This study examines the transient responses of a 50 kW ORC (Organic Rankine Cycle) system subject to change of the water coolant in the condenser with R-245fa as the working fluid. With a moderate increase or decrease of water coolant flow rate, all the major quantities such as the output power, condensing pressure, evaporation pressure show mild transition. The mass flow rate of the R-245fa remains roughly the same during the transient. For a sharp rise of the water coolant flow rate, the ORC system undergoes a rather unusual behavior. The output power is first slightly increased, followed by a sharp decline to barely any output power and remains there until the end of the transient. The output power increased back right after the transient and exceeded its original state. There is almost no R-245fa mass flow rate during the power outage period. Yet the evaporation pressure also experienced a gigantic falloff during the transient period. This peculiar surge phenomenon is related to tremendous change of the total void when condensation takes place that momentarily reduces the R-245fa mass flow rate. The surge effect caused by the water coolant is associated with the rate of variation in the overall heat transfer coefficient.