Reverse cycle defrosting is the most widely used standard defrosting method for air source heat pump (ASHP) units. It was suggested in previous experimental studies that during reverse cycle defrosting, downwards flowing of the melted frost due to gravity over a vertically installed multi-circuit outdoor coil in an ASHP unit has negative effects on defrosting performance. Therefore, an experimental study on draining away locally the melted frost for an experimental ASHP unit having three refrigerant circuits using water collecting trays was carried out and the study results were separately reported. To enable further quantitative analysis on the effects of local draining away the melted frost on reverse cycle defrosting performance of the ASHP unit, a modeling study on the defrosting process, at the two experimental settings of with and without the use of water collecting trays between circuits was carried out and is reported in this paper. Two semi-empirical mathematical models, corresponding to the two settings, were developed. In this paper, firstly the detailed development of the two semi-empirical models is presented. This is followed by reporting the validations of the two models using the experimental data previously reported. Finally, detailed discussions on the potential uses of the two models developed and the limitations of the modeling work reported are included. (C) 2014 Elsevier Ltd. All rights reserved.