Abstract: A method and means for extracting heat from an exhaust stack under highly corrosive and fluctuating conditions. An in-line exhaust gas heat exchanger having selective dual concentric exhaust paths and having a plurality of longitudinal structural stringers to insure against weakening of the exhaust stack. A plurality of heat exchanger coils are located in the outermost exhaust path and means is provided for fully draining the liquid contents thereof. Temperature control apparatus is provided for both regulating the temperature of fluid within the heat exchange coils and for regulating the exhaust stack temperature for controlling critical dewpoint situations.