How to Test a Thermocouple for a Water Heater – Everyone who has owned a gas heater including furnaces or water heaters will know that from to time you have to light the pilot light unless you have the electronic ignition that automatically lights the pilot light.

Sometimes when you try to light the pilot it does not want to stay lit and you know you have a problem. Often times the problem turns out to be a bad thermocouple that needs to be replaced. But how do you know if the thermocouple is bad and the problem isn’t something else?

Fluke 87-V Digital Multimeter

Honeywell CQ100A1013 24-Inch Replacement Thermocouple for Gas Furnaces, Boilers and Water Heaters

How to Test a Thermocouple for a Water Heater

How to Test a Thermocouple for a Water Heater

Thermocouple

Some may be asking what a thermocouple is? A thermocouple is a safety device used in gas heating for many years. A thermocouple is made up of two dissimilar metals. At the end of the thermocouple is a welded end which is at the tip of the thermocouple. The welded end is referred to as a hot junction. The other part of the thermocouple is not welded and this is called the cold junction. When heat is applied to the hot junction a small amount of voltage is produced at the cold junction. This small amount of voltage is measured in millivolts. One millivolt is equal to 1/1000th of a volt and the thermocouple for a gas heating appliance produces enough millivolts to keep a safety solenoid inside the gas valve open.

If the flame goes out that helps the thermocouple produce millivolts that voltage goes away and the safety valve closes. With the safety valve closed the main valve or primary valve inside the gas valve will not open. This prevents the main valve from opening and spewing un-ignited raw gas into the atmosphere where it can be potentially explosive. When the thermocouple goes bad it will not stay lit and the entire furnace will not function as a heater any longer. It is possibly a bad thermocouple or at least in the process of troubleshooting the gas furnace we surmise that the thermocouple is bad and needs to be tested.

How to Test a Thermocouple for a Water Heater – Multi-Meter and Milivolts

low fire two-stage gas furnacesTo make certain the thermocouple is bad you have to test it. For testing a thermocouple you first have to remove it from the furnace, water heater, or gas appliance where it is installed. Once it is removed you will need a multimeter that needs to have the ability to read millivolts and a source of fire. Usually, a lighter will suffice for the test. Attached alligator clips to your multimeter and attach one to the very end of the thermocouple where the connection is made to the gas valve. Attach the other end to the stem of the thermocouple.

The stem is not the tip or any part of the tip but the area between where the connection is made to the gas valve and thermocouple tip itself. Light the fire and hold the tip of the thermocouple inside the fire. As the thermocouple tip heats up you should see a response on the multimeter. Make sure the meter is set for millivolts. You read 25 millivolts all the way up to a maximum of 100 millivolts although a reading that high is unusual. Anything less than 25 millivolts and the thermocouple needs to be replaced.

Conclusion

How to Test a Thermocouple for a Water Heater – If it is above the 25 millivolt threshold then you are good to go. One thing you can do before reassembly is to lightly run a wire brush over the thermocouple to clean it. Only lightly though as it can easily be damaged. You reassemble the thermocouple and then replace it back in the pilot burner and restore everything else back to normal as it was before. How to Test a Thermocouple – Light the pilot light and everything should be good again. Good luck.

Fluke 87-V Digital Multimeter

Honeywell CQ100A1013 24-Inch Replacement Thermocouple for Gas Furnaces, Boilers and Water Heaters

High Performance HVACHow to Test a Thermocouple for a Water Heater

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