How To Test The Thermocouple In A Millivolt Fireplace
Your pilot light has gone out and will not stay lit? Chances are, you are looking at a bad thermocouple. The thermocouple generates an electrical current from the pilot flame. The thermocouple typically produces 20-30 DC millivolts that are sent to the gas valve, keeping the pilot solenoid open.
Tools needed:
- Multimeter to read DC milivolts
- 11/32" wrench (or M8 wrench for vent-free systems)
Before Testing
To begin testing, remove the thermocouple from the gas valve. Look for corrosion at the end of the thermocouple bulb and at the mating surface of the gas valve.
Clean Thermocouple Bulb
The thermocouple bulb can be cleaned by scraping a thin layer of the surface away.
Setting Up The Multimeter
Once the thermocouple has been removed, set your multimeter to DC mV.
- Ensure the pilot flame is lit during testing
- Manually depress the pilot knob to maintain gas flow with the thermocouple removed
- Use a second set of hands or alligator clips to hold the multimeter leads in place
Testing The Thermocouple
Place the black lead of your meter on the thermocouple bulb and place the red lead on the thermocouple wire. Once your leads are in place, light the pilot and wait 30-45 seconds for the thermocouple to heat up. You will see the millivolt readings displayed on the multimeter. We typically look for a reading of 17 mV or greater
Troubleshooting A Pilot Light Problem:
Is the pilot flame contacting the thermocouple tip?
The pilot flame must contact only the top end of the thermocouple. The temperature difference between the tip and the base generates the current. Clean the pilot orifice to ensure there is no flame impingement.
Is the mating surface between the thermocouple and the valve clean?
Corrosion or debris can cause a bad thermocouple connection. This will not allow the proper voltage to the valve. Clean the thermocouple and the mating surface of the valve.
Are there any inline limit switches?
An inline limit switch must be tested to ensure it is not stuck closed. Test the switch’s resistance to confirm it is open.
Is the pilot bracket free of rust and corrosion?
The pilot bracket helps dissipate heat from the thermocouple base. This aids in voltage production. If the thermocouple tests bad with a corroded bracket, replace the pilot assembly.
Next Steps:
If your thermocouple tested in the normal range, it's time to move on to the gas valve.
