DesignSpecification:OutdoorAir and ZoneVentilation:DesignFlowRate

I was importing the DOE reference building EnergyPlus model for medium office into OpenStudio and turned on the Ideal Air Loads in OpenStudio to see if the sizing result would be different between OpenStudio run and EnergyPlus run. From the results, I did see a significant difference (max difference ~2300W) in the "Calculated Design Load" for "Zone Cooling" from the html output file.

Then I checked the IDF file generated by OpenStudio and compared with the original Reference building model. I can see that OpenStudio exported IDF has defined "ZoneVentilation:DesignFlowRate" objects for each zone.

image description

The original EnergyPlus model has only the "DesignSpecification:OutdoorAir" objects for each zone. I tried to copy and paste these "ZoneVentilation:DesignFlowRate" objects from the OpenStudio exported IDF file to the original IDF file and ran the simulation again, it did eliminate most of the difference between the zone sizing load.

My questions are:

Is there a way to set OpenStudio model to not generate the "ZoneVentilation:DesignFlowRate" objects since they are causing the differences?
Even when I matched the input files by adding the "ZoneVentilation:DesignFlowRate" objects to the original EnergyPlus model, there is still small difference in the zone load(~50 Watts). Since they were results from the same version of EnergyPlus engine, What could be causing the differences?

C:\fakepath\OriginalEnergyPlusOutput.PNG

C:\fakepath\OpenStudioOutput.PNG

C:\fakepath\ModifiedEnergyPlusOutput(Added ZoneVentilationDesignFlowRate Objects).PNG

563

TiejunWu

asked 2015-08-18 11:08:15 -0500, updated 2015-08-18 11:51:59 -0500

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One must be careful not to double-count ventilation loads. ZoneVentilation:DesignFlowRate was originally intended to model non-HVAC system ventilation - such as exhaust fans in a gymnasium or open windows. For ventilation provided by the HVAC system, DesignSpecification:OutdoorAir is used to autosize the OA flow rates if the HVAC system has an outdoor air mixer.

DesignSpecification:OutdoorAir can also be used in ZoneHVAC:IdealLoadsAirSystem to include ventilation requirements. This was not an option with the original ideal load system, and so (if I remember correctly) OpenStudio adds a ZoneVentilation:DesignFlowRate object to account for ventilation loads when running with Ideal Air Loads. I believe OpenStudio still works this way.

So, adding ZoneVentilation:DesignFlowRate objects back into the reference building idf is double-counting the ventilation loads. It's fine for comparing the design zone loads back to the OpenStudio Ideal Loads simulation, but it's not what you want when using a full HVAC system.

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MJWitte

answered 2015-08-19 13:49:06 -0500, updated 2015-08-19 13:49:43 -0500

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Mike, Thanks for the clarification. My understanding is that if a zone does not have non-HVAC system ventilation, with Ideal air system, if someone is going to use the calculated design load to size a cooling coil, the result with "ZoneVentilation:DesignFlowRate" is appropriate for use. However, if the calculated design load is used to size the zone supply air flow rate, the calculated design load without the "ZoneVentilation:DesignFlowRate" should be used.

TiejunWu (2015-08-20 13:34:23 -0500) edit

But even with the ZoneVentilation:DesignFlowRate added, the reported zone design cooling load is sensible only, so you'll need to account for the latent loads in the cooling coil sizing.