Simulation of air infiltration and ventilation, and heat transfer processes of buildings are combined in a building simulation program called BUS (version 3.2). The program applies and includes an effective sparse matrix solution method for a linear set of equations. A network assumption is adopted for both air flow and thermal simulation. An iterative method, based on the SIMPLE algorithm, is used for airflow simulation, and the lumped capacitance method is selected for thermal simulation. This building simulation program BUS includes mass balance, momentum, and heat balance equations. This makes it possible to lake into consideration the interaction between ventilation and heat transfer processes, obtain more reliable simulation results than previously, and simulate time-dependent processes.