Which device disconnects voltage to the gas valve if the furnace overheats?

The device that disconnects voltage to the gas valve when the furnace overheats is the high-limit cut-out. This safety feature is designed to prevent the furnace from reaching dangerously high temperatures, which could potentially lead to equipment damage or a fire hazard.

When the temperature inside the furnace exceeds a predefined threshold, the high-limit cut-out activates, interrupting the power supply to the gas valve. This action stops the flow of gas to the burners, thereby preventing further combustion and reducing the risk of overheating.

While the limit switch and thermostat also play roles in the overall operation and safety of the heating system, they do not specifically cut off gas flow based solely on overheating conditions like the high-limit cut-out does. A blower relay primarily controls the operation of the furnace blower fan rather than managing the high-temperature situations that the high-limit cut-out regulates.