Gas furnace - bad thermocouple?

I've got an older "Day and Night" natural gas furnace in my home, model #800-19a. Setting the mercury thermostat turns the blower on, the standing pilot is lit, but the burners do not ignite.
After doing a little online research, I suspect it might be a bad thermocouple, which is a relatively easy fix. But I thought I'd run this by the good, knowledgeable people of this newsgroup first.
Is there anything else I should look at?
-Frank
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I would suspect a flame safety device. This could be a thermocouple or some sort of optical flame sensor. HTH EJ in NJ
Frank Warner wrote:

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I'm not sure my furnace has a flame sensor other than the thermocouple. I'm at work so I can't check this, but will take another look when I get home tonight.
Thanks for the tip.
-Frank
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Frank Warner wrote:

That model may actually be: 80U-19A
Typically, a bad T/C will keep the pilot from lighting (though not always).
Any failure in the 24V control to the gas valve will keep the Main burner from igniting. An AC voltmeter will help tracking the cause.
For example: Your Day/Night should have a Fan/Limit switch like this:
http://tinyurl.com/5m3ws5 Failure of the Limit contact to close will keep the gas valve from opening.
Jim
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Thanks, Jim. My furnace does indeed have a Fan/Limit switch identical to the one you linked. I'll check this part as you recommended.
-Frank
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On Mon, 15 Dec 2008 10:14:20 -0800, Frank Warner

Definitely NOT the thermocouple if the standing pilot is lit. The thermocouple is the protection to turn off the gas to the pilot if it goes out.
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snipped-for-privacy@snyder.on.ca wrote:

Hi, Right, if thermoouple is bad pilot won't stay lit. Thermocouple's function is supplying small amount of gas to keep the pilot lit. When it is calling for heat, measure across gas valve terminal to see 24V AC. If you do, gently tap it to see what happens. Either you're not getting power to valve or the valve is sticking.
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wrote:

They are probably rare now but some older natural gas systems did have a pilot light without a safety cutoff, sometimes fed from a separate small line and valve tapped into the feed lilne ahead of the main gas valve. They heated a thermocouple or mercury bulb which in turn allowed the main gas valve to open. I guess the thinking was that the vent would safely dissipate the small amount of gas if the pilot went out.
Don Young
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Don Young wrote:

Hi, Wow! that must before my time, LOL!
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wrote:

They are not rare systems.
I've still got four older Bryant furnaces in my rental apartments that don't have a safety pilot system.
They have a bimetallic switch heated by the pilot. If the pilot flame goes out, the switch opens and shuts down the main gas valve. However, the pilot gas keeps flowing. One of these furnaces had the pilot blow out over a summer seaon. the tenant never noticed the gas smell. Probably most went up the chimney.
Recently, a local gas company tech told me he still see lots of them. They are pretty common in my area and are not considered unsafe.
Doug
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wrote:

WAY before, if in fact it ever was allowed. There were apparently bimetallic thermo pilots at one time too - but even that has been nixxed for a long time.
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GE had some units like that also that had a temp switch in the pilot flame that broke the circuit to the gas valve if it wasn't red hot. Carrier and it's other companies-- Payne, Bryant and Day&Night-- had one that did the same thing, but was all one assembly, whereas the GE had the same pilot burner as the 100% cutoff, and the switch screwed into it with the same threads as a thermocouple. They were referred to as 90% cutoff. Both were made until around the early 70's. A few other brands used variations of the same setup eons ago. All were for NG only, no LP. Why they used such a thing I have no idea. It really doesn't look like it would have been any cheaper at all, and if it indeed was, it couldn't have been enough to justify the safety issue Larry.
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Help!!! I am having a similar problem with an old hanging furnace. I woke this morning with the blower fann running but no heat! I had to throw the curcuit to get the fan to stop. I have cleaned the thermocouple, it was slightly corroded, but i cant seem to get the pilot to ignite. Does electricity need to be on to have this ignite? Please help!!! Plus i might not even be in the right spot . Any info would be most helpful!!
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On Mon, 22 Dec 2008 18:55:03 -0800 (PST), snipped-for-privacy@gmail.com wrote:

REPLACE the thermocouple if you can't light the pilot..
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snipped-for-privacy@gmail.com wrote:

If the thermocouple goes to the gas valve a bad thermocouple will kill the pilot. Some real old systems had the thermocouple go to a solenoid/relay which interrupted the thermostat circuit to the main burner gas valve. There was a pushbutton to reset the solenoid. If the thermocouple goes to the gas valve Tony's suggestions sound good.

Have you lit the pilot before (do you know how it is supposed to be lit)?
The valves with thermocouple I have seen you turn a knob on the gas valve to a "pilot" position, push down on the knob and light the pilot while holding the knob down. The pilot should light with a bad thermocouple and no power. You need to hold the knob down until the thermocouple gets hot. If you waited long enough and the pilot goes out when you release the knob the thermocouple is probably bad. If the pilot lights turn the knob back to the original position.
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Un-freaking-believable!!!
It was bugs! BUGS!
I took the thermocouple out hoping to polish it down a little. The pilot assembly came out with it. As I checked the parts, little pieces of insects started falling out of the pilot nozzle. Then bigger pieces. Then a whole housefly. Mummified. Probably in there since the furnace was fired up last in February or something. It seems they had flown up there, perhaps attracted by the rotten egg smell, during the long dormant season here on the Central California Coast, and died, partially plugging the pilot.
I cleaned out all the parts, put everything back together, and the burners fired right up!
I still can't believe this. Been in this house twenty years and never had anything like that happen before.
Lurkers with similar symptoms take note.
-Frank
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Frank J Warner wrote:

LOL
Just when you think you've seen and heard it all...
good job.
Jim
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wrote:

That's not a bug, it's a feature. Been hanging around M$ windows to long...
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Frank J Warner wrote:

I've cleaned out entire ant colonies from HVAC equipment. Insects are one of the first things I look for when HVAC systems quit working. I've also removed the dessicated carcases of mice from control circuits because some silly human failed to pop a plug into a 1/2" conduit knock out. Mice will crawl right through those 1/2" holes and get electrocuted which will often short out the power. Hole plugs are readily available and the phone companies and other utilities use insecticide strips inside control boxes to prevent insect infestation. I don't know where I could get the same kind of insecticide strips, I'll have to search.
BTW, those are some incredible knives Frank. I wish I had that sort of artistic skill.
TDD
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