To provide a warm place for the most vulnerable citizens during the 2022/2023 energy crisis, some municipalities have set up public halls as warming shelters. Thus, the present study analyzes the energy performance of a gymnasium in South¬eastern Europe that is repurposed to be used as public warming hall. The study conducted 15 EnergyPlus simulations, covering five states of gymnasium occupancy and three heating, ventilation and air conditioning scenarios. Two scenarios were designed to reduce the possibility of viral disease transmission, in the case public health emergency occurs. The study indicates that gymnasiums with natural ventilation consume more energy than they would with more advanced HVAC systems. This way, when occupancy increases, building energy consumption decreases (from 171-102 kWh per m2). Contrary to that, in more advanced heating, ventilation and air conditioning scenarios, energy consumption slowly increases with the increase of occupancy. Due to the utilization of heat recovery and air re-circulation systems, these scenarios require approximately 60-80% less energy compared to the base scenario. The complex simulations performed in this study provided relatively simple formulas that can be extrapolated to determine hall energy performance for any hall occupancy. These formulas can be used by non-experts and applied to similar buildings in other locations.