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

I have two models implementing two different air side loop as below (plant loop and internal loads are same):
1.VAV system
2.PAU+FCU system

Looking at result, system 2 consume 20% more for cooling which is caused by air side cooling load removal difference as below (26%difference).
1.13,341MWh
2.18,067MWh

When I look at heat gain (People,light,small power,solar heat gain, conduction), I cannot find such difference. 1.12,068MWh
2.11,946MWh

Can anyone tell me what could be the reason causing this difference?

Check for differences in outdoor air quantity and control. The VAV system will bring in outdoor air while the system ooperates. The zone equipment also brings on outdoor air but does not use Controller:OutdoorAir. The outdoor air is fixed with the runtime of the zone equipment.

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rraustad

answered 2017-10-09 07:48:57 -0500

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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?

katsuya.obara (2017-10-09 21:26:24 -0500) edit

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;

rraustad (2017-10-10 07:50:57 -0500) edit

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

katsuya.obara (2017-10-29 20:50:26 -0500) edit

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