Thermocouples: Definition & Uses in Gas Fired Heating Equipment
What is a Thermocouple & Where are They Used on Heating Equipment?
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 or two wires forming a measurement junction at the sensor tip of the device. 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.
A thermocouple can also serve as a process controller or in other applications outlined at THERMOCOUPLE TYPES
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 thermocouple (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.
The thermocouple and safety shutoff do double duty, since on burners that use a pilot flame the thermocouple senses the pilot flame and won't permit the
gas valve to open if the pilot is not lit.
A bad thermocouple itself can prevent a gas furnace or boiler from working - if you can light the flame at the pilot manually but then the flame goes out when you release the manual gas
feed valve, the thermocouple is probably bad.
What's the Difference Between a Thermopile and a Thermocouple?
A thermopile looks like a thermocouple, and does a similar job - sensing temperature, such as a gas flame to function as a safety device. But thermopiles sense temperature without having to contact the object being monitored.
Watch out: thermopiles and thermocouples are not interchangeable. When replacing a temperature sensing device like a thermocouple or thermopile, you should be sure to purchase the proper part.
Thermopiles are made by combining multiple thermocouples together in series in order to produce more electrical current than a basic thermocouple. Externally a thermopile still looks like a single sensing device.
Thermopiles are used to measure temperature without having to contact the object being measured.
A thermocouple reliles on direct contact between its sensor tip and the object whose temperature is being measured.
A thermopile relies on its ability to absorb infrared radiation (IR) emitted from the surface of the object being measured. No direct contact is needed. The sensor surface of a thermopile is usually coated with a "black body" surface to improve its IR absorption. - (Thermometrics Corporation 2017)
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
Watch out: Proper selection and installation of a thermocouple is important for the device to function safely, or for that matter to function at all.
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.
Turn off the gas supply to the equipment. Locate the gas supply control valve, or on a water heater, the combination gas control valve and thermostat and turn gas off there. On a gas line valve the "off" position is with the handle at right angles to the gas pipe. On a gas fired water heater the gas control valve handle will have an OFF position that is aligned with a mark on the body of the gas valve.
Remove access covers as needed: on a gas fired water heater you will need to remove the manifold door at the heater bottom in order to access the end of the thermocouple that mounts on and monitors the pilot flame.
On a gas-fired furnace you may need to remove first the furnace cover and then a flame shield. Keep any mounting screws for re-use.
Remove the old thermocouple at the gas valve by unscrewing its mounting bolt from the gas valve. Typically a thermocouple connects to the bottom of a gas valve using "right-hand" threads.
Watch out: DO NOT FORCE any fittings or you may break something.
You should find that you turn the screw clockwise to loosen a right-hand-threaded connector. The wrench size is typically 7/16". Pull down gently to pull the thermocouple's end out of the mounting well on the gas valve.
Remove the other end of old thermocouple sensor at its mount at the burner or pilot flame.
Typically there is a clip that holds the thermocouple to the pilot flame tube.
Before taking this clip off, take note of where the clip was located on the larger-diameter segment of the copper thermocouple tubing, as you'll want to clip the replacement unit in the same location.
Keep all of the old sensor mounting parts. On occasion you may need one or more of these to properly secure the new thermocouple sensor in place.
Prepare & bend the new thermocouple tubing: gently un-roll the new thermocouple tubing.
To make necessary bends to route the thermocouple into position between the pilot-sensing end and its screw-in connection to the gas valve, use the bending template in the installation instructions.
If you don't have bending instructions, use a round object 2-3" in diameter and keep bends gentle, avoiding crimping or squashing the tubing.
Route the replacement thermocouple into place in the same location and using the same routing as the old one that you removed.
Watch out: if you do not route the thermocouple just as the original it may not operate properly. But extra length should not be a problem if you coil the extra length (or never un-coiled it) and leave it near the point of connection to the gas valve.
Mount the sensor end of the thermocouple at the pilot / flame into its supporting bracket next to the pilot flame tubing. A small nut or clip is used on many appliances to hold the sensor tip in proper position.
Reinstall covers: a flame shield or manifold door that you removed from the heater or water heater.
