Air-balancing is a key technique in HVACs to achieve accurate air supply for better IAQ and energy-saving performance. The existing air-balancing methods are usually based on a hybrid duct system model, including a data-driven duct system model between terminal flow and static pressure, and an ASHRAE damper model between static pressure and terminal damper angle. Most of these methods only focus on how to improve the accuracy of the former part whereas neglecting the latter. However, the ASHRAE damper model cannot exactly reflect the actual dampers characteristics in practice and may lead to unavoidable modelling error. To fix this issue, this paper proposes a full data-driven duct system (FD3S) model to directly reveal the relationship between the terminal flow and terminal damper angle in the whole duct system. Therefore, the desired terminal damper angle under the design flow conditions can be more accurately predicted. Besides, since the proposed method directly models the relationship between terminal flow and terminal damper angle, it avoids installing extra pressure sensors and reduces the operation cost. Moreover, the complexity of the proposed FD3S model is greatly reduced compared with the previous hybrid model. Finally, the experimental results demonstrate the effectiveness of the proposed method.