The paper presents a system identification method that can determine individual room air change rates in commercial building HVAC systems. This is achieved by utilizing a combined low order model based on the room's thermal dynamics and experimental approach. The low order model is shown to capture the dominant dynamics of the room air temperature response to a step change in supply air flow rate in such a way that the actual flow rates can be realized. This is advantageous since it could be used to determine flow rates in each room of large buildings without having to utilize labor intensive flow rate measurements. Experiments were performed on a selection of rooms to confirm that the lumped control volume model accurately represents the thermal response of the room and that the system identification method consistently determines measured airflow rates. The potential of the method presented is to help enable significant building operating cost, energy use and greenhouse gas emissions by reassessing the air flow rates needed for each room in a building.