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Thermocouples on heating appliances: this article describes thermocouples, safety devices used on gas fired heating equipment.
This article explains what a thermocouple (or thermopile) is, how these devices work, where they are installed, and what goes wrong with thermocouples.
We describe how to find the thermocouple if one is used on your heater, and how the thermocouple is replaced.
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A thermocouple is an electrical device that responds to temperature change by a change in voltage output. A thermocouple works to produce a small electrical voltage output by connecting two dissimilar metals. When heated the effect of the two dissimilar metals in contact with one another is the production of an electrical current.
This voltage in turn can be used to cause a gas valve to open or remain open, or to close, stopping the supply of LP or natural gas fuel should a pilot light or gas flame go out on a heating appliance.
Shutting down the gas supply to a heating appliance protects against a gas fire or explosion that could occur if a gas regulator valve remain open without proper ignition of the gas flame.
In general we think that thermocouples are less accurate and less sensitive temperature sensors than thermistors, but these low-cost and reliable temperature sensing devices have been used successfully in heating equipment such as gas fired furnaces, boilers, and water heaters for decades. Thermocouples are also used on gas logs and in gas fireplaces or similar devices.
Thermocouples are produced in a wide range of forms and configurations, in eight or more calibration groups (B,E, J, K, R, S & T) with different temperature ranges including up to very high temperatures such as 3000 oF.
Don't confuse a therm couple (discussed here) with a different type of temperature sensor, solid-state THERMISTORS used in thermostats. Our sketch at above left, illustrating a typical use of a thermopile, a type of thermocouple, in use at a gas boiler, is adapted from Weil McLain.
Where are Thermocouples Used on Heating Equipment?
Thermocouples are used as safety devices that will shut off equipment by shutting down the LP or natural gas fuel supply on some gas-fired heating equipment such as gas fired furnaces, gas fired heating boilers, and gas fired water heaters.
Typically the thermocouple sensor is mounted right in the flame of the pilot light on gas fired heating equipment.
Our photo (left) shows how you may spot the copper tubing of the thermocouple extending between its sensor at the pilot flame of a gas burning furnace and its connection to the gas control valve. [Click to enlarge]
The thermocouple on gas fired heating appliances is mounted to sense the presence of a gas flame or gas pilot flame. The other end of the thermocouple's tubing connects to a port on the gas regulator or gas valve
But not all of these systems use a thermocouple. Some gas fired heating equipment relies on an electronic ignition to ignite the flame. Those devices generally will not use a thermocouple.
If a thermocouple is used you'll see a small copper tube (or in some devices an electrical wire) connecting the flame sensor to the valve.
What's the Difference Between a Thermopile and a Thermocouple?
Not much for practical purposes, except that when replacing a thermocouple on your heating appliance you should be sure to purchase the proper part. A thermopile looks like a thermocouple, and does the same job - sensing a gas flame to function as a safety device.
Thermopiles are made by combining multiple thermocouples together in order to produce more electrical current than a basic thermocouple. Externally a thermopile still looks like a single sensing device.
At THERMISTORS we explain the differences among a thermocouple, thermopile and thermistor in more detail. There we note that millivolt thermopiles are used in lieu of a simpler thermocouple when the device needs to operate a thermostat as well as the gas valve.
Thermocouple Selection & Installation
The manufacturer of the heating appliance in which the thermocouple is used will provide installation instructions that must be followed for safe, reliable use of the heating appliance. Follow the instructions in the manual or guide for your heating appliance and also review the installation instructions and bending instructions that came with a replacement thermocouple.
Some of the photos used here are adapted from detailed thermocouple installation instructions provided by American Water Heater provides at http://www.americanwaterheater.com/support/manuals/nat_tc_instr.pdf 
The connecting copper tubing length for a thermocouple is not usually critical, but the tubing must be long enough to reach without stress from the connection at the gas valve to the thermocouple's sensor mount in the gas flame or pilot flame.
The thermocouple is provided with the connecting tubing coiled neatly in a package. Don't be afraid to un-coil the tubing into a more straight line to ease installation of the device. But do not nick, kink, nor make sharp bends in the tubing. Typical thermocouple installations include instructions for bending the tubing to avoid sharp bends or nicks.
Watch out: don't modify a thermocouple (other than gentle bending as described in the installation instructions). For example do not try to cut or trim the length of the thermocouple. Doing so will almost certainly make it inoperative and thus unsafe. If your gas fired equipment also uses an igniter wire in addition to the thermocouple, they are often routed together and will need to be removed for thermocouple replacement. Take care not to damage the igniter wire and to reconnect it just as it was.
Typical Thermocouple Installation Steps
Installation steps for installing a replacement thermocouple are simple, and are made easier by looking carefully at how and where the old thermocouple was installed and where and how its tubing was routed between the gas valve and the flame sensor position. You can make your job easier by shutting off the gas supply and removing the old thermocouple intact to help select a replacement model.
Replacement thermocouples using "universal mounting" include clips and fittings that will work on most gas fired heaters and water heaters. For example Sid Harvey's Dyna-Couple universal mounting thermocouple, sold in lengths from 18" to 48" will replace Honeywell, Robertshaw, Penn Baso, White-Rodgers and other thermocouples.
