,
OA_Design_Zone,          !- Name
Flow/Zone,               !- Outdoor Air Method
0.00,                 !- Outdoor Air Flow per Person {m3/s-person}
,                        !- Outdoor Air Flow per Zone Floor Area {m3/s-m2}
9,                       !- Outdoor Air Flow per Zone {m3/s}
,                        !- Outdoor Air Flow Air Changes per Hour {1/hr}
Always On Discrete;      !- Outdoor Air Schedule Name