Air-source heat pumps (ASHPs) are widely applied in building energy systems due to their high efficiency and contribution to decarbonisation. However, their performance under cold climate conditions is strongly affected by frost formation on the evaporator heat exchanger, which leads to degradation of heat transfer and increased energy consumption. This study presents an experimental investigation of the influence of frosting conditions on the operational efficiency of an ASHP, combining energy and exergy analysis approaches. A long-term experimental campaign was conducted under real operating conditions during the heating season. Frost formation was observed to intensify at ambient temperatures below approximately 3-4 °C and relative humidity above 80-85%, resulting in increased thermal resistance and reduced heat extraction from the air. The results show that frost accumulation leads to a noticeable decrease in both energy and exergy efficiency. The evaluation based on enthalpy differences allowed quantification of heat exchange degradation and identification of key operating parameters influencing system performance. The findings provide deeper insight into the thermodynamic behaviour of ASHPs under dynamic climatic conditions and contribute to the development of improved modelling and control approaches aimed at enhancing seasonal performance.