Which component is critical for ignition in a gas furnace?

The flame sensor is a critical component for ignition in a gas furnace because it ensures that the furnace only operates when there is a flame present. This safety device detects the presence of the flame and signals the control board to allow the gas valve to remain open, enabling the gas to flow to the burners. If a flame is not detected, the sensor will close the gas valve to prevent gas leakage and potential hazards, like explosions.

In contrast, while the heat exchanger plays a vital role in transferring heat to the air that circulates throughout the home, it does not directly contribute to ignition. The combustion fan is responsible for supplying airflow necessary for combustion but does not initiate the process. The thermocouple is primarily used in older systems for safety by shutting off gas if the pilot light goes out, but it is not specifically involved in the ignition phase of the furnace operation. Thus, the flame sensor's functionality is essential for both safety and the proper operation of the gas furnace's ignition system.