In which scenario would a combustion appliance require immediate service?

A combustion appliance requires immediate service when there are visible flames or unusual noise during operation. This is indicative of potential safety hazards, such as improper combustion, fuel leaks, or malfunctioning equipment. Visible flames, especially when they appear where they shouldn't, can signal incomplete combustion, which may lead to the generation of harmful carbon monoxide. Unusual noises, such as hissing, banging, or popping, can indicate mechanical issues or gas flow problems that could compromise safety and efficiency.

In contrast, excess heat could be a concern but does not necessarily indicate an immediate service need unless accompanied by other symptoms. An appliance that is not in use does not require immediate attention, and a normal smell associated with its operation does not typically warrant immediate service as long as it aligns with standard operational characteristics.