NOx What does this mean?

Nitrogen oxides, or NOx, is the generic term for a group of highly reactive Gases. Nitrogen oxides form when fuel is burned at high temperatures, as in a boiler combustion process.

NOx gasses contain nitrogen and oxygen in varying amounts, and is one of the main ingredients in the formation of ground level ozone and contributes to the formation of acid rain & contributes to global warming.

Boilers now show a NOx rating ranging from 1 to 5, 5 is best.