When you fire up your home's furnace, you likely don't think about all the elements that provide that heat. One essential feature inside every gas boiler is a thermocouple, which serves as a safety. If your pilot light goes out, dangerous gas could enter your house without the thermocouple, which shuts off gas to the main burner when no flame is present. To test the boiler thermocouple, you'll need a multimeter. You'll look at the thermocouple first, then attach the multimeter and note the voltage ranges using the following process.

One hand adjust thermostat valve
credit: SonjaBK/iStock/GettyImages
How to Test a Boiler Thermocouple

Conduct a Visual Inspection

Before you begin work on your furnace, make sure you've turned the gas off using the switch in the gas line leading to the furnace. Make sure the pilot light is out, and allow several minutes for the unit to cool before beginning your work. Then, conduct a visual inspection of the thermocouple. It's located where the flame of the pilot light would be. Look for any discoloration or cracks that might indicate you need to replace the thermocouple. Also, look at nearby wiring and connectors, and make sure they appear undamaged. Once you've verified that all looks okay, move to the next phase of the test.

Attach the Multimeter

To test your thermocouple's voltage, you'll need a multimeter that can read millivolts. You'll then unscrew the bolts holding the thermocouple in place and remove it from the unit. Turn your multimeter on, setting it to ohms, and connect the black lead to the copper of the thermocouple. Hook the red lead to the spot where the thermocouple touches the pilot light. With a match or lighter, apply heat to the thermocouple, and you'll begin to see numbers on the multimeter's screen.

Note the Voltage Ranges

You're looking for voltage between 25 and 30 millimeters. If your reading falls within that range, you likely have another issue. However, if your reading is below 25, it's time to replace the thermocouple. You can purchase a replacement at a hardware store, but you'll need all the details of your furnace before going, since thermocouples are specific to makes and models.

Troubleshooting and repairing a gas furnace can seem intimidating at first, but you can test and replace a thermocouple fairly easily. If you don't feel comfortable working on your own furnace, contact a local heat and air repair service. However, the more steps you can take to diagnose ahead of time, the more you'll save on repairs, since technicians will be able to go straight to the problem upon arrival.