Provided is a method for operating a non-condensing boiler at maximum non-condensation efficiency, and more particularly a method for operating a non-condensing boiler, capable of supplying an appropriate amount of excess air for combustion of a burner, avoiding damaging a heat exchanger due to the condensed water generated at the heat exchanger, and effectively improving the heat efficiency. The method comprises the steps of: sensing a temperature of exchange water introduced into a heat exchanger and a temperature of supply water discharged from the heat exchanger and calculating an average temperature from the two temperatures to obtain a maximum concentration of CO using information stored in a controller; calculating a ratio of excess air (λ) from the maximum concentration of CO using a combustion equation chosen depending on a fuel used; checking from a combustion characteristic curve whether or not the excess air ratio (λ) is within a stable combustion region; when the excess air ratio (λ) is within the stable combustion region, setting this excess air ratio to the control target value, and when the excess air ratio (λ) is less than a lower limit of the stable combustion region, setting a minimum excess air ratio of the stable combustion region to the control target value; calculating a target value of air flow suitable for the current consumption amount of fuel based on the set excess air ratio; and performing feedback control with respect to the air flow target value and a value input from an air flow sensor. With this configuration, no condensation takes place during operation of the non-condensing boiler, so that the boiler can have increased durability, be easily assembled at inexpensive production costs, be made compact, and obtain maximum heat efficiency under a non-condensation condition.