in a method of combustion designed to achieve a further reduction in harmful emissions , especially of nox and co , which is viable at advantageously reduced noise levels and lower cost with any burner of a type comprising a slotted or pierced diffuser of thin sheet metal in receipt of fully or partly mixed gas and air supplies , typically in appliances comprising a heat exchanger such as the boilers of central heating systems , the essential steps are : a ) supplying a uniform flow of fuel - air mixture to the slots ; b ) discharging the mixture from a pierced surface of substantially doughnut geometry comprising a slotted peripheral area and an essentially solid central area of prescribed proportions ; c ) bringing combustion to completion in a slender lamellar flame ; d ) regulating the rate of aeration to a value between 0 . 9 and 1 . 4 , or e ) to values less than 1 . 6 ; f ) effecting a recycle of post - combustion gases .