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Testing a Thermocouple
First, a thermocouple is not a flame rectifier like a modern flame sensor. A thermocouple actually generates a millivolt potential difference when it is heated by a flame—just to get that out of the way for any of you newer techs who are used to modern flame sensors.
With higher efficiency gas-fired equipment being the norm for replacement systems, thermocouples and standing pilots are becoming a thing of the past. Newer appliances do not typically utilize a standing pilot, opting instead for a hot surface or spark to pilot ignition. These ignition systems have benefits over a standing pilot, from increased reliability and longevity to higher efficiency numbers. But there are many appliances in the field that still use a standing pilot, and a good service technician should be able to diagnose a thermocouple issue.
Many of you will say:
“Why even check the thermocouple? It’s a 5-dollar part, just throw a new one in!”
“Why are you so lazy? Do you even HVAC in real life or just on the internet?”
Yes, I know thermocouples are cheap, and I am all for replacing them when they need to be replaced or while replacing a gas valve or pilot assembly. But over the years, I have seen a lot of guys (me included) go on calls for pilot issues, find a pilot blown out, relight the pilot, and then because it’s the easiest, quickest fix, replace the thermocouple, only to have the same customer call in a day or two later with the pilot being out AGAIN. And when the tech goes back and relights the pilot, then what? Is that brand new thermocouple bad after a few days? Probably not. There is probably some other issue, but checking the thermocouple millivolt production is the first step for a proper diagnosis.
So, how does a thermocouple work? Well, I'm no scientist (I’m barely a writer), but I'll tell you what I know. When different metals are joined, and there is a temperature difference between them, a magnetic field occurs between the joints where the different metals meet. The heat of the pilot flame is the source of the temperature difference in a normal pilot system. Through this process, a small amount of current is produced, generally around 30 millivolts. This voltage is sensed by the gas valve and is used to keep the pilot valve internal to the main gas open. If the pilot goes out, the heat that is generating the potential (voltage) is lost; thus, the current stops flowing to the gas valve. The pilot valve is closed, closing off fuel to the pilot assembly. The thermocouple is a safety device. If the pilot flame goes and the pilot valve doesn’t close, the burner compartment and potentially the room the equipment is in can fill up with gas. The consequences of that would require a different article.
When should you check a thermocouple? I am in the habit of checking thermocouples whenever I encounter them, whether it’s on a maintenance inspection or a service call. If you are in the habit of checking them, it usually doesn’t take more than a few minutes. If the millivolt measurement is less than 26-27, I typically recommend replacement.
To check a thermocouple, you need a multimeter that can measure millivolts. It is typically shown as mV or is just the third decimal over on the DC voltage reading. Remember, the meter should be set to DC voltage.
It’s also helpful to have an extra set of hands, but it is possible to perform this check by yourself if you hold your tongue correctly (or just use alligator clips). First, disconnect the thermocouple from the gas valve. Then, light the pilot. Most gas valves have a turn knob that has to be set from On/Off to Pilot. There is usually a push-button that is pressed to manually open the pilot valve, sending gas to the pilot assembly to light the pilot. The trick is to light the pilot and position the meter leads in the proper place to read the voltage. The push-button must be depressed through the whole check. With the thermocouple being disconnected from the gas valve for checks, the pilot valve should not stay open, and the flame should go out when the push button is let up.
Put on meter lead directly on the gas valve side of the thermocouple. Put the other lead on the copper line, as shown by my right hand in the picture above. While holding the meter leads in this position, light the pilot. The thermocouple needs to heat up for 30 seconds to 1 minute in order to obtain a proper reading.
The desired reading is 30 millivolts, with a swing of +/- 5 millivolts. If the readings are in that range, and you have been having pilot failure issues, there is more than likely some other cause. A dirty pilot assembly/orifice is the most common alternative issue I encounter. However, it could be downdraft/flue or combustion air issues, fuel pressure problems, or a failing gas valve. But as stated above, the thermocouple should be eliminated as a potential issue before moving on with a proper diagnosis. Don’t throw parts at a problem and see what sticks. With thorough troubleshooting, you can save a lot of time, headaches, and maybe the customer a little bit of money and frustration.