Mechanical ventilation equipped with heat recovery systems (MVHR) is prevalently used in the Scandinavian climate. These systems considerably reduce the ventilation heat losses during the long and cold winter seasons in such cold climate regions. The humidity in the return air from the building may condense due to exposure to low outdoor air temperatures. This affects the heat transfer rate between the two air streams and causes blockage if the condensate turns into ice. In this study, detailed heat transfer modeling of the heat exchanger enables anticipation of condensation and frosting onsets. Simulation results prove that the relative humidity in the return air and outdoor air temperature are the most decisive parameters for controlling the frosting threshold. Except preheating the outdoor air to above the frosting threshold, increasing the return airflow rate using a variable speed fan can be used for frost prevention or postponing the ice appearance. The results show that increasing the return-to-supply airflow rate ratio effectively lowers the frost threshold for the climate of central Sweden.