Air source heat pump water heater/chiller (ASHPWHC) units, a cooling and beating source for buildings becomes increasingly popular. However, when such a unit is operated as a heating source under low ambient temperature in winter. the formation of frost on the surface of its airside heat exchanger becomes problematic, leading to the degradation of the heat exchanger's performance or even the shutdown of a unit. Therefore it is necessary to have a detailed investigation on the operational characteristic of the airside heat exchanger in an ASHPWHC unit under frosting in order to optimize its structural layout, thus increasing its energy efficiency and operational reliability. This paper first reports on the development of a distributed mathematical model of the airside heat exchanger under frosting in an ASHPWHC unit. The model, firstly seen in open literature, consists of a frosting sub-model and a heat exchanger sub-model. This is followed by an evaluation of operational characteristics of the airside heat exchanger under frosting in an ASHPWHC unit using the model developed. In addition, the impacts of frosting on the operational performance of an ASHPWHC unit were also evaluated. The discussions of optimizing the structural layout of finned tubes used in an airside heat exchanger and improving defrosting control measures are also included in analyzing the simulated results. (C) 2004 Elsevier Ltd. All rights reserved.