When should the thermocouple in a standing pilot furnace be replaced?

The correct choice emphasizes the significance of millivolt readings in evaluating the performance of a standing pilot furnace's thermocouple. A thermocouple generates a small voltage (millivolts) as it converts temperature differences into electrical energy. This generated voltage ensures the gas valve remains open, allowing the pilot light to function correctly.

When the millivolt reading drops to 22mV or lower, it indicates a weakening thermoelectric connection, suggesting that the thermocouple may not be generating enough voltage to keep the gas valve open. This can lead to the pilot light extinguishing frequently, posing a risk for safety and efficiency. Therefore, replacing the thermocouple at this point prevents potential operational issues related to pilot light stability and furnace performance.

The other options are based on misinterpretations of operational indicators. For instance, waiting for the pilot light to go out before replacing the thermocouple might lead to repeated service issues and unsafe conditions rather than addressing the problem proactively. Unusual noises from the furnace could be entirely unrelated to the thermocouple's functionality, as such noises could stem from other components. Lastly, high gas manifold pressure is a separate issue that does not directly correlate with the health of the thermocouple and requires different