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"KLIMA" – Smederevo, was a main participant on Eureka project E! 4117 and our commitment is to design a heat pump for district heating with the best coefficient of performance (COP) to get a higher energy saving. At the beginning our colleagues from Slovenia company “Nafta-Geoterm” and professors from the University of Maribor proposed us to use as refrigerant some types of Freon and at the end they proposed us to use isobutene like a refrigerant because isobutene in their opinion may give us the best COP (about 5). But isobutene is explosive gas and requires special design of heat pump which of course after our analysis increases the cost of heat pump up to three times and in this case there is no reasonable economic. Considering we have a lot of experience with ammonia plants we was suggested to design a heat pump with ammonia (NH3) like refrigerant and guarantee that we will get even higher COP compared to isobutene. After completion of the necessary calculations we managed to design a ammonia high stage heat pump with special design reciprocating compressor about whose design helped us colleagues from the Mycom company. By our calculations with this design of heat pump should be achieved the COP about 6. People from “Nafta-Geoterm” company and Maribor university professors have agreed with our solution and left us to complete the designing job of a heat pump. After delivery of heat pump in Slovenia town Lendava group of our engineers and service has an obligation to put the heat pump in operation and prove the required parameters and efficiency. Testing lasted for several days on which they established the required temperature of water used for heating what was the purpose of heat pump using, whereby the COP reached values of 6.4. Measured value of the COP was somewhat unexpected by our colleagues and this is what makes this construction of the heat pump unique. During design, we met with many problems, for example ammonia condensing in compressor suction line, but we have successfully resolved all the problems. This heat pump design allows us to get 6.4kW of heat with the spent only 1kW of electricity which is a big energy saving.