The paper describes two 3.9 MWth ammonia heat pumps of a District Heating plant which also includes two gas turbine cogeneration units. After nine years of reliable operation the heat pumps still show excellent coefficients of performance of above 4.1. This plant confirms the suitability of ammonia as a working fluid as well as the interest of the integration of cogeneration units and heat pumps interms of operational flexibility and rational use of fossil fuel with significant deductions of CO2 emissions.