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Question DetailsAsked on 1/12/2017

New thermocouple placed 2 weeks ago and today pilot light was out. Could it be the valve ? Help, have no idea ???

About 2 weeks ago noticed no heat coming from vents. Checked furnace & pilot light was NOT on. Friend lit pilot light & heat back on. But recommended to change thermocouple. Done ! Today, after just 2 weeks of new thermocouple being put in no heat , no pilot light again. He lit the pilot light, it turned on fine .. But, he recommended to get another thermocouple. Same brand . Working fine now but, wondering for how long pilot light will stay on? Not sure it may be the valve? Appreciate all answers, read some responses & hard to understand the technical explanations. Furnace may be approx 20 years old. Hoping I don't need a new furnace -- What easy steps to check to make sure parts are working fine? How to check valve since it is attached so securely !!! ... Thanks so much to all who will take the time to help me or your suggestions. Simple as possible , not an expert on heating ! So can't afford a new furnace at all .. Help Please. Thank You

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There are a lot of possibilities - and it sounds like you have aslready read some of the previous responses to this type question in the Home > HVAC link in Browse Projects, at lower left. I am not surprised you found some of them too technical - there are a LOT of things that could be the cause, and of course different types of furnaces and regular and high-efficiency condensing types too make it more complicated.

This sort of on-off sporadic failure is hard to track down. I have listed common causes below, some of which will not apply if yours is a traditional gravity flue (open flue pipe to roof with no fan or dampaer in it like most high-efficiency units have). The sensor or safety thermostat related ones you would need a volt-ohm meter (when power to furnace is on) or continuity testor (power off) to see if a sensor or protective device has cut the power to the unit, which would shut off the pilot light on an electronic controlled furnace with electronic pilot ignition (not always on) - usually not but occasionally will also do so on a standing-pilot (pilot always on) furnace.

1) thermocouple failure - there have been an unusual number of comments here recently about an apparent high incidence of factory thermocouple defects, so replacing it is not a guarantee the new one will work right - about only solution for that is to get a spare and try it too because three in a row is unlikely, especially if changing to a brandname thermocouple. Have also heard of this problem recently from my go-to plumber and HVAC guys - evidently a lot of the chinese cheapo box store thermocouples are defective right out of the package or have "solder buttons" on the end that goes into the gas valve which are too thin (also a problem on many light bulbs these days) so it is not making proper contact, or are sloppy so they short-out the thermocouple in the receiving port. I have run into this several times myself - hard to trace down. If it fails again (I would keep a spare in stock near the furnace) I would go with a Honeywell - only a couple of dollars more and gives peace of mind - I don't recall ever having had a factory defect in a part made by them.

2) if you have a secondary emergency shutoff thermostat - most common on boilers, but some furnaces now have them on the outgoing flue to detect raw hot combustion gases escaping the heat exchanger and overheating the ducting, if that is failing could be shutting down the furnace. (Or if it is legitimately detecting hot gases where they should not be).

3) could be a flame rollout sensor detecting unstable or backdrafting firebox flame rollout, where the flame from the main burner starts moving down or out of the firebox rather than up the flue through the heat exchanger - the sensor detects this and cut power to the burner (and pilot in systems where the pilot is electrically controlled).

4) could be a firebox overtemp sensor detecting excess heat due to unstable or backdrafting flame or excessive gas flow

5) could be original thermocouple just gave up with age (commonly a few to maybe as much as 5-10 years on occassion - I don't trust for more than 2-3), then when the replacement was put in its tip was not inserted far enough into pilot flame to stay properly heated, the thermocouple was not placed correctly relative to the main burner so it is being blown out by the main burner flame kicking on or off, or the draft diverter (the finger-like extension behind the pilot flame) is oriented wrong so the pilot is being affect by drafting that is cooling it enough to shut off the gas flow. See owner's manual for correct setting positions.

6) a common one- like 5) above, but if the "button" end was not properly put all the way up into the receiving hole in the gas valve, it might not be making proper contact. That end of the thermocouple should be straightened out for several inches, then with the connecxting screw pulled back on the thermocouple tubing, insert the "button" into the hole, being certain it goes all the way up in and does not catch prematurely on the threads in the hole. Then, holding it snugly up in place, screw in the retaining nut.

