What is likely causing a wavering flame at one burner in a gas furnace when the blower activates?

A wavering flame at one burner in a gas furnace, especially when the blower activates, often indicates an issue with the heat exchange process or airflow dynamics. When the blower turns on, it increases the airflow through the system, which can affect how gas is delivered to the burners and how efficiently the combustion process occurs.

A failed heat exchanger can create issues such as improper combustion conditions, leading to a wavering flame. The flame may become unstable due to fluctuations in temperature and pressure caused by the failure. In instances where the heat exchanger is compromised, it may not distribute heat effectively, thus impacting the stability of the flame.

The other choices describe various conditions that could lead to heating inefficiencies or operational problems, but they would typically present with different symptoms. For example, insufficient airflow across the heat exchanger generally leads to overheating or shutdown, while a blower motor running too slow may cause inadequate air distribution, not directly causing a wavering flame. A faulty gas valve could disrupt the gas flow but wouldn't necessarily correlate directly with the blower activation affecting one burner. Thus, a failed heat exchanger is the most plausible explanation for the wavering flame phenomenon observed in this scenario.