Thermocouple in the hand before installation (C) Daniel Friedman Gas Flame Thermocouple Sensors Troubleshooting & Replacement
     

  • THERMOCOUPLES - CONTENTS: definition of a thermocouple; how thermocouples are used in heating equipment & water heaters; how to troubleshoot & replace a bad thermocouple
  • POST a QUESTION or READ FAQs about thermocouples used on heating equipment & water heaters
  • REFERENCES

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

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.

Green links show where you are. © Copyright 2015 InspectApedia.com, All Rights Reserved.

Thermocouples: Definition & Uses in Gas Fired Heating Equipment

Thermocouple sketch (C) InspectApedia adapted from WeilMclain boiler installation instructionsWhat 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. 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.[3]

Where are Thermocouples Used on Heating Equipment?

Photograph of rusty gas burnerThermocouples 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?

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

Replacement universal thermocouple from Sid Harvey - InspectApediaWatch 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 [1]

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.

  1. 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.
  2. 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.
  3. Thermocouple connection at the gas valve
  4. 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.

    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.
  5. 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.
  6. 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.
  7. 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.
  8. Thermocouple mounting at the pilot flame
  9. 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.
  10. Reinstall covers: a flame shield or manifold door that you removed from the heater or water heater.
  11. 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.
  12. 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

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

  1. The thermocouple sensor is not properly located and secured in position in the gas flame
  2. 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.

Rusty furnace showing thermocouple & pilot light tubing & assemblies (C) Daniel FriedmanWatch 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

Bruce, Check for these other causes of loss of the gas pilot flame

  • A bad replacement thermocouple or one that was bent, kinked, damaged during installation
  • Thermocouple not properly connected at the control, e.g. the sensor connector not fully screwed-in
  • Thermocouple sensor tip not properly mounted, secured in the flame path
  • 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
  • Dirt or debris or insects (spiders) clogging the pilot orifice or pilot light gas tube
  • 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).
    See THERMISTORS

    Or see THERMOCOUPLES
    This condition was reported in the FAQs by a reader commenting
    at HEAT ANTICIPATOR Adjustment
  • Something else we haven't thought of

What if there is NO Constant-On Pilot Light? Direct-Ignition Gas Fired Heaters

Pilotless ignition gas fired water heater control (C) Daniel Friedman A O Smith water heaters Pilotless ignition gas fired water heater control (C) Daniel Friedman A O Smith water 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.

Or see GAS REGULATORS for APPLIANCES

Suggested citation for this web page

THERMOCOUPLES at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

More Reading

Green link shows where you are in this article series.

...




Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Publisher's Google+ Page by Daniel Friedman

Click to Show or Hide Citations & References