In Geneva, CO2 emissions are mainly related to the heat supply in the residential sector, in particular in multifamily buildings (MFB). In this context, air-source heat pumps (HP) could help to reduce emissions and replace fossil-based heating systems, especially in dense existing urban areas where air is the only available renewable energy source. This study concerns the analysis of an existing non-retrofitted MF building of 1972 (4´047 m2 heated area), whose original fossil heat supply was recently replaced by two industrial air-to-water HPs (2 x 140 = 280 kW). Based on a detailed monitoring campaign covering 15 months of operation, the main findings show that the HP system was able to cover the entire heat demand, except for a short breakdown. Thanks to optimizations in the system regulation and the choice to use only one HP in summer, the COPsys increased from 1.3 in the early stage, up to 3.4 during the last summer. Based on the latter performances, we estimated that the SPF could reach an annual value of 2.3 (instead of the measured value of 1.9).