Abstract: A heat pump system comprises a compressor, at least one expansion valve, an accumulator for storing a volume of liquid refrigerant therein, a liquid refrigerant indicator connected to the accumulator to indicate an appropriate refrigerant charge in cooling and heating modes, and a controller. The controller is configured to determine a target compressor discharge pressure based on measured outdoor air temperature and control the compressor discharge pressure by modulating the position of the at least one expansion valve, wherein the higher the target discharge pressure target, the less liquid refrigerant is left in the accumulator. The accumulator can be sized to always have capacity to hold excess refrigerant during heating operations, and can include a charge level indicator so as to allow proper charge of the system in the field without additional tools.