Pilot won't stay lit


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Old 03-31-09, 07:31 AM
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Pilot won't stay lit

Hi everyone, I have an American Standard natural gas furnace which is about 25 years old. I've only bought this house a year and a half ago, so I don't know of any previous maintnence to the unit.

Last night I could tell that the unit was off, so this morning I checked the pilot and it was out. I tried igniting it and it would light when I turned the red knob. I held it lit for about a minute, but when I released it, it went out immediately.

When I came home from work yesterday I noticed that my brother had the window down the hall from the thermostat open, so the unit might have been running all day long. Is there any kind fail-safe systems in these units that need to be reset or anything.
 
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Old 03-31-09, 07:36 AM
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You probably need a new thermocouple. It should be a small cylindrical part with a stiff wire off the end. It will be mounted where the flame from the pilot hits it.

You can try cleaning it with an abrasive pad or fine sandpaper. If that doesn't work, a new one is a few dollers.

See here for more explanation.
The Gas Furnace Standing Pilot and Thermocouple - How It All Works
 
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Old 03-31-09, 07:43 AM
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Thank You, I was thinking a thermocouple also. Are they rather interchangable? Is this something I am going to have to call american standard and order for a specific make and model?

I appreciate all the help
 
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Old 03-31-09, 07:57 AM
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If cleaning doesn't help, and the pilot flame is full and on the TC, just carefully remove it and almost any appliance or HVAC supply should have one comparable. Take the model number of the furnace with you.
 
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Old 03-31-09, 09:11 AM
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Thank you, I cleaned it real good with some 180 grit sand paper. The pilot still didn't stay on. I'm going to head out now and try to pick one up. I'll post the results when I get back in.
 
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Old 03-31-09, 10:40 AM
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I changed the thermocouple with no different result. The pilot still does not stay on. Are there any other suggestions?
 
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Old 03-31-09, 10:53 AM
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Is the flame nice and clean..all/mostly blue and fully around the TC? No furnace pro here..

The only other thing I could say would be a bad gas valve (the part that the TC connects to)...but I can't tell you anything about testing that. I think you can measure the TC voltage if you have a good meter, but I'd have to defer to a Pro on how to do it.

A slight rap on the valve with a screwdriver handle might help...but I can't really tell you that...lol
 
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Old 03-31-09, 10:53 AM
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You don't clean a thermocouple. You clean the pilot burner and in particular the pilot orifice, which is found after removing the pilot tubing from the pilot burner. Often the pilot orifice can be cleaned with a wire from a wire brush, but some need to be replaced.

Also, the gas valve can be defective. You really need a special adaptor to measure the voltage produced by the thermocouple while it is operating to diagnose these problems reliably.

There can be electrical connections to a limit switch that can be a problem as well.



Seattle Pioneer
 
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Old 03-31-09, 11:17 AM
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Do youhave a multi-meter? You can test the TC to verify it's good. You may have gotten a bad one. Doesn't happen often, but it DOES happen,
Take the TC out of the system (again) & put one probe of your multi-meter on the little silver button at the seating end.
Put the other probe somewhere on the copper tubing.
Hold a match under the top 3/8" & see if you get a reading over 25 mv...
If you don't the TC is bad...
If you get a good reading, check to make sure the seat is clean & engaged properly into your gas valve.
IF that stuff is ok, then the valve is probably the culprit.
You can check the valve using the multi-meter to read the ohms, but without knowing the valve manufacturer, we won't be able to tell you what to look for...
See if you can ID the valve & report back...
HTH
 
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Old 03-31-09, 11:31 AM
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As I said..no expert..but I've seen here and other places that cleaning is a first step.
I know I occasionally have to clean the one on my portable propane heater, but thats likely due to other contaminates building up on it.

If the flame is good, then cleaning the orifice might not be needed, but could be done at the same time easily.

I'll always defer to the experts..no worries here, just helping out.
 
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Old 03-31-09, 12:10 PM
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You guys are the best. I still don't have a working pilot, but you are all pointing me in the right direction. So far today, I cleaned the old TC, nothing changed. Bought and installed a new TC, and nothing happened. I cleaned the Pilot oriface, and I did get a more direct and stronger pilot, but it still goes out immediately.

I am now going to take out the multimeter and check to make sure there is power to the unit and check both TC's. By the way my furnace is an American Standard about 25 years old.
 
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Old 03-31-09, 01:39 PM
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Seattle Pioneer is correct, you don't clean a thermocouple. In fact, you could damage a thermocouple by rough handling.

Gunguy, you DO need to periodically clean a flame rod flame sensor. This should be done with a mildly abrasive cloth or a Scotchbrite pad. A piece of coarse fabric like an old low-thread count sheet or perhaps light weight canvas would likely be fine if you don't have a Scotchbrite pad. Don't use any coarse sandpaper because that will leave tiny grooves in the rod where it will oxidize and reduce the effective area of the rod. All you want to do is to remove the surface oxidation without scratching the rod itself.

Ninety percent of the time a pilot that will not stay lit after releasing the pilotstat button is because of a failed thermocouple or a damaged contact where the thermocouple connects to the gas valve. Of the remaining ten percent, it is usually the solenoid coil inside the gas valve has failed. Sometimes this solenoid is replaceable but usually it requires a new gas valve.
 
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Old 03-31-09, 02:50 PM
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Furd Thank You for the input. OK I did the test on both TC's with the multimeter and they both are working fine. At this point would you guys think it is the valve itself that is not functioning? Or, do you think I should try concentrating on the mating contact between the TC and the valve?
 
