Boilers are important processes in chemical and refinery industries: they are normally operated for an extended period of time, leading to (water/steam) tube leaks because of aging and corrosion. To maintain production in normal and safe conditions, detecting the possible boiler leaks in time is crucial. In this paper, a process model is proposed to describe the boiler tube leak problem. On the basis of this model and the boiler characteristics, a least-squares method with a forgetting factor is derived to detect boiler leakage. The analysis of leak estimation properties for the proposed method is given under certain conditions of boiler operation. The applications based on both simulation and real plant data show that the proposed method is capable of detecting boiler leaks effectively and efficiently.