With the development of district heating networks and the need for decarbonization, heat pumps in district heating are expected to play a major role in the upcoming years. To facilitate the implementation of such systems by sharing lessons learned, this study presents monitoring results of a large-scale heat pump supplying a low-temperature district heating network (LTDH) in Switzerland. Detailed monitoring over a whole year of operation reveals an annual heat production of 8 GWh, of which 85% is covered by the heat pump with an annual SPF of 3.69 (pumping and auxiliaries excluded). However, about 25% of the heat for space heating is produced with very low energy performance. Analyzing the influence of temperatures and flow rates on the heat pump energy performance brings to light potential optimizations of the system and improvements for future LTDH. In particular, the results raise crucial questions related to heat pump sizing and domestic hot water production methods with LTDH, as well as reaffirm the importance of optimizing substations return temperatures when the main heat generator is a heat pump.