I am a new user and new to the HVAC discipline.  When is it most appropriate to create a new shell?  It seems to me that you should create a new shell when you have changes in roof/ceiling height (elevations), different construction materials, or a different type of HVAC system servicing an area. 

I am working on a practice/learning model of a school that is predominantly at one level, but has a mezzanine and a gymnasium.  Currently i have the exterior rooms modeled in one shell, the mezzanine and the rooms beneath it as a secord shell, and the gymnasium as a third.  As I am reading more about the software and the requirements of a building model, it seems that I may have made a poor choice because the choices I made for shell boundaries break up the HVAC zones.

Would it make more sense to assemble the model like this:  one shell for first floor classrooms with a second floor for the mezzanine and a second shell for the gymnasium. 

or

one shell for the first level rooms of one elevation, a second shell for first level rooms of a second elevation, a third shell for the mezzanine, and a fourth shell for the gymnasium.

all zones are serviced by the same type of HVAC system.

Regards,

Matt Clough