What is a primary cause of "Flashback" in gas furnaces?

Flashback in gas furnaces is primarily caused by low manifold pressure. When the pressure in the manifold is insufficient, it can lead to improper mixing of air and gas. In this situation, the gas may not ignite as intended in the combustion chamber. Instead, it can ignite in the burner itself or even travel back toward the gas source. This unsafe condition results from the gas being forced into the system inadequately, which can create irregular burner flames or cause combustion to occur outside of the intended burn zone.

In contrast, high manifold pressure or excessive primary air can result in different combustion issues, such as a noisy operation or incomplete combustion, but they do not directly cause flashback. Excessive combustion air might lead to an overly lean mixture, which can also contribute to different combustion problems, but again, the main contributor to flashback specifically is low manifold pressure. Understanding the balance of pressure and air-to-fuel ratios is crucial for safe and effective furnace operation.