When starting up a 15-year-old natural gas furnace, the burner flames wave in one corner after the fan starts. What might be the cause?

In this scenario, having waves in the burner flames in one corner after the fan starts can indicate an issue with the airflow within the furnace. When the flames are not burning evenly or are waving in a specific direction, it can be a sign that there is improper combustion due to inadequate airflow or obstructions.

A faulty heat exchanger could cause uneven heating and poor airflow, leading to a disturbance in the flame pattern. The heat exchanger's condition is critical because it impacts how air circulates through the system, and if it is compromised or blocked, it can create turbulence in the combustion area. This turbulence results in flame instability, which is visible as wavering in the flames.

While other components, such as the burners or the vent system, might seem plausible, they typically do not demonstrate this specific behavior in the same way. A faulty blower housing might affect airflow but would not specifically cause flames to wave in one corner. Therefore, recognizing that the heat exchanger plays a vital role in ensuring consistent airflow and even combustion helps clarify why this is the most likely cause of the observed issue.