What Is Thermocoupling on a Gas Fireplace?

Gas Fireplace Thermocouple

• The thermocouple on a gas fireplace is essentially a probe that is connected to the pilot light flame. The thermocouple has two different metals connected to a wire that extends from the probe all the way to the gas valve and sends a low electric signal to the gas valve. The signal maintains the gas supply from the gas valve by opening an electromagnet inside the gas valve, which stays open when the charge is present. This function prevents the pilot button from resetting, which will happen if the thermocouple fails and the gas supply shuts off.

Thermocouple Heating

• To work properly, the thermocouple must be heated before the electric signal will pass to the gas valve, allowing the pilot burner to ignite. This is typically performed by holding in the gas control valve for 30 seconds or more once the pilot light is lit. The pilot then creates enough energy for the charge to reach the gas valve, which then stays open until the fireplace is turned off.

Thermocouple Problems

• Symptoms of a gas fireplace thermocouple problem include a popped pilot button or a pilot light that does not stay lit. This indicates that the probe on the thermocouple is no longer working as expected and needs to be replaced.

Troubleshooting

• If the connection between the thermocouple and the gas valve is dirty, the pilot light may not stay lit because the charge is not maintained. Find the connecting nut between the gas valve and the thermocouple and clean with a pencil eraser, then tighten the connection. Another possible problem is the pilot light, which should consume the top 5/8 inch of the thermocouple with the flame when operating correctly. A low flame can typically be corrected by turning the pilot flame adjustment valve in a clockwise direction to increase the pilot flame size. Clean the pilot hood and orifice when the fireplace is completely cool to remove soot that may be blocking the pilot light burner. Never attempt to correct the thermocouple, pilot light or clean the fireplace components if you are not confident in your ability to do so. Contact a qualified professional instead.