What is Gas Thermocouple?

A Gas Thermocouple is a safety device that controls the gas valve of a gas-fired furnace, water heater or other appliance. It shuts off the gas if the pilot light goes out or the electric igniter malfunctions, preventing a fire from starting.

Thermocouples operate through a thermoelectric effect known as the “Seebeck effect.” Two pieces of metal in proximity to each other generate electrical voltage in proportion to their heat. This means that when a thermocouple senses heat from a pilot flame, it creates a difference in the voltage between its two pieces.

When the pilot goes out, the thermocouple produces a low voltage, which shuts off the gas valve. This can be caused by a number of issues, including a dirty or defective thermocouple.

Typically a thermocouple mounts on the end of a pilot flame tubing that is attached to a gas control valve at the bottom of a gas fired water heater or furnace. It connects to the bottom of the valve using a right-hand threaded connector.

Replacing a Thermocouple

When removing a damaged thermocouple from a water heater, furnace or another gas controlled appliance, you will need to remove the manifold door or flame shield at the bottom of the gas control valve to access it. Then unscrew the mounting bolt to release its connector.

Once removed, inspect the thermocouple for damage and visually check the connectors to make sure they are seated properly. If you find any physical damage, such as a crimp or nick on the connector, then you will need to replace it.