The effects of an expansion valve opening on the robustness and system performance of an R32 heat pump system under variable heat source inlet temperatures and hot-water inlet temperatures are studied. The results show that the system has different superheat oscillation ranges under different operating conditions

and the heat source inlet temperature has a significant effect on the initial oscillation superheat

whereas the hot water inlet temperature has no obvious effect on it. Under the variable heat source inlet temperature (15–25 ℃) conditions

the heating capacities show different variation trends with the superheat

whereas the COP shows the same changing trends

and the maximum COP of the system is obtained within the superheat oscillation range. Under the variable hot water inlet temperature (30–40 ℃) conditions

the heating capacities and COP show the same trends with the superheat

and both of them reach the maximum values in the superheat oscillation range. Both the increase in the heat source inlet temperature and the decrease in the hot water inlet temperature can increase the maximum heating capacity and the maximum COP

which are 9.287 kW and 4.646

respectively when the heat source inlet temperature is 25 ℃

and 9.148 kW and 5.665

respectively

when the hot water inlet temperature is 30 ℃.