Which component prevents the heater from operating when airflow is inadequate?

The limit switch is a critical safety device in gas furnaces designed to monitor the temperature of the heat exchanger. Its primary function is to prevent the heater from operating when there is insufficient airflow, which could lead to overheating. When airflow is inadequate, the temperature within the heat exchanger rises beyond safe levels. The limit switch senses this increased temperature and interrupts power to the furnace to prevent potential damage or hazardous conditions, such as fires or equipment failures.

This safety feature ensures that the furnace operates efficiently and within safe temperature limits, protecting both the equipment and the home. Without the limit switch, a furnace could continue running with restricted airflow, posing serious risks to both safety and system integrity.

In contrast, the thermostat is responsible for regulating the desired temperature in a space by turning the furnace on or off based on the set temperature. The espresso switch appears to be a misnomer or irrelevant term in this context, as it does not pertain to gas furnace operations. The return air duct is vital in facilitating airflow back to the furnace but does not serve a safety function when airflow is inadequate. Thus, it is the limit switch that plays the crucial role of ensuring safe operation by preventing the furnace from running under insufficient airflow conditions.