A computerprogram was developed to simulate the performance of residential air-to-air heat recovery ventilators of various effectivenesses operating under frosting conditions, Algorithms were developed to predict the energy performance of the heat recovery ventilatsrs for several frost control and defrost strategies for any prescribed climatic conditions. This paper describes the weather model, the heat exchanger performance/frosting model, and the defrost and frost control algorithms developed for this ASHRAE-funded project.