Efficiency assessment of heat exchangers ensures appropriate use of the available energy. This article presents the concept of a heat exchanger on-line expert system based on qualitative reasoning. Using confluence of the heat exchanger effectiveness defined by the NTU (number of transfer units) concept for the simple parallel-stream heat exchanger, the methodology is developed to be used for the description of the generic behavior of the heat exchanger system. In this respect, particular attention was paid to the recognition of those situations leading to the degradation of the effectiveness of the heat exchanger. The article describes the selection of the diagnostic variables, and their on-line measurements including the logging system for monitoring and acquisition of the data.