Ansys Employee

Pretty much. If you create the "simple" fluid region then do the various subtraction parts you'll be sorted. If you then suppress/delete the current "wall" volume you then label the now outer faces as wall in SpaceClaim (for Fluent Meshing) or in Ansys Meshing if you take that route. 

If you search for HVAC on you'll find a few articles that show models and will give you an idea of what we'd typically include in a simulation.