Water Heater Repair Tutorial: Thermocouples, Piezos and Pilot Lights

In the third article in this four-part series, the author examines the testing of thermocouples, piezos and pilot light issues

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One of the most basic skills a plumber should have is repairing a tank-type water heater.

In the first two articles of this series, I covered initial troubleshooting steps and specifics on diagnosing whether something is a water tank problem or a gas control issue. In this article, I’ll be covering the testing of thermocouples, piezos and pilot light issues.

Pilot won’t light?

Is the knob depressing all the way in the pilot position? Is there no spark? Is the pilot tubing broken or bent to the point of no return? Is the piezo ignitor not functioning properly? Does the pilot light, but then won’t stay lit? 

When in doubt about the ignition chamber, remove the ignition assembly, clean it, reinstall it and check again. If it still won’t light, replace the ignitor. If the pilot light goes out when you release the pilot button (and you are certain you have accurate gas pressure), replace the thermocouple. In this last case of the pilot not staying lit, it usually means there is something defective with the thermocouple or there is an intermittent bad connection with the thermocouple and the gas valve. In my experience, it’s best to replace and retest operation.

If the pilot light comes on and stays on but the main burner never comes, again start with the basics. Is the gas valve thermostat setting too low to even call for hot water? Do you have enough gas coming to the unit? Then the issue probably is the gas valve, but we need to run the next few tests before making that determination.

In either case, you need to test the thermocouple.

Testing the thermocouple

In order to test the operation of a thermocouple, you have to understand what is going on here. The tip of the thermocouple in the fire is one kind of metal, and the portion of the thermocouple that is screwed into the gas valve is a different kind of metal. The two are connected with a small piece of copper. Because they are dissimilar metals, a small millivolt current runs from the tip to the connection, which tells the gas valve to turn on. 

The correct way of testing a thermocouple is “closed circuit testing,” which means testing it while it’s in operation. Let the unit fire for a few minutes. If it stays lit, use your multimeter with gator clamps and set to millivolts. You want to clamp one probe on the back of the combination gas valve on the top terminal, and you want the other probe only on the copper portion of the thermocouple. If the reading is 10 millivolts DC or higher, the thermocouple is OK. If the reading is lower than 10 millivolts DC, the thermocouple is defective (for some Honeywell gas valves). If you can’t get the pilot light to stay lit, then unfortunately you will have to hold the pilot light down during the test in order to get the reading. That said, there are many different manufacturers and types of thermocouples that have their own charts for acceptable millivolt readings with variables. This is only one example.

If you want to try and test the thermocouple using the “open circuit” method, make sure the flame stays on and remove the red wire from the thermal switch. Set your meter to millivolts, then connect the positive side of your meter to the switch and the negative side of your meter to the jacket base of the water heater (the silver base, preferably on one of the screws down there). Normal thermopile operation is between 350mV – 850 mV. If your readings are outside this range, replace the pilot assembly.

The piezo

The piezo is a spark maker. This assembly runs from the spark button down into the burner assembly and provides a spark to ignite the gas. If you do not see a spark in the sight glass after pressing the button many times, it is time to check the piezo.

Remove the wire from the piezo button and connect one end of a jumper wire in its place. Hold the other end of the jumper wire onto bare metal and press the spark button. If you do not get a spark, replace the piezo.

An important note: You don’t need gas to visually see the ignitor sparking in the sight glass. Sometimes a tech will hold the pilot gas button down the entire time not getting a spark. In a streak of bad luck, the sealed combustion chamber is now completely filled up with gas and has oxygen entering the bottom of the water heater, and then the spark will work halfheartedly making a giant boom and singe your eyebrows off or worse. Keep that in mind.

The final installment of this series will cover temperature sensors, erratic combustion, and thermal switches.

About the Author

Anthony Pacilla is a registered master plumber for McVehil Plumbing in Washington, Pennsylvania. He has 22 years of experience in the plumbing and HVAC trades, and has a bachelor’s in business and economics from Thiel College. 


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