An upright generally cylindrical casing forms a gas passageway communicating with the exhaust of a flue for receiving its heated exhaust gases. A burner, axially mounted below the casing in the heated gas stream, heats the gases and refractory material contained by the casing to a combustion temperature. Baffles in the casing direct the heated gases toward refractory vanes contained by the casing. Additional air is admitted to the depending end of the casing to support combustion of unburned hydrocarbons.