DesignBuilder CFD doesn't include the concept of infiltration as cracks cannot be graphically defined and there can be no porous airflow through CFD surfaces as there can in EnergyPlus. Because airflow through cracks in simulations using Calculated natural ventilation can be significant, importing EnergyPlus opening airflows from simulations that include crack leakage can sometimes cause flow balance problems, especially in zones with only a single opening. It is therefore usually advisable to exclude the effects of infiltration from simulations that are to be used for setting boundary conditions for CFD. A warning message to this effect is given if you attempt to import simulation data generated using Calculated natural ventilation and infiltration.
Another reason for excluding infiltration from simulations to be used for setting up CFD boundary conditions is that the flow is likely to balance much more easily without the need to add large balancing flows to one or more openings.