What is the primary function of the limit switch in a furnace?

The primary function of the limit switch in a furnace is to de-energize the gas valve if the furnace overheats. This safety component is designed to protect the system from excessive temperature, which can lead to dangerous conditions such as overheating and potential fire hazards. When the temperature reaches a predetermined limit, the limit switch interrupts the electrical circuit to the gas valve, effectively shutting off the gas supply and stopping combustion within the furnace. This action prevents damage to the furnace and enhances safety for occupants by reducing the risk of potential incidents.

The other options address functions that, while important in the operation of a furnace, do not pertain specifically to the primary purpose of the limit switch. For example, controlling blower motor speed and regulating airflow are governed by different components, such as fan speed controls and thermostats, and monitoring the temperature of the flue gas is typically the function of a different type of sensor or device. Thus, the limit switch uniquely serves the crucial role of ensuring safety by responding to overheating conditions.