While heat pump (HP) systems could be a main solution to achieve the energy transition, their operation in actual condition of use still face specific issues, in particular when integrated in multi-energy systems with fossil complement. Based on detailed monitoring (hourly data, entire year), this paper concerns the monitoring and simulation of the heat production system of a new low energy multifamily buildings complex in Geneva - Switzerland, comprising a centralized HP on waste water (200 kWth) and complementary gas boiler (600 kWth). For SPF optimization reasons and in order to take advantage of the lower space heating temperatures, a seasonally differentiated operation mode was planned for the HP, with a lower temperature production setpoint in winter than in summer (when only domestic hot water is needed). However, this does in fact limit the operation of the HP to short periods, and leads to a lower HP share (45%) than expected. By way of numerical simulation, we show that alternative operation modes could induce a higher share of HP production, up to 70%. Despite a possible drop in SPF (from 3.2 in the current situation to 2.7 when maximizing the heat pump production), we highlight that this could actually lead to a better system performance in terms of CO2 emissions, when taking into account the related reduction in gas consumption of the complementary gas boiler, as well as the actual hourly electricity mix for the HP.