the disadvantage of the defrosting determination criterion of time-coil temperature was theoretically analyzed and experimentally verified. considering the different characteristics of variable refrigerant flow air conditioning system (vrf ac) from the common air conditioners, the defrosting criterion model of time-coil temperature-condensation temperature has been built. in this model, there exist four temperature regions based on different frosting characteristics at different ambient temperatures, i.e., t-ao(degrees c) is an element of (-infinity, -9], (-9, 0], (0, 4], (4, 21]. the relations between the defrosting judgment temperatures and ambient temperatures have been determined by experiments. and the minimum defrosting cycle when t-ao(degrees c) is an element of (-9, 21] and t-ao(degrees c) is an element of (-infinity, -9] were respectively determined to be 60 min and 90 min by experiments. moreover, the defrosting accumulative coefficient, the defrosting judgment time constant and the restriction of the condensation temperature have been introduced into the model. thus the complete criterion model can avoid delayed and premature defrosting, even false defrosting and inaccessible defrosting. a large number of experiments have verified the feasibility of the model. (c) 2012 elsevier b.v. all rights reserved.