The thermocouple on your gas furnace is a heat-sensitive device that transmits a small electrical signal to the gas valve to keep it open when the pilot is on. The thermocouple lasts for 10 years or more, and when it fails, the pilot won't stay lit and the fireplace won't work. Replacing it is a relatively simple job that you can accomplish with a wrench. Before you replace it, though, it's a good idea to test it with a voltmeter to make sure it's actually malfunctioning. If it checks out, the problem with your fireplace could be a blockage in the pilot tube or a faulty gas valve.
What's a Thermocouple?
All gas appliances with standing pilots, such as furnaces and water heaters, have a thermocouple. It's a cylindrical probe about 4 inches long and a quarter to a half inch in diameter that's attached to a 1- to 2-foot length of copper tubing. The probe is positioned next to the pilot opening so that its tip is in the flame when the pilot is on. The other end of the tube connects to the gas valve by a threaded connector.
The tip of the probe contains a heat-sensitive material that generates a small electric voltage – around 25 millivolts – when it's heated by the pilot flame. The voltage sends a signal to the gas valve to stay open. Without this signal, the valve closes automatically to prevent the release of unburnt gas. When you light the pilot, you usually have to hold in the gas valve manually for about 10 seconds to give the thermocouple time to heat up. Once it has reached the required temperature, you can release the valve control and the pilot will continue burning.
Test Before You Replace
If you can't get the pilot to stay lit, the chances are good that the thermocouple is faulty, but it's best to check before you replace it. You can perform a simple test with a voltmeter. Set the meter to read millivolts, then hold one probe on the metal copper tube that leads from the gas valve to the pilot orifice and the other probe on the connector that holds the thermocouple tube to the gas valve. Have a helper light the pilot and keep the valve control depressed to heat up the thermocouple probe. About 10 seconds after the pilot starts, you'll get a reading of 25 millivolts if the thermocouple is working. If you get a smaller reading or none at all, the thermocouple needs to be replaced.
Procedure for Replacing a Thermocouple
The only tool you need to replace a thermocouple is a wrench. Use it to unscrew the connector that holds the thermocouple to the gas valve, then unhook the probe from its clip and pull out the entire thermocouple assembly. Before you do this, be sure to turn off the line valve on the gas supply.
Take the thermocouple to the hardware store to purchase a new one. Many styles are available, so look for one that matches. Once you have the new thermocouple, replace it by snapping the probe back into the clip to hold it at the proper distance from the pilot flame, then screw the other end into the gas valve. Test it by lighting the pilot and making sure the flame stays lit. If you can't get the flame to stay on, try moving the probe slightly closer to the flame until the flame does stay lit.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.