In this study, two-phase flow simulations have been performed for the waterbox of an Mercedes-Benz HVAC- system by using the commercial CFD program STAR-CD.The waterbox, which is the first component in Mercedes-Benz HVAC-systems, is located underneath the engine hood just in front of the HVAC fan. The flow in the waterbox is a typical two-phase flow with the air as the continuous phase and the water as the dispersed phase. The numerical simulation is performed by using the Lagrangian model as implemented in STAR-CD. The influence of the water droplets on the airflow as well as droplet break-up and different interactions of the droplets with the walls can be taken into account.Several geometrical modifications of the waterbox have been investigated by numerical simulation. The computational results can be used to get a better understanding of the physics of the flow in the waterbox and to optimize the pressure drop and the water separation. The comparison of the computational results with measurements showed a good agreement for the pressure loss and corresponding trends for the effect of the water separation.