The present invention is designed to reduce running costs in refrigerant based air-conditioning, refrigeration and heating systems by using a combination of thermodynamic and hydraulic control to manage the on and off states of the compressor, which is the main energy consuming component. Thermodynamic or temperature control is used to manage comfort levels within the room or space being cooled. Hydraulic control is used to determine when the compressor has completed its useful work in delivering a supply of high-pressure liquid refrigerant. Once temperature and hydraulic conditions are satisfied the compressor can be turned off; thereby delivering a significant reduction in running costs.