Although HVAC (Heating Ventilation, and Air Conditioning) layout design has transitioned from 2D drawings to 3D BIM models, the traditional manual process of sizing and placing terminal units is still a time-consuming task, which can lead to inappropriate airflow and wastage of resources due to human error. The purpose of this paper is to develop a highly robust method for sizing and placing HVAC terminal units in rooms, while also systematically avoiding obstacles based on the locations of air terminal units and FCUs (Fan Coil Units) to which they belong. The method employs a “scan” approach to solve the problem of insufficient robustness caused by the traditional grid division method. Rule-based iterative algorithms are utilized for the sizing of terminals, airflow verification, and obstacle avoidance to generate a complete set of FCU and CAV (Constant Air Volume) system terminal layouts within a building. The method was tested for the automated design of HVAC terminal layouts in six different buildings and successfully completed the task within seconds, demonstrating the method’s immediacy and robustness. Moreover, airflow organization tests showed that the terminal layouts generated by the algorithm had a 95% pass rate, indicating the effectiveness of the method.