Good HVAC control schemes in buildings help reduce energy use and maintain occupant comfort. To this end, PID (proportional integral derivative) controllers are widely used in commercial buildings to keep variables, such as temperature and pressure, at predefined setpoints. These controllers, when they do not include an autotune feature, must be carefully tuned. Also, interactions occurring among the control loops must be considered during the first fine-tuning phase, so one loop response is not improved to the detriment of another. This study concentrates on PI (proportional integral) control of dual duct systems. Three tuning methods were selected and tested on a small scale dual duct system with four decentralized control loops. Each controller was tuned according to the best performing set of tuning parameters, and the system was then operated under design conditions to observe the interactions among the control loops. The effects of interactions were examined through a poorly tuned controller in each loop, one controller at a time.