This paper describes the design process of a complete HVAC module using computational fluid dynamics (CFD). CFD gives a detailed insight on the flow characteristics of HVAC components even in early design stages. Due to a close coupling with CAD/CAE systems the number of prototypes, costs, and development time can be reduced. Optimised air duct designs lead to reduced pressure losses, and turbulence levels, that consequently decrease flow induced noise. Simultaneously the air distribution of the duct outlets is uniformed. Thermal analysis gives information about the heat transfer and hot/cold air mixing process inside the HVAC module.