apparatus capable of fitting over the upper exposed end of a chimney liner and operable in the event of a chimney fire to extinguish the fire , the invention can be fitted to existing chimney structures without alteration of the chimney or liner structure . the present apparatus includes an air flow restriction plate which is held in a position allowing free flow through the chimney in normal use situations , the plate being held in place by a heat fusible element . the extremely high temperatures existing during even the first moments of a chimney fire cause melting of the fusible element and therefore closing of the air flow restriction plate to restrict air flow through the chimney and thereby quickly extinguish combustion .