In order to deepen the understanding of the main phenomena governing the membrane based energy recovery ventilators performance, we present a three dimensional numerical study of the mechanisms of heat and mass transfer in a cross-flow heat exchanger with balanced and unbalanced flow. This work involves the use of a control-volume method and solves the set of Navier-Stokes equations in the air streams and as well as in the membrane core. The results are compared to literature available data and the agreement is seen to be satisfactory. The effect of operating parameters such as the Reynolds number, volume flow rates and temperature on the performance of the heat exchanger are also investigated. (C) 2014 Elsevier Ltd. All rights reserved.