A method of improving natural draft fire tube burner efficiency by controlling combustion air flow. The method involves positioning in a fire tube an air damper body comprising a fixed plate having a plurality of air flow openings and a rotatable plate having a plurality of air flow openings. Air flow through the air damper body is controlled by adjusting the relative rotational position of the fixed plate and the rotatable plate. The air damper body has an air inlet face, an air outlet face, and an outer circumference. The method involves establishing and maintaining a relationship between an air damper open area, firing rate and excess air to maintain stack oxygen below 3% in accordance with a formula A2+A3