A method and apparatus for operating a dryer used to reduce the moisture content of sheet material. A drying chamber is provided and includes a plurality of drying sections and a single point exhaust system. A seal section located at the input end of the dryer includes an exhaust passage through which a gas sample is drawn by a sampling fan. Gases within the seal section are a combination of ambient air drawn through restricted passages at the entry to the seal section and exhaust gas that bleeds into the seal section from the drying chamber. A controller monitors the temperature of the sampled gases and ambient air and adjusts the rate of exhaust flow from the main exhaust system as a function of the temperature differential. A heating system is also provided for heating the seal exhaust gases above the pitch condensation temperature for the flow.