A complex heating and cogeneration plant including two 3.9 MWth ammonia heat pumps and two 3 MWel gas turbines with heat recovery boilers and three storage tanks of 66 GJ has been in operation since 1986. The heating surface is actually about 280’000 m2 and is still increasing. It is divided into two parts, a medium and low-temperature district heating network. To optimise on the best possible performance from the heat pumps, the network water is delivered directly to the radiators of the building without intermediate heat exchangers. This plant is still a pioneer installation for many of its characteristics. This report is a result of an experimental evaluation during the eleventh season of heating. Energy balances of the whole plant throughout the season are discussed as well as the performances of one fully instrumented heat pump. Results confirm the interest of this type of technology combination even if the performances of the heat pump exhibit a decrease of COP from an everage of 5 to about 4.4. This drop is mainly attributed to a drop in compressor performance, fouling in the evaporator, and the presence of inert gases.