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To study the influencing factors of NOx emission in gas-fired heating and hot water combi-boilers,a boiler with the maximum heat input of 26.0 kW was selected,and influencing factors including flue restrictor diameter,fan power,nozzle aperture,nozzle ejection distance and air relative humidity on NOx formation were determined.The NOx test rig has been built and the concentration of NOx at the rated heat input and the NOx weight value(NOx)pond with different heat input in the dry flue gas have been tested respectively according to the test methods in Chinese national standard GB 25034-2010.The results show that with the increase of the diameter of flue restrictor at exhaust outlet,the NOx concentration at the rated heat input and the NOx weight value(NOx)pond with different heat input in the dry flue gas decreased by 26.9％and 5.9％;with the increase of the diameter of flue restrictor at air intake inlet,the NOx and(NOx)pond decreased by 36.5％and 16.0％;with the increase of fan power,the NOx and(NOx)pond can be decreased by 48.4％and 16.1％;with the increase of ejection distance of nozzle,the NOx and(NOx)pond decreased by 7.7％and 6.8％;with the increase of aperture of nozzle,the NOx and(NOx)pond increased by 5.2％and 2.3％;with the increase of air relative humidity,the NOx decreased by 16.4％and the(NOx)pond basically remains unchanged.The analysis of the influence factors of NOx emission can be provided as reference for the optimization design of combi-boilers with low NOx emission.