By 2035, Geneva aims to cover up to 30% of its heating needs with district heating networks (DH), 80% of which will be supplied by renewable or recovered energies. To integrate these energies effectively, DH temperatures need to be reduced, while respecting the individual requirements of substations (SSTs). We investigate the issues and potentials of temperature reduction on Geneva’s main DH, which supplies 359 GWh/year at supply/return temperatures of 110°C/70°C. Using the excess flow method and a measurement campaign targeting a selection of SSTs, coupled with analysis of the number of transfer units (NTUs), the study identified priority substations for optimization and the source of high return temperatures. Contrary to expectations, building distribution systems show relatively low return temperatures between 40°C and 50°C, while temperatures on the primary side of their heat exchangers vary between 60°C and 90°C. This indicates that the heat exchangers are undersized, with NTU values between 0.5 and 1.5, well below the Scandinavian reference design value of 4. Replacing or adding a second heat exchanger would, in most cases, make it possible to achieve the desired temperature reduction without modifying the buildings’ secondary distribution systems, which are generally not under the responsibility of the DH operator.