A technician notices flames "roll out" during the main ignition of a standing pilot furnace. What is the suspected issue?

When flames "roll out" during the main ignition of a standing pilot furnace, it indicates that there is an issue with the safe venting of combustion gases. This phenomenon can occur if the venting system is obstructed or malfunctioning, preventing proper exhaust of combustion gases. As a result, the flames can be pushed back into the furnace, causing them to roll outwards from the burner area.

This scenario typically points to a faulty vent, as it suggests that the venting pathway is either blocked or otherwise unable to adequately expel gases from the furnace. Proper venting is crucial for both efficient operation and safety, as it ensures that hazardous gases do not enter the living space and that the furnace can operate as designed.

While other issues, such as improper gas pressure or a defective heat exchanger, could lead to combustion problems, they would not typically present with flames rolling out during ignition in the same manner, as these issues primarily affect fuel delivery or heat transfer, not the venting process itself. Therefore, the identification of a faulty vent system is critical in this case to prevent potential hazards and ensure safe operation.