The ever increasing environmental awareness introduces the trend of increasing the efficiency of power plants via implementing waste heat recovery. This study reports two numerical models suitable for analyses of a condensing heat exchanger. The numerically predicted results are compared with the results acquired experimentally with the use of a coal-fired water condensing heat exchanger installed in the local laboratory of thermal engineering. The first applied mathematical model is a stationary 1D model based on our own modification of the Colburn-Hougen model. The second one is a proposed 3D multi-phase flow model based on the Euler model of mixture expanded by the model of condensation of water vapour from the flue gases. A thorough comparative analysis of the results of both the numerical models and the experimental ones showed very good correlation. The differences between all the acquired results for the temperatures of flue gases and heated water at the outlet of the heat exchanger did not exceed 4 degrees C. In cases of heat recovery with the occurrence of condensation, the latent heat has a significant influence on the flue gases and water temperatures at heat exchanger outlet. (C) 2018 Elsevier Ltd. All rights reserved.