If flames "roll out" during the main ignition of a standing pilot furnace, what is the likely issue?

When flames "roll out" during the main ignition of a standing pilot furnace, this typically indicates that there is an issue with the venting system. The phenomenon of flame rollout often occurs when the products of combustion are not being properly expelled from the furnace, causing them to back up into the combustion chamber. This can be caused by a blockage or obstruction in the venting system, which can prevent proper airflow and negatively impact the furnace's operation.

A faulty vent can lead to inadequate draft, which is essential for proper combustion and exhaust of gases. When combustion gases cannot escape effectively, it may result in flames rolling out from the combustion chamber instead of being contained within it. This is not only inefficient but also poses a significant safety hazard, as it can lead to the risk of fire or the release of harmful gases into the living space.

In contrast, high or low gas pressure would typically manifest in other ways, such as inconsistent burner operation or failure to ignite, rather than flame rollout specifically. Similarly, insufficient combustion air could potentially lead to incomplete combustion, but it would not normally cause a flame rollout situation in the manner described. Thus, the presence of flame rollout primarily points to a venting issue.