Abstract: The disclosure relates to burners with a burner nozzle and a primary air nozzle, the nozzles being jointly axially offset in relation to a secondary air nozzle. The primary air nozzle is in the form of, or is mounted to, an elongate tube which, in its turn, is shiftably mounted on and extends through an openable scuttle and a coronal portion, located inside the scuttle, of a primary air supply duct. The tube is axially shiftable between an operative position and an inoperative position. The burner and its burner nozzle are mounted in the elongate tube and are disposed, together with the tube to move as a unit out from the coronal portion on opening of the scuttle. The elongate tube has at least one air passage which, in all adjustment positions of the tube, maintains communication between the primary air nozzle and the coronal portion of the primary air supply duct.