Because of the requirement of annual operation

an air source heat pump water heater has complex and variable working conditions. Therefore

there are higher requirements for air source heat pump water heater system control. Based on the flexible adjustment characteristics of the electronic expansion valve

this study proposes a simple control method of the fixed discharge superheated temperature. Meanwhile

the control methods of a fixed valve opening and fixed superheated temperature were used to conduct experimental research and analyze their internal process. When the ambient temperature is 22 ℃，the experimental results show that the discharge temperature dramatically decreased under the control methods of a fixed valve opening; however

its coefficient of performance (COP) is the lowest and its compression risk with liquid is high. Although the COP is the highest under the control method of a fixed superheated temperature

the discharge temperature is also the highest. Under the control method of a fixed discharge superheated temperature

its COP is slightly lower than that of the fixed superheated temperature control method. However

the discharge temperature is effectively reduced by nearly 7 K on average. Meanwhile

compared to the fixed valve opening

its average COP increased 4.9% and the risk of compressor suction with a large amount of liquid refrigerant can be avoided. Therefore

this method effectively combines the advantage of the other two methods.