Equal OA for ASHRAE 90.1 baseline and proposed in OpenStudio?

asked 2024-11-22 16:30:18 -0500

mattkoch
1201 ●11 ●10

updated 2024-11-24 17:29:02 -0500

14460 ●91 ●25 http://bigladdersoftware.com/

It sure would be nice to know how to configure in OpenStudioApp 1.7.0/OpenStudio 3.7.0 the ASHRAE 90.1-2010 Appendix G (Performance Rating Method) requirement for equal OA flows in the baseline and proposed building for a building with multiple variable speed AHU serving multiple simple reheat VAV terminal units. Aside from giving spooky results (like 100% OA) applying VRP to baseline and proposed would likely not give the same OA.

So for now, I am going by what the engineer tells me the OA/ft2 and OA/person requirements are (after their own VRP calculations), implement them as a DesignSpecificationOutdoorAir object for each space (can't apply them to a thermal zone), use ZoneSum everywhere and set OA flow rates in ControllerOutdoorAir and AirLoopHVAC to "autosize". This is me hoping that somehow the OA/ft2 and OA/person numbers in combination with identical occupancy schedules for baseline and proposed will yield said equal OA flows.

I'd welcome any thoughts advice. Thank you.

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answered 2025-05-21 08:12:55 -0500

mattkoch
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In OpenStudio, one can only define/assign DSOA at the space level, but all (?) OA calculations are made at the zone level. As far as I have learned, when a zone has several spaces, and there are different DSOA for each of these spaces, then EnergyPlus randomly picks one of these DSOA to proceed with. The pick can and does differ from run to run, so, that is why I got random results from run to run. I was able to address this by calculating the average DSOA for all spaces of a zone, then assign this average DSOA to each of these spaces. That way, whatever EnergyPlus's pick-of-the-run may be, it'll always be the same DSOA, and at the zone level, the average DSOA should be all that matters anyway. Then I use the same DSOA for proposed and baseline, and my sizing and 8,760 h OA numbers indeed match.

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EP3 is another EnergyPlus user-interface that exposes the EnergyPlus functionality directly. EP3 allows you to set the DSOA for zone sizing at the Sizing:Zone level, and another (optional) object at the AirTerminal level for demand control ventilation.

kwalkerman ( 2025-05-21 09:37:14 -0500 )edit

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answered 2024-11-24 03:30:51 -0500

Twose10
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Hi mattkoch,

I tested what you're trying to accomplish. Unfortunately, I couldn't find a simple way to automatically model the outdoor air in the AHUs based on DesignSpecificationOutdoorAir. The automatic approach requires using the E+ editor. If you're familiar with that tool, I can guide you through the process.

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Hello Twose10, and my apologies for the late reply. I am in the middle of a LEED project and just discovered last night that multiple runs of the same identical model give multiple results in OpenStudio. Not exactly encouraging. As far as I can tell, it has to do with which space's DesignSpecificationOudtoorAir (DSOA) is selected to represent a thermal zone. If not all spaces of the same thermal zone have the same DSOA then trouble arises The selection appears to be random.

mattkoch ( 2024-11-26 06:58:12 -0500 )edit

Be that as it may, yes, I would be very interested in learning more about your approach and have you guide me through the E+ Editor process. Perhaps it will also give me ideas for an OpenStudio script. How would you like to go about this? Is this something you have written up and can share, or do we need to touch base on the phone? Thanks.

mattkoch ( 2024-11-26 07:01:30 -0500 )edit

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Equal OA for ASHRAE 90.1 baseline and proposed in OpenStudio?

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Equal OA for ASHRAE 90.1 baseline and proposed in OpenStudio?