Which of the following can indicate improper fuel combustion?

Excessive soot formation is a clear indicator of improper fuel combustion. When fuel is not burned completely, it can lead to the production of soot, which is carbon byproduct resulting from incomplete combustion. This can occur due to several factors, such as incorrect air-to-fuel ratio, poor mixing of fuel and air, or inefficient burner operation. The presence of soot not only signifies inefficiencies in the combustion process but can also lead to significant operational issues, including reduced efficiency, increased maintenance costs, and potential safety hazards like chimney fires.

In contrast, low emissions typically signify more complete and efficient combustion because less unburned carbon and other pollutants are being released into the atmosphere. Similarly, high efficiency reflects that the fuel is being converted to energy effectively with minimal waste, which is the opposite of poor combustion conditions. Stable pressure readings are generally a sign of a well-functioning boiler system, indicating that there are no disturbances that might indicate improper combustion. Therefore, excessive soot formation stands out as the critical indicator of combustion issues.