How to Troubleshooting Thermocouples on Gas Fired Heating Equipment
First confirm that the problem is the thermocouple
Typically the gas control is pushed-in or held in a spring-loaded position to force gas through the pilot light assembly to permit manually lighting the pilot flame.
If you are able to light the pilot on the gas fired appliance but when you release the gas control from it's "LIGHT" position the pilot immediately goes out, if there was a good solid flame that was clearly touching and heating the thermocouple, that is, the thermocouple was properly positioned in the pilot flame, and if you are sure that you held the control in the LIGHT position long enough for the thermocouple to heat up normally (30 seconds is plenty), then I suspect that the thermocouple is defective.
The two most common defects I have seen that give trouble with a thermocouple are
Check the gas flow from source through the pilot light orifice before replacing a thermocouple.
Watch out: Before trying a new thermocouple I'd suggest checking for debris in the pilot light orifice or tubing. In gas fired equipment that remains shut down for long intervals we sometimes find spiders or insects have nested in the equipment, even mud-dauber wasps, blocking proper gas flow or gas appliance venting.
Photo at left: red arrows indicate the thermocouple tubing and assembly; blue arrows indicate the pilot light tubing & assembly on a horribly rusted Holland brand gas furnace.
We have run into this problem and also read other accounts of it concerning the Gaffers and Sattler
Model S 80 FDF gas fired furnace and similar gas fired heating equipment but this debris clog problem is widespread and may show up on just about any pilot-lit gas fired appliance, even gas log fireplaces and portable heaters.
Check for debris blockage right at the pilot light, inside the gas tube between the pilot light and the gas control valve, and using a fine wire (don't scratch or enlarge the orifice) try cleaning the orifice at the pilot light itself.
Reader Question: Question: why does the pilot flame keep going out on my hot air furnace?
What would cause standing pilot flame to keep going out . thermocouple was replaced on my air furnace - Bruce 11/17/12
Reply: checklist for thermocouple problems
Bruce, Check for these other causes of loss of the gas pilot flame
What if there is NO Constant-On Pilot Light? Direct-Ignition Gas Fired Heaters
Many modern gas fired heating appliances, boilers, furnaces, water heaters, use an electronic ignition or spark to light the pilot.
On these appliances there may still be a thermocouple to confirm that there is a good gas flame when the burner is on - since we don't want to continue supplying gas if there is no flame (doing so risks an explosion).
If your appliance has a flexible electrical wire that connects to a sparking device at the burner or burner pilot, the appliance has no continuously-on pilot - you do not have to light the pilot manually.
In our photo at far left the yellow and blue wires is the safety sensors & direct ignition wiring connected to an igniter that will be found at the pilot light.
In addition to finding electrical wires leading to an igniter at the gas burner or gas pilot, you will also see a tag such as that shown on this gas valve on an A.O. Smith gas fired Direct Igntiion water heater (photo at close left).
Continue reading at GAS BURNER FLAME & NOISE DEFECTS or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
Why does the thermocouple on our boiler keep failing?
We have a 1907 rowhouse with a basement gas boiler fueling hot water radiators on the two floors above.
The boiler, a Utica Model 4GB-100, keeps blowing the thermocouple that senses the gas flame (one or two each heating season for the past six or seven years).
Now that we know what to expect, we don't mind replacing the dratted things periodically, but we wonder if this signals something we should be worried about. The long-term future of the heating system is not a big issue, since we likely will not own the house much longer. Our concern involves the living creatures inside it.
We don't mind paying you a reasonable fee for guidance/suggestions. But we're hoping you don't tell us to call in a local professional. If the experiences of our neighbors are any indication, no one in our area wants to deal with these old "inefficient" systems, only to sell us a brand new one (preferably one that requires moving the boiler to the opposite end of the basement, thus requiring lots of pipes to be relocated).
We appreciate any advice or information you can supply. If you need more information, please don't hesitate to write or holler at us. - K.L. 10/23/2013 Oh, and we LOVE InspectAPedia.
Reply: things to check for repeat failures of a thermocouple or thermopile
A bit of detail might help me help you. Here are some diagnostic questions: Over what time have how many thermocouples failed? Give me a rough calendar.
Can you tell me the brand and part number you've installed?
Can you give me some sharp photos of the heating system - both in macro that I can see the heater's general environment and close-up of the control installation? I particularly want to see a photo of the position of the thermocouple sensor in the actual gas flame. Have you bought all of the replacement parts at the same place?
Did you see anything interesting on the old thermocouples like a kink, sharp bend, mechanical damage?
And no, there is no fee for my consulting as long as it doesn't become too consuming for me. Utica gas boiler information for your product series is at http://www.uticaboilers.com/products_gasboilers_mgb.asp
Take a look at the installation instructions at THERMOCOUPLES.
Also take a look at page 27 in this Utica MBG gas boiler installation manual http://www.ecrinternational.com/secure/upload/document/2783.pdf
at the pilot flame diagnostic drawing where you will see that placement of the thermocouple properly in the flame is important: let's also be sure that the problem is the thermocouple and not its position or the pilot flame.
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