In this work the thermodynamic performance of a heat pump used for recovering the waste heat from a data centre for district heating purposes was evaluated by calculating its avoidable irreversibilities. The supply and return water temperatures of the district heating network were taken as 50/25 °C and CO2/propane (R744/R290)-based mixtures were considered as green working fluids for the heat pump. The investigated mass fractions of the R744/R290-based mixtures were equal to 0.60/0.40, 0.70/0.30, 0.80/0.20, 0.85/0.15 and 0.95/0.05. The results obtained highlighted that the values of heat rejection pressure providing the lowest total exergy destruction rates and the highest coefficient of performance (COP) values coincided at the selected operating conditions. The scenario involving the mass fraction of R744/R290-based mixture of 0.85/0.15 featured the lowest value of the total exergy destruction rate (i.e. 34.50 kW) and the highest COP (i.e. 5.68). The system with pure R744 provided 36.99 kW of the total exergy destruction rate and COP equal to 5.40. The conventional exergy analysis indicated that both the compressor and the expansion valve had the largest values of exergy destruction rate, i.e. the highest priority for improvement. As for the outcomes in terms of removable irreversibilities, it was found that for the R744/R290 mass fraction of 0.85/0.15 the compressor is the component with the highest priority for thermodynamic improvement, revealing a potential decrease of its exergy destruction rate equal to 9.20 kW. The results obtained from the analysis of removable irreversibilities showed that the mutual interactions between the compressor, gas cooler and evaporator are weak, while the inefficiencies caused by the expansion valve are mostly related to the the gas cooler. The improvement of the components (individually or simultaneously) for the heat pump using pure R744 could decrease the exergy destruction rate by between 5.66 kW and 19.65 kW, resulting in COP values from 6.65 to 8.72. In case of using R744/R290-based mixture with 0.85/0.15 mass fraction the reduction in avoidable irreversibilities within the components could decrease the exergy destruction rate by from 4.95 kW to 19.29 kW, leading to COP values from 6.31 to 9.34. To conclude, taking into account the flammability safety and the potential system thermodynamic improvement, the mixture involving the 0.85/0.15 R744/R290 mass fraction was found to be the most preferable.