What safety device is used to prevent overheating in a gas furnace?

The limit switch is a critical safety device in a gas furnace designed to prevent overheating. Its primary function is to monitor the temperature within the furnace and ensure it does not exceed a predetermined safe level. When the furnace reaches this set temperature, the limit switch automatically interrupts the power to the burner, shutting it down to prevent excessive heat buildup, which could pose a fire hazard or cause damage to the furnace components.

In addition to maintaining optimal temperatures, the limit switch also serves as a backup safety measure, providing an essential layer of protection for both the system and the home. If the furnace malfunctions, such as a failure in the airflow due to a blocked vent or malfunctioning blower, the limit switch will activate to prevent overheating, thereby enhancing overall safety.

Thermostats, blower motors, and gas valves play important roles in the operation of the furnace—they control temperature settings, manage airflow, and regulate gas flow, respectively—but they do not specifically function to prevent overheating in the way that a limit switch does. Thus, the limit switch is the correct answer as it directly addresses the safety concern of overheating in gas furnaces.