Control of the electronic expansion valve in an air-source heat-pump water heater has considerable influence on the system performance and should be the focus of investigation. In this study

the effect of the opening of the electronic expansion valve on the performance of an air-source heat-pump water heater was analyzed. The results show that

when the system operates with the same valve opening

the heating capacity and coefficient of performance (COP) exhibit a declining trend after an initial ascent. For different valve openings

the system COP is in direct proportion to the opening when the average cistern temperature is 20-30 ℃

whereas it is in inverse proportion to the opening for an average cistern temperature of 45-55 ℃. The system COP trend lines for each valve opening indicate a cross each other. As compressor suction with little liquid refrigerant can improve the system performance

the intersection occurs close to the point at which the superheated temperature reaches 0 K

but does not coincide with it. Although the maximum optimization ratio on system COP for the method of controlling the electronic expansion valve via the average cistern temperature (optimization #1) can reach 24.8%

its feasibility is poor. The maximum optimization ratio on system COP for the method of controlling the electronic expansion valve via the superheated temperature in the compressor suction (optimization #2) is lower than that for optimization #1

at 4.2%. However

the results of the confirmatory experiment verify the feasibility of optimization #2.