In cold climates heat recovery in the ventilation system is essential to reduce heating energy demand. Often the low exterior air temperature results in condensation and freezing in the heat exchanger and defrosting strategies have to be employed. In order to investigate different design options and defrosting strategies there is a need for a dynamic model that takes into account condensation and freezing. This article presents a dynamic model of a counter flow air to air heat exchanger taking into account condensation and freezing and melting of ice. The model is implemented in Simulink and results are compared to measurements on a prototype heat exchanger for cold climates.