An adequate mass flow distribution on instrument panel air vents is fundamental for passengers thermal comfort in an automobile. Computational fluid dynamics (CFD) is a useful tool to achieve this goal, since one can identify flow patterns and details that cannot be seen during experimental tests. Despite the software and hardware developments in the last years it is still necessary and clever to make simplifications in the numerical models: the simpler and still representative the model the better it is. In this context the present work aims to study the influence of the HVAC (Heating, Ventilation and Air Conditioning) system geometry on mass flow distribution simulation. A total of 8 different domains were studied: Complete HVAC system in recirculation position, Complete HVAC system in external intake position, HVAC cut in the filter area, HVAC cut in the evaporator region and one model without HVAC system. On the first tree studies it was also verified the influence of the evaporator geometry. All the models with HVAC system presented similar results, and closer to experimental when compared with the one without the HVAC. The evaporator geometry had no significant influence in order to justify its modeling. The model with the HVAC cut in the evaporator region (without the evaporator) has demonstrated to be the best combination between accuracy and simplicity.