Abstract: A method for controlling nitrogen oxides (NO.sub.x) levels in a gas produced by a fossil fuel fired unit comprises establishing an optimum gas temperature range for the unit and monitoring a gas temperature of the unit. The heat exchanger surfaces of the unit are then cleaned until the monitored gas temperature is within the optimum gas temperature range. An optimum fuel combustion rate range is also established for the unit and a fuel combustion rate of the unit is monitored. Air is provided to the unit until the monitored fuel combustion rate is within the optimum combustion rate range.