How outdoor air fraction is decided time by time?

asked 2018-05-29 03:55:50 -0500

2085 ●1 ●4 ●14

I am modelling VAV system.
When I check the System Ventilation Requirements for Cooling from html table result, percentage of outdoor air to supply air is not 100% (for example, VAV 1F has 43% outdoor air fraction compared to total supply air in the air system) as shown in image above. However, when I look at hourly data, Outdoor Air Fraction is 100% anytime when the air system is operating as shown in middle image. This lead to additional outdoor air intake more than required(e.g. in summer, outdoor air volume is more than 5kg/s which is much bigger than 2.15m3/s summarized in System Ventilation Requirements for Cooling as shown in bottom image)
I want to keep outdoor air intake to minimum requirement. Does anyone know how to achieve this?
image description

answered 2018-05-29 12:07:00 -0500

mleach
556 ●5

I would expect this is being caused by economizing. You could try setting Economizer Control Type to NoEconomizer in the Controller:OutdoorAir object. You could also set the Maximum Outdoor Air Flow Rate directly (also in Controller:OutdoorAir).

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@mleach I confirmed that all Controller:OutdoorAir set to NoEconomizer. I can set Maximum Outdoor Air Flow Rate. But which means that Autosize for Maximum Outdoor Air Flow Rate' inController:OutdoorAir` does not work correctly...

katsuya.obara ( 2018-05-29 19:01:20 -0500 )edit

The only other thing I can think of is exhaust. Do you have Unbalanced exhaust on this system? I believe EnergyPlus will try to increase OA flow to balance Unbalanced exhaust flow as necessary. Also, I'm not sure if it is possible to have minimimum fraction of OA schedule values greater than 1, but that could also cause an issue. For reference, there is a prioritized bullet list for how OA rate is determined in the Controller:OutdoorAir section of the E+ Input Output Reference. It is at the end of the general object description.