Computational fluid dynamics (CFD) has immense effect in building system performance. There have been few studies on the potential use of CFD in building concerning the evaluation of external and internal flow. In this study, CFD module of DesignBuilder (state of the building energy simulation software) has been designed to predict airflow and temperature distribution in buildings. Conventional CFD packages (FLUENT,PHOENICS, CLIMA 3D etc.) for building airflow analysis is time consuming and intensive task to setting up the correct geometry and boundary conditions. In this study, a simplified simulation method has been followed by providing the geometry and boundary conditions in the CFD module of Design Builder. Calculated temperatures, heat flows and flow rates of air from the building system have been provided boundary conditions simply by specifying the time/date of the CFD analysis. The study employs SIMPLER algorithm, which belongs to one of the most widely used families of CFD solution methods. Turbulence is modelled using standard k-e model. A wide range of boundary conditions such as supply diffusers, extracts, temperature patches,etc. have been assigned to room surfaces. Component library hasbeen used to include building systems (radiators, fan-coil units) and occupants within the analysis. Using time dependent CFD simulation, it is possible to control time dependent thermalcomfort by changing the boundary condition. In this study, CFD simulation integrated with whole building energy model will be used to obtain more accurate prediction of the building performance.