The paper presents research results for NOX reduction for moderate incomplete combustion, its impact on the environment and boiler reliability, optimal combustion conditions criteria, and its implementation in operating steam and hot-water boilers. The authors pay additional attention to the cost efficiency criterion while analysing the best method for reducing NOX. After conducting a numerical calculation for combustion processes under certain defined conditions, the authors suggest a new method for emission reduction by suppression of thermal NOX formation using the extended Zeldovich mechanism. Research shows up to 40% NOX reduction potential for existing boilers depending on fuel type and operating conditions with CO concentration in stack gases lower than standard levels due to chemical incomplete combustion.