Reconnect the end of the thermocouple to the gas control valve where the old thermocouple end was un-screwed. You should find that you turn the screw counter-clockwise to tighten a right-hand-threaded connector.
Tighten the new thermocouple first by hand, gently, taking care not to cross-thread the connecting nut. When the connector is turned fully tight by hand, use the 7/16" wrench to make a final quarter turn.
Watch out: do not over-tighten the connector at the gas valve. Do not cross-thread the connector. Do not apply thread sealant to the connector.
Watch out: If any gas connections were removed and replaced (usually not necessary if you are only replacing a thermocouple) then check for leaks using a chloride-free soap and water solution.
Watch out: failure to place the tip of the thermocouple in proper position to sense the gas pilot flame is a common source of heater troubles and failure to operate.
Watch out: failure to properly seat the thermopile or sensor end of the thermocouple at the gas valve (the end opposite of the flame-tip) into its recess in the gas control can also prevent proper gas heater operation:
you may have trouble lighting the gas flame or keeping it lit, and symptoms will be the same as if the thermocouple assembly were defective. In the photo below my wrench and the round white arrow show the connection to which I'm referring.
If you keep the thermocouple tubing straight for at least an inch at this connecting end there's a better chance that it will seat properly in the gas control.
Note that in the photo below I am loosening the connector.
Turn the screw clockwise to loosen a right-hand-threaded connector such as this one, and turn the screw counter-clockwise (opposite direction as that white arrow) to tighten the connector.
Turn on the gas and check for leaks. You should not see any bubbles. If you see leaks turn the gas supply back off immediately. Any leaks must be fixed immediately. More help is at
Do not turn on nor try to use gas fired equipment if there is a gas leak. Doing so risks fire, explosion, injury, even death.
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
The thermocouple sensor is not properly located and secured in position in the gas flame
The thermocouple connecting tubing has been mechanically damaged, crimped, cut or nicked.
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.
(Gaffers and Sattler was an appliance brand (kitchen ranges, heating equipment, air conditioning) owned by Maytag and actually preceded "Maytag" as a company name.
The Gaffers Sattler and Maytag Washing Machine Company was founded in 1893 by businessman Frederick Maytag. G&S cooking ranges were identified as a subsidiary of MagicChef in 1969. Magic Chef found its way back to Maytag in 1986.
If you are looking for parts for Gaffers and Sattler equipment check with Maytag. Separately a Gaffers & Sattler appliance company still does business in Murray UT. )
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
Check for these other causes of loss of the gas pilot flame
Thermocouple sensor tip not properly mounted, secured in the flame path
Thermocouple's indicating end not fully secured into the gas valve.
A bad replacement thermocouple or thermocouple tubing that was bent, kinked, damaged during installation
Thermocouple not properly connected at the control, e.g. the sensor connector not fully screwed-in
Sudden drafts. We often find this problem in southern climates where heating equipment may be installed outdoors or in a shelter exposed to wind.
Low gas pressure, weak flame
Inadequate combustion air - very dangerous, can be fatal
Dirt on the equipment burner, thermocouple, connections, wiring
Dirt or debris or insects (spiders) clogging the pilot orifice or pilot light gas tube
Symptoms of Bad thermocouple or dirty thermistor on gas fired equipment: Short cycling or unexplained on-off cycling of heating equipment has also been traced (by one reader) to a dirty or failing thermocouple (possibly the reader meant a dirty thermistor).
A less likely problem is a failed gas control valve.
Something else we haven't thought of
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).
Types of Thermocouples
The thermocouples used in building mechanical systems are generally type K: thermocouples used in heating boilers, calorifiers, geyers, gas burner applications (flame sensors). Type K thermocouples are widely used in industrial applications involving water, mild chemical solutions, hospitals and the food industry.
Here, courtesy of Pyrosales, an Australian provider of thermocouple devices, is a complete list of the types of thermocouples and their applications.
