The effects of starting mode and outlet temperature of heat sink (OTHS) on the starting performances of direct-heated heat pump water heater using natural blend were investigated experimentally. The results indicated that both starting mode and OTHS had notable impact on starting time of transient performance parameters. Both warm start and lower OTHS helps to quickly obtain hot water at the target temperature, mainly because the system startup time in the warm start was 16.73 % shorter than that in the cold start and the system gained the shortest startup time (1015 s) in low temperature condition. The transient heat sink outlet temperature had a rapid temperature rise section and a slow temperature rise section (STRS), and the STRS consumed 56.58 %–66.67 % of the system starting time, so the improvement of STRS is the key to realize the fast startup. The effects of startup mode on the trends of transient performance parameters were memorably different, but the OTHS had no apparent effect on their trends. The transient refrigerant pressures and transient refrigerant temperatures had the fluctuations. The transient suction pressures had the minimum values (0.3925–0.5575 MPa), and both cold start mode and higher OTHS resulted in a decrease in the minimum value.

Experimental study on startup characteristics of instant heat pump water heater with non-azeotropic natural blend