The staged-air angle was adjusted from 45 degrees to 20 degrees on a down-fired 600 MWe supercritical utility boiler suffering from superheat and reheat steam temperatures operating at below their designed values. This modification was expected to elevate the flame kernel position and increase the combustion share in the upper furnace, thereby raising superheat and reheat steam temperatures. To evaluate the effect of this modification on coal combustion, NOx emissions, and the aforementioned steam temperatures, normal full-load industrial-size measurements were taken within the furnace before and after the angle reduction. A comparison of the data from the two angle settings revealed that there were slight changes in superheat and reheat steam temperature levels as well as NOx emissions. Unexpectedly, the angle reduction caused several adverse effects, including a reduction of gas temperatures in the furnace, a sharp increase of carbon in fly ash, and a decrease in boiler efficiency. In conclusion, the staged-air angle reduction failed to improve the low steam temperatures and, simultaneously, resulted in poorer coal combustion within the furnace.