Thermocouple — pilot-flame safety sensor for gas heat

A thermocouple is a flame-sensing safety device that uses heat from a pilot flame to keep the gas valve open in certain appliances.

Thermocouple diagram — labeled parts, dimensions, and installation context

What It Is

A thermocouple is a small metal probe connected to a gas control valve by a narrow tube or threaded lead. When the pilot flame heats the probe tip, it generates a tiny electrical signal that tells the gas valve the flame is present.

If the pilot goes out, the signal stops and the gas valve closes. That simple design prevents unburned gas from continuing to flow into the room.

Types

The main residential versions are standard standing-pilot thermocouples and universal replacement thermocouples in different lengths. They are used in older or simple gas appliances that rely on a pilot rather than electronic ignition.

Where It Is Used

Thermocouples are used in standing-pilot water heaters, wall heaters, older furnaces, gas fireplaces, and some gas ranges or patio heaters. They are not used on every gas appliance because many newer systems use flame rectification sensors instead.

How to Identify One

Look for a small metal probe positioned directly in the pilot flame next to the pilot tube. It usually connects back to the gas valve with a thin copper-colored line.

Replacement

Replacement is common when the pilot will light but will not stay lit after the control knob is released. The new thermocouple has to be the correct length and must sit properly in the pilot flame for reliable operation.

Thermocouple — FAQ

What are the signs of a bad thermocouple?

The classic symptom is a pilot flame that lights briefly but goes out as soon as you release the control knob. Dirt in the pilot assembly or a weak pilot flame can cause the same symptom, so the whole pilot system should be checked.

Can a thermocouple be cleaned instead of replaced?

Sometimes light oxidation can be cleaned off the probe tip, and that may restore operation for a while. If the probe is worn out or the signal is weak, replacement is usually the lasting fix.

Is a thermocouple used with electronic ignition?

Usually no. Many modern appliances with electronic ignition use a separate flame sensor or integrated control board instead of a standing-pilot thermocouple.

Why does the pilot still go out after replacing the thermocouple?

The pilot flame may be too small, the new probe may be mispositioned, or the gas valve itself may be failing. Venting problems and dirty pilot orifices can also interfere with stable flame sensing.