HVAC Control Logic

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I think what you want is a combo of a SetpointManager (tells the equipment what the setpoint should be while it is running) and an Operation Scheme (defines when particular equipment runs).

1. On the chilled water loop supply outlet node, put a SetpointManager:Scheduled with a 13C temp schedule
2. Add a `PlantEquipmentOperationOutdoorDryBulb' to the chilled water loop. This object is available in OS 1.8.5 and onward. Because it is not yet available through the GUI, you'll need to add it via a measure.

Here is some code that shows how to do it:

chiller_op_scheme = OpenStudio::Model::PlantEquipmentOperationOutdoorDryBulb.new(model)
chiller_op_scheme.setName('Operate Chiller Above 13C')
chiller_op_scheme.addEquipment(lead_chiller)
chiller_op_scheme.addLoadRange(13.0,[]) # No equipment operates from -50C to 13C
chiller_op_scheme.addLoadRange(70.0,[lead_chiller]) # Chiller operates from 13C to 70
chilled_plant.setPrimaryPlantEquipmentOperationScheme(chiller_op_scheme)

I would take a look at the following post, which provides some general guidance for hydronic loop operation.

Control Strategies for Plant Loops

To accomplish the situation you have described above (in E+), one would use 'PlantEquipmentOperationSchemes' to enable/disable equipment under certain conditions (lockout chiller operation for OA temperatures below 10C in your scenario). I can't speak to DesignBuilder, but OpenStudio 1.8.0 does not expose the E+ operation Scheme 'PlantEquipmentOperation:OutdoorDryBulb' needed to lockout chiller operation based on OA dry bulb.

Once the equipment is enabled, a typical scheduled SetpointManager will establish the temperature setpoints of the active loop (13C in your scenario).