This paper presents an on-line method which detects steam generator tube leaks and the heat exchanger in which the leak occurs. This method (the Tube Failure Model) has been demonstrated by direct testing experience. It is based on the Input/Loss Method, a patented method (1994–2004) which computes fuel chemistry, heating value and fuel flow by integrating effluent measurements (CEMS data) with thermodynamics. This paper explains the technology supporting the detection of tube failures, the method of identifying the location of the failure, and cites direct experience of detecting tube failures at two power plants. Most importantly, this paper presents the results of direct testing at the Boardman Coal Plant in which high energy steam/water lines were routed from the drain headers of all major heat exchangers into the combustion space. When allowed flow, these lines were used to emulate tube leaks from any of the major heat exchangers. Their flow rates and locations were then compared to Tube Failure Model predications. This testing is considered significant as for the first time Δheat rate effects of tube failures will be directly determined; and, further, this testing will provide the Tube Failure Model its on-line proof-of-process.