The present invention is concerned with a method of operating an oil evaporation burner for heating boilers which, for NOx reduction, are operated with an exhaust gas return from the combustion chamber of the heating boiler into the burner. In oil evaporation burners of the afore-described type the exhaust gas return during the start-up phase presents certain difficulties inasuch as the burner frequently goes off during the start-up phase. To overcome such difficulties the flow of exhaust gas as returned during the start-up phase is penumatically discontinued by the formation of blocking curtains from air as supplied.