Which symptom indicates low water levels in a boiler?

The symptom that indicates low water levels in a boiler is the occurrence of a loud banging noise. This noise, often referred to as "water hammer," is caused by the sudden collapse of steam bubbles in areas where water has dropped below the safe operating levels. When the water level is too low, steam pockets can form and then rapidly condense, resulting in a shocking pressure change that creates a loud noise and can potentially cause damage to the boiler and piping system.

The other choices do not directly indicate low water levels. An improvement in steam quality generally suggests sufficient water levels and proper operation, while rapid boiling can occur in various conditions and does not specifically signify low water. A significant drop in temperature is more likely associated with inadequate fuel supply or heat transfer issues, rather than specifically pointing to low water levels. Understanding the relationship between water levels and these symptoms is crucial for maintaining safe boiler operations.