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 strage tanks has been in operation since 1986. This paper is a result of an expreimental evaluation during the eleventh season of heating. Energy balances throughout the season are discussed as well as the performances of one fully equipped heat pump. Results confirm the interest of this type of technology combination even if the performance of the heat pump exhibit a decrease of COP from an average of 5 to about 4.4. This drop is mainly attributed to a drop in compressor performance, fouling in the evaporator, and presence of inert gases.