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Old 03-31-09, 04:24 PM
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You can try cleaning the contact end of the thermocouple with a Scotchbrite or a pencil eraser and then look at the contact point in the gas valve (use a flashlight) and see if there appears to be any corrosion. You can use a small screwdriver to gently scrape the contact in the gas valve a bit to clean it up.

If this doesn't work then replacement of the gas valve is probably called for.
 
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Old 03-31-09, 05:08 PM
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Originally Posted by furd
Ninety percent of the time a pilot that will not stay lit after releasing the pilotstat button is because of a failed thermocouple or a damaged contact where the thermocouple connects to the gas valve.

Ummm. Well, in my experience about 75% of the time the problem is a dirty pilot burner, another 15% of the time a bad thermocouple. 5% of the time a bad gas valve, 5% of the time a poor electrical connection at a limit switch or other miscellaneous problem.

I do find that thermocouples tend to be changed out far more often than they are actually no good.

The correct way to diagnose such problems is to use a thermocouple adaptor which allows millivolt readings while the thermocouple is in the gas valve circuit and the pilot light is lit.
 
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Old 03-31-09, 05:54 PM
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Gun, Gaksy, Seatle and Furd thank you again for all the advice. I haven't figured it out yet, but I have ruled some thing out now. My next area of concentration will be for me to try and either fix or rule out the contact point between the TC and the gas valve.

In a worse case scenario, if I have to replace the whole gas valve. Do anyone know the price range? Where is the best place to look for one? Or, should this be left to a pro? I am very confident in working on just about anything, but since I have never dealt with this before I don't know if there are special tools required.
 
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Old 04-01-09, 03:36 PM
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Thermocouple

You said you tested the thermocouples & they tested good. What did you measure?
Using a multimeter set to the DC millivolt (mv) scale, you need to clip one lead to the very end of the small tube (the half ball which goes into the gas valve) & the other end to the tube. Light the pilot & hold the button down. In less than a minute you should read about 30mv. If you get the 30mv or close to it, the thermocouple is good & the problem is likely the gas valve.
 
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Old 04-01-09, 07:06 PM
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This is the kind of adaptor you need to reliably diagnose thermocouple problems:

Product Group Details


They only cost $5 or so ---you can probably find one at an appliance parts store or a HVAC parts dealer.

It allows the thermocouple circuit to be tested while the circuit is under load.
 
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Old 04-01-09, 08:29 PM
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Adaptor

Testing under load is superior to simply checking output. Stange, I've tried to get one of those from my local supply houses for years. Every time I ask I get looked at like I have three heads or am told they are out of stock.
 
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Old 04-02-09, 05:22 AM
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Thanks Seattle, I'm going to try and find one this morning.
 
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Old 04-02-09, 08:49 AM
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Originally Posted by Grady
Testing under load is superior to simply checking output. Stange, I've tried to get one of those from my local supply houses for years. Every time I ask I get looked at like I have three heads or am told they are out of stock.

The ONLY way to test is under load. Taking an open read is useless, it only confirms your meter is working. I've seen open reads of 25 and under load 0-1.

Grady, I've got an extra adapter if you want it, it's an older one, which means it's a good one. Not like the one out now. In fact I still have my first one which is about 40 yrs old, made by General Controls. Let me know.
 
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Old 04-02-09, 04:01 PM
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Originally Posted by SeattlePioneer
This is the kind of adaptor you need to reliably diagnose thermocouple problems:

Product Group Details

Could you explain how you do it? I'm curious myself. I saw the part from Robert Shaw, and it says it allows a high limit to be connected. It doesn't say anything about this being a testing device.

What does it mean to have it 'under load'? What is the load? Testing by having a flame on a disconnected thermocouple (at the gas valve end), and taking a reading, as Grady suggested, is not a good indicator? Why not?
 
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Old 04-02-09, 06:55 PM
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This kind of adaptor provide you with access to the energized interior wire of the thermocouple.

So you can connect it to a limit switch, for example. As long as the limit switch is closed, the gas valve will operate. If the limit switch opens, the circuit is broken and the gas valve shuts off.

To use it as a test point, you'd short the two connections together. That allows you to measure the millivolts while the thermocouple is energizing the gas valve magnet.

If, for example, you measure 1 MV when the thermocouple is hot, that indicates you have a bad thermocouple.

If the read is 10-15 MV, and the magnet wont hold, that indicates that the magnet is defective since adequate voltage is being supplied but the magnet isn't functioning.

Sometimes you can get a read of 20-30 MV by testing the thermocuple by itself, but that read will drop to 1 MV or so (inadequate to operate the magnet) when the thermocouple is connected to the magnet. This adaptor allows that kind of problem to be detected.
 
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Old 04-03-09, 06:26 AM
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Then, if the limit switch wires are shorted together, simulating a furnace that should operate(and the gas valve be open) - then where do you put your voltmeter leads?
 
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Old 04-03-09, 10:25 AM
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If you are using that kind of adaptor to test the millivolt circuit, short the two leads together. Then test the milivolt level between one of the shorted together leads and the copper tubing that forms the ground on the millivolt circuit.
 
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Old 04-06-09, 05:15 PM
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Can someone help me as well? OUr furnace quite heating today, and we are going to have a 28 degree night. The pilot won't stay lit on our Bryant Furnace. The flame from the aluminum tube only stays lit when I tap on the gas valve that the aluminum tube exits from. It seems that there is not enough gas flame is present to heat the thermocouple and light the furnace. Thanks for any help you can give.
 
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Old 04-06-09, 05:21 PM
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Pilot problem

From what you describe it sounds like the pilot burner is plugged or partially so. You can remove & clean it. Pilot orifices are quite small & cleaning sometimes doesn't work. You may have to replace the orifice.
 
 

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