What safety device is typically used to prevent overheating in gas furnaces?

The temperature limit switch is a crucial safety device in gas furnaces, designed specifically to prevent overheating. This device monitors the temperature within the furnace and is programmed to interrupt the operation of the furnace if temperatures exceed a safe threshold. When the temperature limit switch detects that the furnace is running too hot, it will shut off the burner, stopping the combustion process to ensure that the heat exchanger and other components do not become damaged or pose a fire risk.

In contrast, the other options serve different functions. A pressure switch is employed to monitor the pressure of the gas supply or the airflow through the unit, ensuring that conditions are safe for operation. A flame sensor detects whether a flame is present during combustion, providing a safety mechanism to shut off the gas supply if the flame goes out. The gas valve controls the flow of gas to the burner but does not specifically prevent overheating; it's more focused on regulating gas supply for combustion. Thus, the temperature limit switch's role as a preventive measure against overheating makes it the most appropriate choice in this context.