Under what condition should a thermopile be replaced?

A thermopile should be replaced when it fails to produce sufficient voltage because this indicates that it is not functioning effectively to generate the necessary voltage for the operation of the gas valve and other crucial components in the heating system. The thermopile is designed to convert temperature into electrical energy and produce a specific voltage range. If it cannot produce this voltage, it will not reliably control the gas flow or maintain safe furnace operation, leading to potential overheating or unsafe conditions.

While visible damage and high temperatures can be concerning, the primary indicator of a thermopile's performance is its voltage output. A noisy furnace might suggest other issues in the system unrelated to the thermopile's performance, such as airflow restrictions or mechanical problems. Therefore, inadequate voltage production is the most definitive reason for needing to replace a thermopile, ensuring the system operates safely and efficiently.