The commercial general - purpose Computational Fluid Dynamics (CFD) code PHOENICS is used to study the indoor environmental conditions of a large, mechanically ventilated, athletic hall. The indoor space of the building was simulated in the PHOENICS environment and computations were validated against experimental data obtained during a ten-day campaign in the hall. Data included measurements of airflow characteristics at different indoor locations under different ventilation conditions. Having obtained good agreement from comparing computed and experimental results, different ventilation scenarios of air-conditioning, heating and cooling, were applied in the model to investigate the air velocity and temperature patterns prevailing in the hall in each case. Computed results showed dynamic airflow and temperature patterns significantly altering with the different considered scenarios. Temperature stratification was evident, while distinct recirculating vortices characterized the airflow fields originating from the ceiling air inlet fans of the ventilation system.