Gas Furnance Pilot goes out after burner cycle

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Old 01-08-10, 11:02 AM
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Question Gas Furnance Pilot goes out after burner cycle

I think I have an unusual problem - please bear with me -

FrigiKing Forced Air Central (natural gas) Furnace -
Model # UG 80D8 E1
80,000 BTU
Gas valve controller: Model 36C53 Type 200

A few days ago I found the standing pilot was out in the morning.

I merely re-lit it successfully -
but it seemed that it kept going out.

So searching on the web I found replacing the thermocouple was probably what was needed - which I did.

But even though the pilot will light and stay lit with a healthy blue flame that engulfs the thermocouple tip -
the pilot will still eventually go out
(just like before the thermocouple was replaced).

I was relighting the pilot several times a day, and obviously in the morning as the pilot went out overnight.

I eventually watched the burn cycle.

While the burner is on working the pilot does have a flame - but does not engulf the thermocouple.

When the burner cycle is finished and goes off the pilot is weak and does not reach the thermocouple - then after a few second obviously the pilot goes out.

The thermocouple is to the side of the pilot and the flame is deflected by a piece of bent metal (looks intact) to engulf the thermocouple.

When the furnace is hot right after a burn cycle is the time when the pilot light looks weak may even have yellow in the flame and does not reach the thermocouple - so I cannot even relight the pilot for it to stay on.

However when the furnace is not hot - eg: 1/2 minute after the burn cycle, I can easily light the pilot and it has a good healthy blue flame that engulf the thermocouple and all is well - until the next burn cycle when for whatever reason the pilot becomes weak and does not reach the thermocouple and therefore goes out.

So it is not the thermocouple -
it is the pilot flame is weak but only after a burn cycle.
All other times it seems just fine - pilot stays healthy and lit - but goes out after a burn cycle - it is not blown out, it is just weak but only after a burn cycle.

Any suggestions on what the problem and solution may be please?

FWIW - some pictures -
Gas Furnace -


Pilot light area and actual flame -




Thanks,

Vincent
 

Last edited by UnknownVT; 01-08-10 at 11:46 AM.
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Old 01-08-10, 11:10 AM
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The usual problem is that the pilot burner orifice is dirty and needs to be cleaned. That involves removing the brass ferrule that holds the pilot tubing in the pilot burner with a 7/16" wrench, which allows the pilot orifice to be removed.

Some pilot orifices can be cleaned with a wire from a wire brush, others may need to be blown out with some compressed air or replaced if the holes are too small and can't be cleaned properly.
 
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Old 01-08-10, 11:42 AM
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Originally Posted by SeattlePioneer View Post
The usual problem is that the pilot burner orifice is dirty and needs to be cleaned. That involves removing the brass ferrule that holds the pilot tubing in the pilot burner with a 7/16" wrench, which allows the pilot orifice to be removed.
Thank you for your quick response - interesting.

As mentioned the pilot will stay lit and with a healthy looking blue flame which does engulf the thermocouple - as shown in my pics above.

It is only when the burner is On that the pilot flame diminishes and when the burner cycle goes off the pilot flame is weak with some yellow in flame.

But once the furnace cools down - (as short as 1/2 minute) the pilot can be re-lit and once again gives good healthy blue flame that covers the thermocouple.

So do you still think it's a dirty pilot orifice -
or could it be anything else?

Thanks,

Vincent
 
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Old 01-08-10, 12:48 PM
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Clean the pilot orifice and we'll see.
 
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Old 01-08-10, 01:26 PM
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Originally Posted by SeattlePioneer View Post
Clean the pilot orifice and we'll see.
Thank you for your kind patience -
and I apologize for being a doubting Thomas.

I cleaned the orifice using a wire brush and gently poked one of the brush's wire through the hole.

Sure enough that did the trick - I watched the burner come on and the pilot was still very much covering the thermocouple. Turned the burner off and the pilot was still fine/blue covering the thermocouple

Thank you very much - to think it was that simple -
no amount of my "logic" would have found that.

I am very grateful.

Thank you,

Vincent
 
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Old 01-08-10, 01:57 PM
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No problem.

Most of the problems with pilot lights are caused by dirty pilot orifices, not bad thermocouples. So 1) cleaning the pilot orifice will usually solve the problem and 2) even if it doesn't it probably needs to be cleaned anyway.

If that doesn't solve the problem, you go on to theory #2 to test.
 
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Old 01-08-10, 02:34 PM
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Originally Posted by SeattlePioneer View Post
Most of the problems with pilot lights are caused by dirty pilot orifices, not bad thermocouples. So 1) cleaning the pilot orifice will usually solve the problem and 2) even if it doesn't it probably needs to be cleaned anyway.
ha, I wish I knew that before being so blase and changing out the thermocouple.

Well hopefully I have learned.

Thank you for the kind advice, and patience.

Vincent
 
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