Gas Heat ESCO Practice Exam

Which factor primarily causes burner flashback in a gas furnace?

1. high inlet air temperature

2. incorrect air-to-fuel ratio

3. low airflow

4. low manifold pressure

The correct answer is: low manifold pressure

The primary cause of burner flashback in a gas furnace is related to the incorrect air-to-fuel ratio, which directly influences the combustion process. When the air-to-fuel ratio is not balanced, it can lead to incomplete combustion, causing the flames to travel back into the burner. This situation typically arises when the ratio allows for excess fuel without sufficient air to burn it properly. While low manifold pressure can contribute to a misfire or ineffective combustion process, it's the imbalance in the air-to-fuel ratio that fundamentally creates the conditions for flashback. Other factors like high inlet air temperature or low airflow can also affect combustion efficiency but are not the primary drivers of flashback incidents, as their effects are more indirect. By maintaining the correct air-to-fuel ratio, a gas furnace can operate safely and efficiently, minimizing the risk of burner flashback.