Best way to model return air plenum heated with radiators

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Hi Internet.

I am trying to model a building with a slightly odd configuration in the top floor plenum.

The building has return air-path through the plenum but the plenum is also heated with fin tube radiators to account for losses through the roof (so much simpler than insulation).

Currently thinking of stacking two "sub plenums" on top of each other, separated by an air wall.

Looking for any thoughts on thisapproach and if it is as simple as modeling the heated plenum as a normal zone but un-ticking the box that says "part of total floor area".

thanks.