Which of the following is NOT a sign of a backdraft?

When assessing conditions indicative of a backdraft, it's important to recognize the specific signs associated with this dangerous phenomenon. A building that shows a thick yellowish smoke and appears to be breathing—expanding and contracting—are classic indicators that conditions may be right for a backdraft. Similarly, the absence of smoke can also suggest that there are smoldering combustibles within a confined space, leading to an imminent backdraft when fresh air is introduced.

In contrast, heavy flames visible from the exterior do not align with the characteristics of a backdraft. A backdraft typically occurs when there is insufficient oxygen in a fire's environment, causing combustion to stall. When heavy flames are visible, this typically indicates that there is a sufficient oxygen supply allowing for active combustion, making it less likely for the conditions leading to a backdraft to be present. Thus, heavy flames are not a sign of a backdraft, which is why this answer is correct.