Is it possible to combine plenum with room below?

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Not an answer - too many characters in my initial comment.

Not an Asset Score user, so can't help you on that end - other UMH users would undoubtedly offer better advice. In a nutshell, you're downloading an Asset Score-generated .osm file, and plenums have been added - correct? Are these plenums added as AirLoopHVAC objects? If so, it becomes a more challenging operation (e.g. editing/removing nodes by hand), than if plenums were simply modelled as dead air spaces. Hard to guess without the .osm file ... can you share (Google Drive)?

There are UMH posts on merging OSM spaces (here, here), yet they involve using SketchUp.

A suggestion: Rather than seeking to merge plenum spaces with the occupied rooms they serve below, why not post-process EnergyPlus-reported outputs accordingly (e.g. script adding EnergyPlus-reported plenum air volumes to conditioned space air volumes)? Less messy, less risky. Of course, this depends on what objectives/metrics/targets you're after ...