Abstract: A butterfly damper with a plurality of blades which can be locked in the open position, and which, when released, rotate about a hinge to the closed position. A pawl member, attached to one blade, extends away from the blade and through an opening in a second blade when the blades are in the open position. A mounting member attached to the second blade restrains the pawl member thereby holding the blades in the open position. A bimetallic link attached to the mounting member causes the pawl member to be released when the temperature of the air passing through the duct increases beyond a preselected level. A closure spring attached to the blades acts on the blades tending to force them apart against the action of the mounting member and the pawl member when the blades are restrained in the open position. The damper blades can be insulated to prevent the heat from a fire on one side of the damper from being conducted by the damper to the other side when the blades are in the closed position.