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Question DetailsAsked on 1/17/2017

How to install a thermocouple in a gas heater

The pilot is on but the burners do not come on... would installing a new thermocouple fix the problem?

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2 Answers


I hope not - the pilot light should not stay lit if the thermocouple is not working right, except if you are holding down the pilot light button (or similar bypass for lighting the pilot). If it did stay on with the thermocouple failed, you could have it go out and be pumping out gas, creating an eventual explosion hazard.

Of course, check no one has turned the control knob to PILOT - and try turning the knob to PILOT then back to ON (or such similar phrasing) - occasionally as the gas valve ages the gas control valve gets a bias in it, and if the knob was not fully on might have worked originally but now needs to be turned fully on to actually be on.

Also, of course, be sure the thermostat says it is actually calling for power - digital ones have a "furnace on" indicator, usually a little "flame" image that pops up below the temperature reading. If this does not come on when the temperature is below the ON setting, or if you temporarily jack the temperature setting up several degrees as a test, check thermostat for operation and for functional batteries. Most thermostats will NOT call for power if batteries are dead or dying, even though the temperature display may still show (because it takes a lot less power than the furnace controller takes to kick on).

That said and assuming those did not solve the issue - if the thermocouple is putting out low voltage, it can OCCASIONALLY allow the pilot to light but not be enough power to energize the main burner gas valve. Very rarely, with older gas valves, occasionally the coil or spring in the gas valve goes bad and will not allow the main gas to come on either - but that is down close to the end of a troubleshooting checklist. Be sure the tip third or so of the thermocouple is fully immersed in the hot part of the flame (blue/green part) so it is up to proper temp. Certainly, if you know how to do it (instructions in owners manual) changing it out is a cheap thing to test - at worst you end up with a spare $5-10 thermocouple for future replacement use.

One other possible cause - some thermocouples/thermopiles use the outer part of the tubing as a conductor, or are sensitive to static buildup (you are only talking millivolts of power here), so be sure the thermocouple is not touching the furnace anywhere but at the end connections.

My guess would be some safety cutout sensor activated, which cuts the power to the gas controller or the control board/unit, depending on brand and type of cutout. Some self reset after the heat shuts off, some have to be reset manually and a fairly rare few have to be replaced after being triggered. Also, on some units, if such a sensor triggers (and you swhould find out WHY, not jusat reset it),, the control board needs to have a reset button pushed before it will operate normally. Could be a flame rollout sensor, firebox overtemp sensor, drafting pressure sensor, eductor fan motor operation sensor, flue damper sensor, flue temperature sensor, heat exchanger air pressure sensor, etc.

If you have an electronic or newer electrical controlled unit, you may have LED lights which display status - the pattern of which are on or off or which ones are blinking (see manual for codes) can tell you what the detected problem is in many cases. Newer units sometimes have LED displays which actually display an alphanumeric or numeric trouble code.

If you are not able to figure it out yourself, then Heating and A/C would be the Search the List category to find a well-rated and reviewed company to call to have a technician comne out and troubleshoot it. if it has not been cleaned in the past year or less, you might schedule that at the same time (if they are not swamped with trouble calls) to save some cost.

Answered 3 years ago by LCD



This is Erick in Member Care. Thanks for your interest in Angie's List!

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Answered 3 years ago by Member Services

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