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The global warming potential of many working fluids used nowadays for vapor compression refrigeration systems and heat pumps is very high. Many of such fluids, which are used in currently operating refrigerators and heat pumps, will have to be replaced. In order to avoid a redesign of the system, it would be very helpful if efficient and ecological alternative working fluids for a given plant could be found. With modern process simulation tools such a selection procedure seems possible. However, it remains unclear how detailed such a model of a concrete plant design has to be to obtain a reliable working fluid ranking. A vapor compression heat pump test-rig is used as an example and simulated by thermodynamic models with different levels of complexity to investigate this question. Experimental results for numerous working fluids are compared with models of different complexity. Simple cycle calculations, as often used in the literature, lead to incorrect results regarding the efficiency and are not recommended to find replacement fluids for existing plants. Adding a compressor model improves the simulations significantly and leads to reliable fluid rankings but this is not sufficient to judge the adequacy of the heat exchanger sizes and whether a given cooling or heating task can be fulfilled with a certain fluid. With a model of highest complexity, including an extensive model for the heat exchangers, this question can also be answered.