<p>Modern split air conditioning (SAC) units offer a fan-only mode that operates without activating the energy-intensive compressor, making them a promising component in mixing ventilation systems. However, there is currently little research on the mechanisms and effectiveness of aerosol mixing when SAC units are used indoors. This article presents high-fidelity computational fluid dynamics (CFD) simulations to investigate airflow patterns and aerosol particle behaviour in a classroom equipped with three indoor SAC units operating in fan-only mode, alongside two exhaust fans and an open door. Lagrangian particle tracking is employed to analyse the spatio-temporal distribution of aerosols and quantify the mixing effectiveness of the SACs. The simulations reveal that, without active mixing ventilation, large recirculation regions form, including a stagnation region that spans the height of the room. Under isothermal conditions, the flow is primarily driven by the entrainment of fresh air by the isothermal AC jets; SAC units that fail to entrain fresh air lead to the formation of stagnant ‘dead zones’. Under non-isothermal conditions, buoyancy-driven rise of warmer outdoor air entering through the door enhances fresh air intake by the SACs, allowing their jets to distribute fresh air more effectively across the room. Additionally, analysis of airflow near the door shows an outward flow that promotes a higher rate of fresh air inflow into the classroom. Lastly, we quantify the improvement in ventilation in different cases and assess the likelihood of spread of infection within various regions of the room, considering a single localized source of infection.</p>