Thermocouple Types & Applications
Type B thermocouple
Metal industries: steel, iron
Type C thermocouple
Very high temperatures, space vehicles, nucleaer reactors, industrial heating, research, vacuum furnaces
Type E thermocouple
Very cold temperatures, sub-zero, oxidizing or inert (non-corrosive) environments, cyrogenic, pharmaceutical, chemical
Building mechanical systems, oil & gas heating, water heaters, gas burner flame safety, water, mild chemicals, gases, dry areas, engines, hospitals & food industry. Type K (chromel-alumel) are the most-widely-used general purpose thermocouple
Vacuum & controlled atmospheres, high temperature kilns, ovens, furnaces, gas turbine engine exhaust monitor, metal industries: aluminum & smelting
Type R thermocouples
Heat treatment, process control, semi-conductor, glass industries, metal industries: ferrous & non-ferrous
Type S thermocouples
Heat treatment, process control, semi-conductor, glass industries, metal industries: ferrous & non-ferrous
Type T thermocouple
Food monitoring, environmental sensors, used in strong magnetic fields
-200 - 350°C
-328 - 660 °F
"Which Thermocouple is Right for Your Application" [infographic], Pyrosales Pty Ltd., New South Wales, 4 Wordie Place, Padstow NSW 2211, PO Box 309, Australia, Tel: 1 300 737 976 & Tel: +61 2 9790 1040, Email: firstname.lastname@example.org Website: https://www.pyrosales.com.au, Tel: NSW : 02 9199 5755 VIC : 03 9988 0945 QLD : 07 3171 8672 CHILE : +56 22 5031596
CHINA : 86 22 23708751 USA : 847 656 5258 retrieved 2017/07/21,
Pyrosales USA, 51 N. Ayer Street, Harvard IL 60033-2859 USA, Tel: 847-656-5258 Email: email@example.com
original source: https://www.pyrosales.com.au/blog/cat/thermocouple/post/which-thermocouple-is-right-for-you
Website excerpt: Pyrosales has operations in Sydney, Melbourne and Brisbane as well as in South America, China and the United States.
"Thermocouple Technical Chart: Temperature Ranges", Thermometrics Corporation, 18714 Parthenia St
Northridge, CA 91324 USA Tel: (818) 886-3755 also DBA
AeroGear Telemetry serving aerospace & military applications, retrieved 2017/07/21, original source: http://www.thermometricscorp.com/thermocouple.html
... calibration types are designed to deliver as close to a straight line voltage curve inside their temperature application range as possible ... The smaller the diameter,the faster the thermocouple responds. Grounding the junction also improves response time by approximately 50 percent based on the sensor achieving 63.2 percent of the final reading or to the first time constant.
Calibration types for thermocouples are specified by the American Society for Testing and Materials(ASTM) according to their temperature versus EMF characteristics in accordance with ITS-90,in standard or special tolerances.
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"Residential Gas Water Heaters: Replacement Parts List, Models FCG/PCG 75-100 & CGX/PCGX/SGX/PSGX-75 Series 270", [PDF], A.O. Smith Water Products Company, 5621 W. 115th St., Alsip IL 60803 USA, Tel: 1-800-433-2545, Website: www.hotwater.com, Email: www.hotwater.com/parts, retrieved 15 March 2015, orginal source: http://www.hotwater.com/lit/partslists/psd3549.pdf
"Which Thermocouple is Right for Your Application" [infographic], Pyrosales Pty Ltd., New South Wales, 4 Wordie Place, Padstow NSW 2211, PO Box 309, Australia, Tel: 1 300 737 976 & Tel: +61 2 9790 1040, Email: firstname.lastname@example.org Website: https://www.pyrosales.com.au/LI>
"Thermocouple Technical Chart: Temperature Ranges", Thermometrics Corporation, 18714 Parthenia St
Northridge, CA 91324 USA Tel: (818) 886-3755 also DBA AeroGear Telemetry serving aerospace & military applications, retrieved 2017/07/21, original source: http://www.thermometricscorp.com/thermocouple.html
 American Water Heater, "Thermocouple Replacement Instructions - Natural Gas Models (All Models Except 30T)", American Water Heater, Johnson TN, Website: http://www.americanwaterheater.com, Tel: 1-800 999-9515, (no address given online) retrieved 10/23/2013, original source:
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