This article presents results of research concerning the possibility of reducing the level of toxic nitric oxides (NOx) emission to the atmosphere. The research has been conducted on DKVR 20-13, PTVM-50 and DE 25-14 gas boilers. The complex character of this issue requires individual consideration regarding each boiler configuration. Each case requires consideration of characteristics and details of all elements constituting the boiler-furnace unit. The main problem was to establish the reference level to which the reduction of nitric oxides occurs. The actual maximum emission of nitric oxides was assumed as this level. It was verified with the maximum allowable emission of nitric oxides for each boiler. Three levels of the potential influence of emission on the atmosphere have been taken into account. This experimental research allowed for proposing an effective method, which led to reducing nitric oxides emission by around 30%.