What is the role of the thermocouple in a gas furnace?

The thermocouple plays a critical role in a gas furnace by detecting the pilot flame. Its primary function is to ensure that the gas valve remains open only when there is a flame present. When the pilot light is lit, the thermocouple generates a small electrical voltage due to the heat from the flame. This voltage acts as a signal to keep the gas valve open, allowing gas to flow to the burner for combustion.

If the pilot flame goes out for any reason, the thermocouple cools down, which stops producing the electrical voltage. As a result, the gas valve closes, preventing any unburned gas from leaking into the furnace and creating a potential safety hazard. This mechanism provides a crucial safety feature, ensuring that gas is only supplied when there is a reliable ignition source, thus protecting against the risk of gas accumulation.

The other options do not accurately describe the function of a thermocouple. Igniting the gas is the role of the ignition system, temperature control is managed by a thermostat, and filtering combustion byproducts is the responsibility of a different component, such as a flue or venting system.