Abstract: A multi-blade damper for an exhaust duct through which heated air flows after passage through heat exchange coils carrying a hot, liquid heat exchange medium which is subject to solidification if cooled too greatly, for example, sodium. When the heat exchange medium is very hot, air is forced through the duct by high volume fans and the damper blades are opened widely. When the source of heat to the heat exchange medium is shut down, however, the flow of the heat exchange medium and its temperature are greatly reduced. Even if the fans are shut off at this point, the natural draft of air through the duct may so cool the coils of the heat exchanger that the medium will freeze in the coils. Under these conditions, some of the damper blades are closed to majorly reduce the air flow. However, during the transition from hot operating conditions to stand-by conditions and during stand-by periods, the heat exchange medium must be kept hot enough to prevent solidification.