If you were to use the MAPDL solver directly (rather than via Mechanical) you would be permitted to solve with this one convection BC. In a steady state thermal analysis, you would find, as Dave noted, that the calculated temperature of the entire structure would be the bulk temperature of the convection BC.
Is this facade attached to the exterior surface of a wall? Maybe you can include a section of the wall in your model, with the facade attached to it in whatever way it is phyically attached. On the interior surface of the wall you could specify another convection boundary with the bulk temperature being whatever the air temperature of the room is (22 C?). You would then see a resulting temperature ditribution between the interior surface of the wall and the exterior surface of the facade.
Cheers,
--Bill
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