Large scale recirculation due to unwanted free convection currents of the fluids in a vertical countercurrent shell-and-tube heat exchanger in industry obviously was the reason of severe reductions in heat transfer performance by up to 40% against the design expectations. The apparatus was operated with the cold end on top. Applying a simple one dimensional calculation of flow and heat ransfer, with backflow of the tube side fluid in some of the tubes, shows that the reductions predicted from the model are not too far from the observed ones. In comparing experimental plant data to the predictions of the model, it became obvious, that the influence of free convection on the shell side heat transfer needs to be taken into account additionally to fully explain the experimental observations. An effective remedy, as earlier suggested is to change flow directions in shell and tubes, i.e. to operate the heat exchanger with the hot side on top. In case of the industrial heat exchanger presented in the paper, the change of flow directions in shell and tube side has been carried out in the meantime. This change actually solved the problem, and resulted in the designed performance with efficiencies around 95%.