Abstract: Cyclonic furnace for efficient burning of such waste products as rubber tires or industrial or domestic refuse with minimum atmospheric pollution and high heat output has fixed circular hearth with central outlet for slag and ash discharge, material to be burnt being fed onto the hearth periphery; combustion air ports opening tangentially into its combustion chamber; and automatic control means regulating e.g. material infeed and inflow of combustion air including a facility for monitoring combustion conditions in the furnace, preferably by measurement of CO and CO.sub.2 levels, and comparing those conditions with previously recorded conditions automatically to predict changes and make anticipatory adjustments of the furnace controls giving extremely rapid control reaction and enabling combustion air requirements to be cut to a minimum, of which the following is a specification.