What is a likely cause for a gradual increase in stack temperature?

A gradual increase in stack temperature is often indicative of heating surface fouling. When the heating surfaces, such as boiler tubes or heat exchangers, accumulate soot, scale, or other deposits, they become less efficient at transferring heat. As a result, more heat is retained within the combustion process rather than being transferred to the water or steam. Consequently, this inefficiency leads to an increase in the temperature of the gases exiting the stack.

In this context, increased fuel consumption might lead to higher temperatures, but it does not account for the gradual nature of this increase, particularly if the system is maintaining a consistent operational level. Excessive airflow can also lead to lower stack temperatures because it results in more complete combustion and better heat dispersion. Low water levels generally cause overheating situations but are more associated with safety risks or operational disruptions rather than a steady, gradual increase in stack temperature. Thus, heating surface fouling is the most fitting explanation for a gradual rise in stack temperature due to its impact on heat exchange efficiency.