The thermocouple on your gas fireplace is a heat-sensitive device that keeps the pilot gas tube open while the pilot is lit. It does this by generating a small electric current -- on the order of 10 to 30 millivolts -- as a signal to the gas valve. Thermocouples wear out and are replaceable, but you should conduct a few simple tests before doing that, because your gas pilot may be going out for another reason. The thermopile is a related device that you should test at the same time -- its function is to open the gas to the burners.
Check the Pilot
Locate the pilot, often behind a removable grill running along the bottom of the fireplace. Light the pilot using the recommended procedure for your fireplace. The usual procedure is to turn the gas valve to "Pilot," push in the knob and depress the red ignitor button until the pilot lights. If you have an older fireplace, you may have to light the pilot with a long match.
Hold the knob in for 60 seconds, then release it. If the thermocouple is working, the pilot should remain lit. If the pilot goes out, relight it and check the flame. It should be about an inch in length and predominantly bluish in color, and it should be in contact with the tip of the thermocouple.
Clean the pilot tube with compressed air, following the manufacturer's instructions, if the pilot flame is small or intermittent. Move the thermocouple tip closer to the flame if it was not within the flame's reach with the pilot was lit. Relight the pilot and try the test again.
Test the thermocouple with a multimeter if the pilot goes out again. Set the meter to measure millivolts and, to make measurement easier, fit the leads with alligator clips.
Locate the gas valve -- it's usually behind a grill on the front of the fireplace -- and find the copper thermocouple wire leading from it. The best way to do this is to follow the wire back from the tip of the thermocouple, which is right next to the pilot light. Unscrew the thermocouple fitting and remove it from the valve, using a 7/16-inch wrench. Leave the pilot end of the thermocouple in place.
Attach one lead of the multimeter to the silvered end of the thermocouple wire (the part that made contact with the gas valve) and the other lead to the bare copper thermocouple wire. Light the pilot, make sure the flame is in contact with the thermocouple tip and hold in the gas valve for 60 seconds. You should get a meter reading of 20 to 30 millivolts (or within the low range of the thermocouple's rating; voltages vary by product). If the reading is less than 20 millivolts, it's time to replace the thermocouple.
Things You Will Need
- Long match
- Can of compressed air
- Multimeter with alligator clips
- 7/16-inch wrench
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