Which condition is categorized as Class A?

Class A conditions are typically those that pose an immediate and serious risk to safety, often requiring urgent attention. In the context of this question, leaking gas that cannot be stopped falls squarely into this category, as it presents a direct and significant hazard of explosion or fire. Such a condition would necessitate immediate action to mitigate risks and protect safety.

This classification is in contrast to other conditions listed. Properly vented appliances, while important for safe operation, indicate that there is no immediate threat, which does not meet the criteria for Class A. Similarly, defective heat exchangers that operate normally do not pose an immediate danger if they are functioning, regardless of their defects. Ventilation systems that are functioning without obstruction likewise indicate a safe condition, not one that requires urgent intervention. Therefore, leaking gas that cannot be stopped is justified as a Class A condition due to its serious nature and the potential consequences it poses.