How to trim down air flow at night for a make up air unit in OpenStudio?

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You can limit the maximum OA fraction using the "Maximum Fraction of Outdoor Air Schedule" in the Controller:OutdoorAir object attached to the AirLoopHVAC:OutdoorAirSystem on the Airloop.

Here is what this looks like in an OpenStudio Measure:

# Find the AirLoopHVAC:OutdoorAirSystem on the Airloop
# Find the Controller:OutdoorAir object on the OA system
# Set the Maximum Fraction of Outdoor Air Schedule to one
# that has night-time maximums set to the desired fraction.
air_loop.supplyComponents.each do |supply_component|

  # Skip any supply component that isn't an AirLoopHVAC:OutdoorAirSystem
  next if supply_component.to_AirLoopHVACOutdoorAirSystem.empty?

  # Convert the supply component to a Controller:OutdoorAir
  oa_system = supply_component.to_AirLoopHVACOutdoorAirSystem.get

  # Get the OA controller that belongs to this OA system
  controller_oa = oa_system.getControllerOutdoorAir

  # Set the Maximum Fraction of Outdoor Air Schedule
  controller_oa.setMaximumFractionofOutdoorAirSchedule(max_oa_frac_sch)

  # Tell the user what happened
  runner.registerInfo("Set the max OA fraction schedule for #{air_loop.name} to #{max_oa_frac_sch.name}.")

end

The OA controller will not alter the supply flow rate. The minimum flow fraction on a VAV fan only affects the fan power input calculation. EnergyPlus fans determine their flow rate based on the request from the terminal unit(s), or the fan can also be controlled using AvailabilityManager:NightVentilation (but that is intended for night cooling, not continuous ventilation). For this application, the best approach is to change the terminal unit(s) to AirTerminal:DualDuct:VAV:OutdoorAir. This terminal unit can be used in a single-duct mode (see EnergyPlus example file DOASDualDuctSchool, terminal unit "Corner_Class_1_Pod_1 Dual Duct Box Component"). It will set the supply flow rate for the terminal unit based on the inputs in a DesignSpecification:OutdoorAir object. Use the "Outdoor Air Flow Rate Fraction Schedule Name" field to control the day/night fractions. I don't know whether these objects can be accessed directly in the current version of OpenStudio. @aparker?