This work presents an actuator fault tolerant control (FTC) system applied in a countercurrent double pipe heat exchanger. The heat exchanger has two input variables which are, cold and hot water flow rates. The main actuator manipulates the cold water flow rate in order to control the hot water temperature. The goal of this work is to keep in continuous operation the counter-current double pipe heat exchanger, even if the main actuator is stuck-open. In case of failure on the main actuator, it is proposed to control the process with the second input variable (by manipulating the hot water flow rate). To achieve our aim, we designed an FTC system based in a fault detection and isolation (FDI) system and a Model Following Control (MFC). To develop the FDI system an adaptive nonlinear observer to estimate the flow rates in the heat exchanger for both streams (hot and cold) was implemented. The FDI is performed by the comparison between the flow rate estimation (by the adaptive nonlinear observer) and the flow rate provided by the control law. The nonlinear flow rate estimation is needed because the real plant does not have digital flowmeters to measure the flow rates online. The FDI system allows to detect and isolate the fault actuator then, the control signal and the failure accommodation is realized by the MFC. The results showed that it is possible to keep the heat exchanger in continuous operation even if the main actuator is stuck-open. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.