When operating on the AIRmix zone of combustion, such as during startup or turndown, the O2 is determined by the required air/fuel ratio. This is dependent on several factors, such as the fuel composition and the air preheat temperature. There is no operator control of excess air until the COOLmix zone is engaged. If control of excess air is desired when the firebox is below AIT, then the burner can be supplied with a second set of COOLmix tips that do not require the AIT permissive at the sacrifice of NOx. Predicted emissions during this mode of operation are approximately 15 ppm for typical refinery conditions, regardless of APH temperature.