A monitor for a heat-pump circuit having indoor and outdoor heat exchangers (16, 20), a compressor circulating a working fluid between the indoor and outdoor heat exchangers, and an expansion device between the indoor and outdoor heat exchangers, the monitor comprising: a return-air temperature sensor adapted to measure a first air temperature of air upstream of the indoor heat exchanger; a supply-air temperature sensor adapted to measure a second air temperature of air downstream of the indoor heat exchanger; a working-fluid temperature sensor disposed between the expansion device and the indoor heat exchanger and adapted to measure a working-fluid temperature of working fluid flowing between the indoor heat exchanger and the expansion device when the heat-pump system is operating in a heating mode; and a processor in communication with the return-air temperature sensor, the supply-air temperature sensor and the working-fluid temperature sensor, characterized in that the processor is configured to determine a first difference between the second air temperature and the working-fluid temperature, a second difference between the second air temperature and the first air temperature, and a third difference between the working-fluid temperature and the first air temperature, and in that the processor is configured to determine a working-fluid-charge condition of the heat-pump system based on a first comparison of the first difference with a first predetermined value and one of: a second comparison of the second difference with a second predetermined value and a third comparison of the third difference with a third predetermined value; wherein the processor determines the working-fluid-charge condition based on the first comparison and the second comparison if the first difference is higher than the first predetermined value; and wherein the processor determines the working-fluid-charge condition based on the first comparison and the third comparison if the first difference is lower than the first predetermined value. a return-air temperature sensor adapted to measure a first air temperature of air upstream of the indoor heat exchanger; a supply-air temperature sensor adapted to measure a second air temperature of air downstream of the indoor heat exchanger; a working-fluid temperature sensor disposed between the expansion device and the indoor heat exchanger and adapted to measure a working-fluid temperature of working fluid flowing between the indoor heat exchanger and the expansion device when the heat-pump system is operating in a heating mode; and a processor in communication with the return-air temperature sensor, the supply-air temperature sensor and the working-fluid temperature sensor, characterized in that the processor is configured to determine a first difference between the second air temperature and the working-fluid temperature, a second difference between the second air temperature and the first air temperature, and a third difference between the working-fluid temperature and the first air temperature, and in that the processor is configured to determine a working-fluid-charge condition of the heat-pump system based on a first comparison of the first difference with a first predetermined value and one of: a second comparison of the second difference with a second predetermined value and a third comparison of the third difference with a third predetermined value; wherein the processor determines the working-fluid-charge condition based on the first comparison and the second comparison if the first difference is higher than the first predetermined value; and wherein the processor determines the working-fluid-charge condition based on the first comparison and the third comparison if the first difference is lower than the first predetermined value.