EnergyPlus: How to Override RTU Intake Temperature Without Modifying EPW?

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What I am trying to do is :

I want the RTU / VAV system to see a different outdoor air temperature at the OA inlet, such that: Tinlet = OA + deltaT

The idea is to modify the actual outdoor air temperature entering the RTU / OA mixer (for example, to represent a rooftop microclimate effect), without changing the EPW weather file, because I only want the OA stream going into the air loop to be affected. Other calculations like the building envelope or infiltration should remain unchanged.

I tried using EMS,

EnergyManagementSystem:Actuator,
  VAV1_OAInlet_T_Act,      !- Name
  VAV_1_OAInlet Node,      !- Actuated Component Unique Name
  Outdoor Air System Node,    !- Actuated Component Type
  Drybulb Temperature;    !- Actuated Component Control Type

But when I look at the output:variable 'System Node Temperature', I’m seeing that the temperature difference between the OA node and the VAV_5_OAInlet Node is only around 3°C. it fluctuates by about ±3°C instead of staying exactly at +3°C. Why is that happening, and how can I make it apply an exact +3°C offset?

I’ve shared the download link to my IDF file here.

https://drive.google.com/drive/folder...

Thank you!