What is a common symptom of a failing boiler?

Unusual noises, such as banging, hissing, or rumbling sounds, are common indicators of a failing boiler. These noises can result from several issues within the system, such as the buildup of steam bubbles in water or the expansion and contraction of metal components due to temperature changes. For instance, banging can be a sign of "water hammer," caused by steam moving through the piping too quickly or the presence of trapped water. Hissing may indicate a leak in the system or issues with pressure regulation. These auditory alerts signal that the boiler is not operating as intended and may require inspection or maintenance to prevent further damage or failure.

The other options point to issues that might arise in different contexts but are not as readily identifiable as symptoms of boiler failure. Excessive steam generation can happen under normal operating conditions, and while overheating could lead to failure, it’s often a result of other underlying problems rather than a direct symptom. Constant pressure fluctuations can indicate a malfunctioning pressure control system but are less specific to the boiler’s condition compared to the clear and discernible sounds that indicate mechanical issues or build-up within the system.