This article concerns a long term in situ monitoring of an existing district heating (DH) that is supplied by gas boilers equipped with heat recovery system and a borehole heat exchanger field connected to a heat pump (HP). The original goal of this concept is to be able to stop the gas boilers during summer and use only the HP for DH heat supply. The monitored annual energy flows of the system show that gas supplies 94% of the DH energy demand, while geothermal energy supplies 1.5% and electricity 4.5%. In fact, HP heat source is mainly waste heat from the gas boiler (82%) and only 18% from geothermal energy. In reality, the gas boilers are not stopped in summer because several issues associated with heat demand (domestic hot water - DHW) were underestimated: i) DHW demand is not a base load, in fact it has three characteristic loads peak ii) Legionella bacteria management and its associated high temperatures cannot be underestimated; iii) DH substations architecture is key to achieve temperature levels in the DH that are compatible with the HP operation iv) lower flow rate in DH compared to flow rate in HP condenser leads to decrease in HP production. Finally, a numerical model allowed to analyse different scenario from the following was observed: firstly, the DH extension significantly increases gas needs, which in turn increases waste heat recovered from gas, which is then used instead of geothermal energy; secondly, resizing of the HP power enables an increase of geothermal energy of +270 MWh/yr, meaning that geothermal energy has the potential to cover 16% of the annual heat demand. This research, based on long term in situ monitoring, allows acquiring knowledge on energy innovation when applied in real field conditions: this knowledge is further developed via a feedback process between academics and field workers.