What on the BD is an indication of unburned fuel vapors within the firebox?

The presence of fogged mirrors in the context of firebox operation indicates unburned fuel vapors. When there are unburned fuel vapors, it can result in moisture and condensation forming on surfaces, including mirrors that may be present in the firebox area. This can signal inefficient combustion, where not all of the fuel is being consumed, leading to moisture buildup.

In a well-functioning firebox, you would expect complete combustion of fuel, producing minimal unburned gases and resulting in clearer reflections on mirrors. Therefore, the visibility of fogging on mirrors serves as a practical indicator of potential combustion inefficiencies that may require attention.

Other indicators, such as soot buildup, flame distortion, and excessive smoke, relate more generally to combustion issues, but fogged mirrors specifically highlight the presence of unburned fuel vapors in a visual manner, making them a direct sign of inefficiencies in the combustion process.