Cooling consumption difference between VAV system and PAU+FCU system for air side loop

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I would like to confirm my understanding. Do you mean that
Since HVACTemplate:System VAV is variable air volume against HVACTemplate:DedicatedOutdoorAir which is constant air volume, OA introduction in VAV may be much smaller than PAU. That may lead to the result I am having?

Yes, that is a possibility. I am suggesting you investigate the amount of outdoor air introduced by each system. Plot the node standard density flow rate for each system using:

Output:Variable,OANodeName,System Node Standard Density Volume Flow Rate,timestep;

Your comment was right. VAV system was not introducing OA which leads to zero ventilation load.