7) similar to 6) above, but if you tighten the retaining nut more than snug you can smush out (technical term there) the solder button on the end and short out the thermocouple up in the gas valve

8) some brands of thermocouples/thermopiles use the tubing as part of the circuit, or are sensitive to static electricity - so make sure the tubing is not touching metal anywhere along the line - only where it plugs into the pilot assembly, and where it goes into the gas control valve. Also make sure it was not kinked or sharply bent anywhere in handling - that will easily short-circuit it internally. Smooth curves are fine, kinks not.

9) could remotely be unstable flame or gas feed to burner from dirty burners, which can result in funny drafting in the firebox which can blow a pilot light out or deflect it so cools the thermocouple enough to shut off the gas flow

10) if it has been windy recently (and when the failures occurred), could be your flue is not properly protected from the wind (wind divertear or baffled flue cap) so flue drafting or backdrafting is putting the pilot out.

11) if a high-efficiency furnace with damper and eductor (exhaust) fan in the plastic flue pipe (commonly but not always leading horizontally directly outdoors rather than up through roof, though some do that), then there are damper position and/or temperature and pressure sensors, of various types in different brands, which detect if the damper has opened and the eductor fan turned on after the burner turns on - some of these only shut down the burner, most will also shut off the pilot if it is an on-demand electronically-ignited pilot (on only when there is demand for firing, not constant on), as would be normal for a high-efficiency furnace.

12) could be gas control valve internal springs going bad, which can cause intermittent shutoff of the gas flow - but that would be one of the last things I would be looking at, and more usually affects the main burner than the pilot. I would guess in probably 90% or more of cases the gas control valve outlives the furnace.

13) probably rarest - bad electrical connection at the gas valve, or in the control unit of the furnace, or even in the wiring to the furnace or at the breaker, sporadically losing contact and causing the furnace controller and/or gas control valve to lose power, shutting off the valve. Transformer at the furnace failing can also rarely cause this.

As to how to "check" gas valve - other than checking the electrical leads to it are securely attached (may be a wiring "harness" connector or just push-on tab connectors), and items 6) and 7) and 8) above regarding inserting the thermocouple into the valve, not much you can do yourself to check it. And "repair" of a residential gas valve like that is illegal - if it has gone bad you replace it for about $50-200 depending on brand and model, plus $100-200 labor roughly.

If these did not help any or are too technical, then you need a Heating and A/C tech (your earch the List category) to diagnose it. Could be about $100-150 normal minimum trip charge, and on up from there depending on how hard to diagnose and what repair parts are needed when he does find the problem.

Clearly, if 20 year old furnace, you need to set a $ limit with the tech - ask for diagnosis only and maybe set a limit of $100-200 or whatever on repair cost after diagnosis without your further permission, so you don't possibly get into a possibly many hundreds o even thousands of $ repair which might better go into a furnace replacement instead, if the problem is that serious (not likely in this case). That decision, if it comes down to that, depends largely on your feeling of confidence in the existing unit, how serious furnace outages are to you (i.e.. how cold your winters are), how much time the house is unoccupied so a furnace failure could result on pipes freezing or such before it is detected, the general condition of the furnace, the condition of your budget to be able to afford a replacement furnace, and how inefficient your old furnace is compared to a new higher efficiency unit considered in combination with how much your normal annual heating cost is. More on the economics issues in some of the previous questions in Home > HVAC link in Browse Projects at lower left, if you are interested.

Answered 3 years ago by LCD



This is Erick in Member Care. I'm happy to help!

I've found some highly rated Heating & A/C providers from the List to assist:

All Temp Heating & Air Conditioning
4363 W Montrose
Chicago, IL 60641
(773) 481-2000

Calumet Heating & Air Conditioning
14435 Kolin Ave
Midlothian, IL 60445
(708) 385-8051

A Custom Heating & Air Conditioning
5772 W Higgins Ave
Chicago, IL 60630
(773) 539-8175

As an Angie's List member, you can find these same results and additional providers by logging into your membership and searching for Heating & A/C in the "Search the List" tab.

If you would like information on additional providers through this forum, please let us know. You can respond to this thread or submit a new Answers post. You can also reach us at We're happy to help!


Answered 3 years ago by Member Services

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