In this paper, the eco-friendly zeotropic mixture, R744/R1234yf was chosen as the alternative refrigerant in the instant heat pump water heater system. The cycle performance of heat pump was analyzed theoretically under the nominal working condition of heat pump system. The prediction results show that the new mixture has significant advantages for the instant heat pump with a large heat-sink temperature lift, and there exists an optimal mass fraction of R744/R1234yf, at which the maximum COPh should be obtained. The optimal mass fraction of R744/R1234yf is 17/83 and the system COPh should be attained to 4.29.

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