Which of the following is an indication of potential boiler issues?

Unusual noises during operation are indeed an important indication of potential boiler issues. Boilers are designed to function quietly and efficiently, so the presence of banging, popping, or hissing sounds can signal various problems such as water hammer, air trapped in the system, or issues with the combustion process. These noises can indicate underlying issues that may lead to more significant problems if not addressed promptly.

On the other hand, consistent water levels suggest that the boiler is maintaining its designed water volume, which is a sign of normal operation. Regular pressure readings that fall within the normal operating range indicate that the boiler's pressure control systems are functioning correctly. Similarly, a stable combustion flame ensures that the fuel is burning properly and efficiently, showing that combustion is occurring within the designed parameters. These factors demonstrate that the boiler is likely operating correctly, in contrast to unusual noises which are a warning sign that should be investigated further.