What is a common indication of insufficient secondary air during combustion?

A common indication of insufficient secondary air during combustion is the presence of black smoke coming from the stack. When secondary air is inadequate, not all of the fuel burns completely, leading to incomplete combustion. This incomplete combustion produces carbon particles, which emit as visible black smoke. The presence of this smoke signifies that the combustion process is not achieving the necessary temperatures or airflow needed for optimal fuel combustion, resulting in wasted fuel and potential environmental pollution.

In contrast, clear smokeless emission typically indicates efficient combustion where fuel is fully burned. White smoke, on the other hand, often results from water vapor or condensation, which is not directly related to secondary air insufficiency. Lastly, the smell of unburned fuel does suggest inefficiency in the combustion process, but the specific visual cue of black smoke is a more direct indication of insufficient secondary air during combustion.