The probable cause of an increase in temperature of combustion gases at the boiler outlet is likely what?

The increase in temperature of combustion gases at the boiler outlet is primarily associated with the condition of the heating surfaces within the boiler. When heating surfaces are dirty, they become less efficient at transferring heat to the water within the boiler. This inefficiency forces the combustion gases to increase in temperature as the system struggles to achieve the desired heat transfer. The dirty surfaces impede the normal flow of heat, resulting in hotter exhaust gases.

While other factors can affect boiler performance, the presence of dirty heating surfaces directly impacts the temperature of the gases being expelled. In contrast, excessive fuel flow could lead to incomplete combustion and potentially lower gas temperatures, while too much steam pressure or insufficient air supply could disrupt combustion efficiency but may not directly correlate with increased gas temperatures at the outlet in the same way that dirty surfaces do.