NOx emission reduction from gas boilers has become a key issue in improving air quality. Combustion air humidification technology is gradually being used to reduce NOx emissions. However, the NOx emission reduction effect of gas boilers at a higher combustion air humidity has been studied less. A flue gas with low NOx emissions and a waste heat recovery system using combustion air humidification technology are proposed in this study. In the ultra-low NOx mode, the effect of high combustion air humidity on NOx emission reduction and efficiency of the gas boiler were studied experimentally. In the waste heat recovery mode, the effects of the heat network backwater temperature on the NOx emission reduction and system efficiency were studied experimentally. Results showed that an increase in air humidity can significantly reduce the NOx concentration formed by combustion. The ultra-low NOx mode reduces NOx emissions from 130 mg/m3 to 23.3 mg/m3 and affects the boiler efficiency slightly. In the waste heat recovery mode, NOx emissions can be reduced to 39.9 mg/m3 when the backwater temperature of the heat network is 55 °C. This condition improves the efficiency to 93.8%. The analysis results provide suggestions for the selection of the operation modes.@en