A thermocouple is a type of flame safety that prevents the gas valve from dispensing the gas into the burners without the presence of a flame. These are found mostly in older standard pilot appliances including (mostly older) furnaces, water heaters, and unit heaters. A typical thermocouple will produce upwards of about 30 millivolts. To give you an understanding of how small of a voltage that is, 1,000 millivolts is 1 volt.
One end sits in the flame, and the other threaded end is connected to the gas valve. With an applied flame to the hot junction, a small voltage (millivolts) is generated to power the electromagnet coil and hold the solenoid open to allow the flow of gas to continue. If the flame goes out, the solenoid will close and stop the flow of gas as the thermocouple will not generate the amount of voltage (millivolts) to hold it open. The thermocouple must shut de-energize the coil within 90 seconds. The thermocouple will power the safety solenoid, but the main gas valve will still need its 24v circuit to operate, which in the residential and light commercial space is usually 24v.
A thermocouple has two dissimilar metals. One end is known as the hot junction, and the other is the cold junction as seen above. The hot junction sits in the pilot flame no more than a half-inch. Since the other end does not touch the flame, there’s a temperature difference between the two, and it needs to be at least 400°F. The temperature difference causes something called the Seebeck effect.
The Seebeck effect is the temperature difference between two dissimilar conductors causing a voltage difference between the two. With heat from the pilot to one of the two conductors (hot junction), heated electrons move to the cooler one (cold junction). A small amount of mV (millivolts) is generated powering the pilot solenoid valve to stay open in the gas valve
There are two ways to test a thermocouple:
Open Circuit Test
The open-circuit test will show the potential millivoltage the thermocouple can generate with no load. Since this is with no load, I frequently stick with the closed-circuit test. An open circuit test on a proper thermocouple will yield 25mV to 30mV.
- Make sure your meter can read up-to 30mV
- Connect one of the meters leads to the copper sleeve of the thermocouple, and the other lead to the end contact of the thermocouple. With the flame hitting the hot junction of the thermocouple (first half-inch), you should read 25mV to 30mV
Closed Circuit Test
The closed-circuit test shows what millivoltage the thermocouple can keep while under load. The closed-circuit test on a proper thermocouple will yield 12mV to 15mV.
The main difference between the open circuit and closed circuit test is the thermocouple needs to be connected to the gas valve for this test with a test adapter.
- Make sure your meter can read at least 15mV
- Remove the thermocouple nut connector from the gas valve. Connect the thermocouple test adapter, but do not overtighten. The adapter just needs to be snug
- Connect one of the meters leads to the copper sleeve of the thermocouple, and the other lead to the contact on the side of the test adapter. Light the pilot as usual and watch the mV generated
- 12mV to 15mV would be a reliable rating. If you read below 12mV, replace the thermocouple
A thermopile is also known as a powerpile. The thermopile is just multiple thermocouples connected in series to generate more millivoltage than you can with a single thermocouple. Most thermopiles are 10 or more thermocouples connected in series.
The most common issues with thermopile reading low millivoltage output are loose wiring connectors. With thermopile applications, you need to have good wiring connections since you’re dealing with such a small amount of voltage.
Your wiring connections need to be clean and free of corrosion.
Abnormally long wire lengths can cause an issue, but it’s uncommon to run into this issue.