The paper presents an analysis of the air – conditioning, heating and cooling system of an administrative space within a comercial building. The system is based on the application of a heat pump and an air – handling unit with dual – duct air distribution. Design calculations of heat losses and gains were carried out, and the required amounts of fresh and recirculated air were determined. The impacts of various parameters (the ratio of fresh to recirculated air and the ratio of warm to cold air streams) on system operation, as well as energy efficiency and user comfort, were analyzed.