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I am modelling a run-of-the-mill VAV system with outdoor air. The OS:AirLoopHVAC has an OS:AirLoopHVAC:OutdoorAirSystem with an OS:Controller:OutdoorAir. The OS:Sizing:System of the OS:AirLoopHVAC has "100% Outdoor Air in Cooling" and "100% Outdoor Air in Heating" set to "No", but most other settings are defaulted.

I observe curious results when I modify "Minimum Outdoor Air Flow Rate" of the OS:Controller:OutdoorAir. When it is set to "autosize", I get very high energy usage, and when I set it to "0", I get more reasonbale energy usage. Now, when I enable DCV, or when I enable an economizer to "FixedDryBulb" between 50 F and 60 F, then there is no difference between "autosize" and "0".

So, I am wondering what "Minimum Outdoor Air Flow Rate" really does, and what it is autosized to (as opposed to set to "0"), and also why it only has an impact when there is no DCV or no Economizer, or put another way why there seems to be no difference between "autosize" and "0" when there is DCV or an Economizer?

By the way, I leave any other OA fields in the various air loop objects on "autosize" and drive OA by setting "Outdoor Air Flow per Floor Area" and "Outdoor Air Flow per Person" in a OS:DesignSpecification:OutdoorAir for each OS:Space.

I am modelling a run-of-the-mill VAV system with outdoor air. The OS:AirLoopHVAC has an OS:AirLoopHVAC:OutdoorAirSystem with an OS:Controller:OutdoorAir. The OS:Sizing:System of the OS:AirLoopHVAC has "100% Outdoor Air in Cooling" and "100% Outdoor Air in Heating" set to "No", but most other settings are defaulted.

I observe curious results when I modify "Minimum Outdoor Air Flow Rate" of the OS:Controller:OutdoorAir. When it is set to "autosize", I get very high energy usage, and when I set it to "0", I get more reasonbale energy usage. Now, when I enable DCV, or when I enable an economizer to "FixedDryBulb" between 50 F and 60 F, then there is no difference between "autosize" and "0".

So, I am wondering what "Minimum Outdoor Air Flow Rate" really does, and what it is autosized to (as opposed to set to "0"), and also why it only has an impact when there is no DCV or no Economizer, or put another way why there seems to be no difference between "autosize" and "0" when there is DCV or an Economizer?

By the way, I leave any other OA fields in the various air loop objects on "autosize" and drive OA by setting "Outdoor Air Flow per Floor Area" and "Outdoor Air Flow per Person" in a OS:DesignSpecification:OutdoorAir for each OS:Space.

I am modelling a run-of-the-mill VAV system with outdoor air. The OS:AirLoopHVAC has an OS:AirLoopHVAC:OutdoorAirSystem with an OS:Controller:OutdoorAir. The OS:Sizing:System of the OS:AirLoopHVAC has "100% Outdoor Air in Cooling" and "100% Outdoor Air in Heating" set to "No", but most other settings are defaulted.

I observe curious results when I modify "Minimum Outdoor Air Flow Rate" of the OS:Controller:OutdoorAir. When it is set to "autosize", I get very high energy usage, and when I set it to "0", I get more reasonbale energy usage. Now, when I enable DCV, or when I enable an economizer to "FixedDryBulb" between 50 F and 60 F, then there is no difference between "autosize" and "0".

So, I am wondering what "Minimum Outdoor Air Flow Rate" really does, and what it is autosized to (as opposed to set to "0"), and also why it only has an impact when there is no DCV or no Economizer, or put another way why there seems to be no difference between "autosize" and "0" when there is DCV or an Economizer?

By the way, I leave any other OA fields in the various air loop objects on "autosize" and drive OA by setting "Outdoor Air Flow per Floor Area" and "Outdoor Air Flow per Person" in a OS:DesignSpecification:OutdoorAir for each OS:Space.

I am modelling a run-of-the-mill VAV system with outdoor air. The OS:AirLoopHVAC has an OS:AirLoopHVAC:OutdoorAirSystem with an OS:Controller:OutdoorAir. The OS:Sizing:System of the OS:AirLoopHVAC has "100% Outdoor Air in Cooling" and "100% Outdoor Air in Heating" set to "No", but most other settings are defaulted.

I observe curious results when I modify "Minimum Outdoor Air Flow Rate" of the OS:Controller:OutdoorAir. When it is set to "autosize", I get very high energy usage, and when I set it to "0", I get more reasonbale energy usage. Now, when I enable DCV, or when I enable an economizer to "FixedDryBulb" between 50 F and 60 F, then there is no difference between "autosize" and "0".

So, I am wondering what "Minimum Outdoor Air Flow Rate" really does, and what it is autosized to (as opposed to set to "0"), and also why it only has an impact when there is no DCV or no Economizer, or put another way why there seems to be no difference between "autosize" and "0" when there is DCV or an Economizer?

By the way, I leave any other OA fields in the various air loop objects on "autosize" and drive OA by setting "Outdoor Air Flow per Floor Area" and "Outdoor Air Flow per Person" in a OS:DesignSpecification:OutdoorAir for each OS:Space.

EDIT: <<<

I was wrong about which version has higher energy usage. Here is a summary table, also including other cases:

.

I would have expected the (Proposed) "Model" and "Model_Test" cases to have the same MJ/m2.

Also, for the "Model" case, which is the primary suspect, here are OA and MA flows for the two AHU when Min OA is "autosize":

And here are those when Min OA is 0.0, i.e. the "Model_Test" case:

I thought it was very curious that the MA flows should be so very different, and that the OA flows for the "0.0" case are pretty much as expected from the OA/Area and OA/People settings, while for the "Autosize" case, they are quite a bit higher.