For optimal combustion, air and fuel should be in a perfect ratio known as the stoichiometric ratio, ensuring complete combustion without leaving unused fuel or excess air. However, in practice, a controlled amount of excess air is introduced to avoid unburnt fuel, soot formation, and flame instability. While this helps with combustion, too much excess air reduces boiler efficiency as the unused air, which enters the boiler at a lower temperature, absorbs heat from combustion. The extra air escapes with the flue gases, lowering the temperature of the exhaust gases. This cooling effect of excess air reduces the heat available to the superheater, ultimately causing a decrease in the superheater outlet temperature.

The economiser transfers heat from flue gases to the feedwater entering the boiler. If the economiser inlet temperature decreases, it means that colder water is being introduced to the economiser. When more cold water is circulated, the steam formed is at or slightly below the saturation temperature due to excess water cooling. As this saturated, lower-temperature steam passes through the superheater, the heat transfer to the steam is reduced. This results in a decrease in the superheater outlet temperature.

When tube banks are badly fouled by soot, carbon, or unburnt fuel deposits due to improper combustion or lack of sufficient excess air, the heat exchange between the hot gases and the water inside the tubes is reduced. This means less heat is absorbed by the water in the generating tube banks, leaving the flue gases hotter than normal. These hotter gases then flow over the superheater directly, raising the superheater outlet temperature as more heat is transferred to the steam in the superheater.

When there are excessive amounts of total dissolved solids (TDS) in the boiler water, impurities accumulate in the water. These solids can form deposits and scale on the boiler tubes, including the superheater tubes. Such deposits reduce the efficiency of heat transfer in the superheater, leading to fluctuations in the superheater outlet temperature. Overheating and localised hot spots can occur, causing varying temperature profiles across the superheater system. Additionally, steam impurities may carry over to the superheater tubes, further exacerbating temperature inconsistencies.