Refrigerant charge faults have a great adverse effect on the performance of heat pumps and must therefore be detected and diagnosed early in real time. In this study, the effect of refrigerant charge faults on a water-to-water heat pump is experimentally investigated in cooling mode and heating mode at various outdoor entering water temperature conditions. The study showed that refrigerant undercharge affects the performance of water-to-water heat pump more in heating mode than in cooling mode. Results from the study are used to develop a refrigerant charge fault detection and diagnosis (FDD) algorithm that works using correlations and rule-based refrigerant fault characteristic charts. The FDD algorithm is able to detect refrigerant charge faults in the water-to-water heat pump within an error threshold of ±4.5% and ±1.1% in cooling mode and heating mode respectively.