Red Seal Gasfitter — Class A Practice Exam

What is a common cause of yellow flame in gas appliances?

1. Excess fuel use

2. Incomplete combustion due to insufficient airflow

3. Faulty gas supply

4. High humidity levels

The correct answer is: Incomplete combustion due to insufficient airflow

A yellow flame in gas appliances is primarily indicative of incomplete combustion. This occurs when there is insufficient airflow or oxygen mixing with the gas. When combustion is incomplete, the fuel (in this case, gas) cannot fully react with oxygen, leading to the formation of carbon soot, which produces a yellow color in the flame. Instead of a clean blue flame, which signifies efficient combustion and proper fuel-to-air ratios, a yellow flame suggests that the combustion process is not functioning optimally. While excess fuel usage can contribute to poor combustion quality, the critical factor is the availability of oxygen. Low airflow leads to a lack of adequate oxygen needed to fully combust the gas, which is the direct cause of the yellow flame. It is essential for gas appliances to maintain proper air intake to achieve efficient and safe operation. Thus, recognizing the role of airflow in combustion is crucial for both troubleshooting issues and ensuring the safe operation of gas-fired appliances.