How to define the infiltration rate in a central zone which is composed by internal walls?

edit

How you define infiltration is your choice. You might want to do it only for perimeter (=exposed to the outdoor) zones, or to put a constant air change in the entire building for example. There are good reasons to choose one or another.

There are several choices in ZoneInfiltration:DesignFlowrate's Calculation Method for example (I'm linking to this object since it's the most commonly used, but there are a few other infiltration objects).

If you use Flow/ExteriorAreaor Flow/ExteriorWallArea, any core/central zones that don't have exterior surfaces will get a resulting infiltration of zero. If you use any of the other methods (Flow/Zone, Flow/Area, AirChanges/Hour) it won't care whether it's a core or a perimeter zone.

EDIT:

Here's an example to set the Flow/Zone to 500 CFM / 0.246 m3/s, with a constant value (not depedent on temperature or wind), where Core Zones is a ZoneList listing all the zones to apply it to, but you could also just use the name of one Zone.

ZoneInfiltration:DesignFlowRate,
    Core Infiltration,   !- Name
    Core Zones, !- Zone or ZoneList Name
    Always On Sch, !- Schedule Name
    Flow/Zone,      !- Design Flow Rate Calculation Method
    0.246,       !- Design Flow Rate {m3/s}
    ,               !- Flow per Zone Floor Area {m3/s/m2}
    ,               !- Flow per Exterior Surface Area {m3/s/m2}
    ,               !- Air Changes per Hour
    1,              !- Constant Term Coefficient
    0,              !- Temperature Term Coefficient
    0,              !- Velocity Term Coefficient
    0;              !- Velocity Squared Term Coefficient