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My furnace pilot light keeps going out so I changed the thermocouple. Now the furnace will come on and heat up to the degree set on my thermostat, then the pilot light will go out and the furnace will not come back on.

Could the thermostat be bad and cause the pilot light to go out?

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Management of the pilot light is not within the thermostat's purview. It has nothing to do with it. It either calls for heat or not, the furnace does the rest. Sounds like you can successfully relight the pilot and it works until the furnace again runs, then shuts down? Then the thermocouple works, so I don't know what else could be the problem. – bcworkz Dec 30 '12 at 21:56

6 Answers 6

If the pilot light keeps going out, the culprit is probably one of two things, with a third being unlikely but possible:

  • Pilot light line flow set too low, or pilot line dirty or obstructed - The pilot light is just a low-flow branch of the gas line that is always on, maintaining the ignition source for when the burner is turned on. The actual flow of gas to the pilot light can usually be adjusted, to compensate for normal variances in system pressure. In your case, this is probably set too low, which means if the overall system pressure drops (perhaps due to added demand from your furnace or gas stove), the pilot light gets starved. Failing that, the valve or pilot light nozzle may be dirty or clogged, obstructing flow more than you want. This is the most likely cause if you only notice this issue with one appliance, while others with pilot lights are working fine.

  • Gas Regulator Faulty, Failing or Improperly Adjusted - The meter outside your home has a regulator (the disc part with the cylinder protruding from one side) to "step down" the pressure in the gas main to a proper pressure for your home's appliances. The regulator, like most, basically works by opening and closing very quickly, letting higher-pressure gas into the system until the pressure "downstream" reaches the desired level, before closing off to prevent overpressure. The moving parts in a gas reg see a LOT of use over the reg's rated life, and the one on your meter may be on its last legs. This is normally something that would affect all gas appliances in your home; in addition to the WH pilot going out, you may notice other pilots snuffing, like on your furnace, or you may notice variations in flame height of your stove burners while cooking. In any case, you can call your utility company and ask for a replacement gas regulator; this is a safety feature and part of their service system beyond the gas meter, so maintenance of it is their job.

  • High NG demand causing system fluctuations - This is winter, mild though it may be in your area. Furnaces are running, HWHs working overtime, and more people will be using their stoves and ovens during this season than in summertime. As a result, NG usage is at its annual peak. Your area's "gas grid" may be nearing its capacity, which may be causing low pressure on the service side intermittently failing to replenish the downstream pressure of your home's gas system. Ask your neighbors if they've had issues with intermittent low gas pressure; it may not be just you. If it's affecting the entire area, there's not a whole lot you can do unless you've experienced actual monetary damage that you can prove was caused by the service failure. So, check everything else first, especially everything within your home's system (which the gas company isn't liable for).

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The thermocouple that verifies that the pilot light is burning (as opposed to releasing unburned gas) could be going bad. And you could also have either a blockage in your flue or a high temperature limit switch that detects or triggers a false positive that there's too much heat building up in your flue and shuts off the gas for safety. And finally, there could be a blockage in your pilot light itself that prevents it from properly working.

The first thing I would try replacing is the thermocouple.

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You have one of three things going on

  1. the pilot has low flame due to being dirty or otherwise needs adjustment, but it worked fine before so dirt is the most likely culprit.
  2. Make sure the flame hits the thermocouple end very well. It works best if the tip of the thermocouple get hot enough to glow slightly. This ensures that it has the best capacity to generate that necessary DC millivoltage to hold in the plunger when pushed in. The new thermocouple could also be bad YES new out of the box parts are sometimes bad but unlikely
  3. The power unit inside the gas valve is weak. The thermocouple generates two types of DC millivoltage. Loaded and unloaded It should be about 30 mV unloaded. You will need a heating professional well versed in these older type systems with the proper test equipment and accompanying chart to determine this.
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confusing answers.... I have the same problem,and a licenced furnace guy told me over the phone why it's the thermocouple, It mostly went over my head , but since he,s an expert and new right off what it was ,I'm buying a new therocoupler.

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My expert explained it differently , I'm switching mine out tomorrow I told him my proplem , and he said " I know already what the problem is " then said why its the thermocouple . He turned down a $50 sevice call plus $100 per hr.minimum charge ,and said call me if you ever have a serious problem.

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another possible situation could be that when the furnace lights it is consuming a massive amount of combustion air. If the furnace room is sealed tight it will blow that pilot out 1 suggestion would be too cut a vent in the wall were the combustion air necessary can be drawn in from the next room.

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