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Refrigerant mixtures such as R410A (R32/R125: 50%/50%) and D2Y60 (R32/R1234yf: 40%/60%) have specific mass fractions of each constituent.In practical,the mass fractions may not be maintained due to refrigerant leakage,charging andeven systems normal operation.This paper discusses the effects of themass fraction shiftof refrigerant mixtures on the system-level performance such as COP,power consumption,discharge temperature and operating pressures.The information is also helpful to develop a quantitative understanding on the deviations/uncertainties of those values if predicted by using the original mass fractions.Two refrigerant pairsare discussed,namely R32/R134a and R32/R1234yf.A validated three-ton capacity heat pump system model was used to simulate the systems steady state operation with refrigerant mixtures having different R32 mass fractions.It was found that the cooling capacity can be off up to 5% and 4% when R32 mass fraction shifts by 10% for R32/R134a and R32/R1234yf,respectively.The system COP deviation is dominated by the cooling capacity deviation.Both pair shows a significant operating pressure which is up to 9% deviation as R32 mass fraction shifts by 10%.Both refrigerant mixtures shows the decrease in discharge temperature,which could be by up to 0.9K for every 10% decrease in R32 mass fraction.