Hydraulic balancing is key to ensure proper operation of the heating system. When the heating system is not hydraulically balanced, heat is unevenly distributed across dwellings resulting in a wide temperature spread, overheating, and consequently, wastage of energy. In this paper, we study to which extent hydraulic imbalance affects the thermal energy demand of buildings. Furthermore, key variables and interactions that influence the thermal performance of buildings are identified. In the first part, a dataset, including building features, boiler capacity and energy consumption of 49 multifamily buildings in Geneva (Switzerland), is analysed. Applying regularization regression, we find that higher variation in indoor temperature leads to larger energy consumption. Buildings constructed before 1980, having large boiler capacity and large heated floor area, are more likely to be hydraulically imbalanced and to consume more energy, indicating higher energy saving potential from hydraulic balancing. The second part consists of a case study of four multifamily buildings in Geneva where hydraulic balancing was implemented. We monitor data associated with temperature levels and energy consumption and find that hydraulic balancing significantly reduces the temperature spread avoiding overheating. The yearly energy savings by hydraulic balancing at an outdoor temperature of 0°C are estimated at 9% in Geneva.