We report a novel defrosting control method for refrigeration systems including refrigerators and heat pumps that commonly use time control or time-temperature control to determine the defrosting start-time. However, these methods do not provide accurate defrosting start-times due to variations in the operating conditions. Here, we describe the use of the effective mass-flow fraction (EMF) to determine the defrosting start-time; this method qualitatively detects variation trend of heat transfer rate based on only temperature measurements. The performance of the EMF control was compared with that of time control under varied frosting conditions of an experimental system. The time control method determined the defrosting start-time with an error of 50%, whereas the EMF control precisely determined the defrosting start-time with an error of 10%. (c) 2015 Elsevier Ltd. All rights reserved.