Heat recovery is adjustable: A portion of the treated flow can be diverted through a bypass valve directly to the discharge stack. During high concentration operation, reduced heat recovery enables operation at maximum loading. Conversely, maximum heat recovery enables energy efficient operation with a low concentration source vapor.  A stack damper automatically applies pressure to the discharge circuit when the heat exchanger bypass valve is opened to support bypass flow. When the heat exchanger bypass valve is closed, the damper is fully open. When the Catox is off, the damper closes to reduce heat loss.