Like the other component parts in your furnace, the thermocouple can wear down over time, producing lower voltage than it should when heated. And the worst part is that you can have a bad thermocouple without even knowing.
Therefore, inspecting and testing your thermocouple should be part of your furnace maintenance. Be sure to inspect before you test, however, to ensure that there are no obvious problems that could affect the readings from testing!
How Does a Thermocouple Work?
The thermocouple is a small electrical device, but it is a critical safety component on your furnace. The thermocouple responds to changes in temperature by producing an electrical current that causes the gas valve that supplies the pilot light to open when the temperature is high or to close when there is no direct heat source.
How to Inspect Your Furnace’s Thermocouple
You will need a wrench, multi-meter, and a flame source, like a candle or a lighter, to perform the test.
Step 1: Inspect the thermocouple
What does a thermocouple look like and how do you find it? Your furnace’s thermocouple is usually located right in the flame of the furnace’s pilot light. Its copper tubing makes it easy to spot.
The thermocouple is made up of a tube, a bracket, and wires. The tube sits above the bracket, a nut holds the bracket and wires in place, and underneath the bracket, you will see the copper lead wires that connect to the gas valve on the furnace.
Some thermocouples will look slightly different, so check your furnace manual.
Failed Thermocouple Symptoms
Once you have located the thermocouple, do a visual inspection. You are looking for a few things：
The first is signs of contamination on the tube, which can include discoloration, cracks, or pinholes.
Next, check the wiring for any signs of wear or corrosion like missing insulation or bare wire.
Finally, visually inspect the connectors for physical damage because a faulty connector can affect the reliability of the test reading.
If you cannot see or detect problems proceed with the test.
Step 2: Open circuit test of the thermocouple
Before the test, turn off the gas supply because you must first remove the thermocouple.
Remove the thermocouple by unscrewing the copper lead and connection nut (first) and then the bracket nuts.
Next, take your meter and set it to ohms. Take the two leads from the meter and touch them—the meter should read zero. Once this check is done, turn the meter back to volts.
For the actual test, turn on your flame source, and place the tip of the thermocouple into the flame, leaving it there until it is quite hot.
Next, attach the leads from the multi-meter to the thermocouple: put one on the side of the thermocouple, and attach the other lead at the end of the thermocouple that sits in the pilot light.
A working thermocouple will give a reading of between 25 and 30 millimeters. If the reading is less than 25 millimeters, it should be replaced.
Post time: Dec-17-2020