This paper analyses the mechanical ventilation of an auditorium in England in terms of thermal comfort and indoor\ud air quality. The auditorium has a capacity of 1200 seats and a displacement supply air system. To evaluate the thermal comfort and indoor air quality, monitoring and a questionnaire survey were carried out during a performance, also CFD simulation was employed. Based on the results, it was identified that the supply air velocity is highly variable with some places over-ventilated which could cause a cold draught around the feet of the audience. Conversely, a low supply air velocity leads to an increase in temperature. It was also identified that the temperature gradient is more than 3K from people's feet to their heads. Results from monitoring the CO2 concentration identified that there is an unintended recirculation between the main inlet and outlet. In addition, results based on CFD assessments revealed that the location of the supply causes the non-uniform air velocity. These findings are useful to improve the existing and new mechanical systems in theatres as well as reducing the energy consumption in this sector.