Traditionally two methods for controlling drainage of the evaporator during hot gas defrost are used: Pressure control, which keeps the pressure in the evaporator constant during defrost and liquid drain control, which uses a float to drain condensed liquid from the evaporator. The energy consumption using the two methods is quite different, as the pressure control method bypasses a certain amount of hot gas at the end of the defrost period. This paper presents a model, which quantifies the energy consumption during the two methods, and discuss the influence of operating conditions and evaporator design. The results from the model are validated against laboratory measurements on two evaporators with different pipe arrangements. Detailed laboratory test on ammonia defrost system has been conducted as a part of the ELFORSK project 347-030.