Thermocouple Keeps Burning out


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Old 12-24-09, 07:11 PM
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Thermocouple Keeps Burning out

I have a Weil-McLain, model #: CG-5-SPON series 11 natural gas hot water boiler and every year it burns out it's thermocouple

It usually goes out in february but this year it decided to go last week in the middle of the night (cause of course it can't pick a nice convenient time)

I use honeywell 30" universal thermocouple, and I had a pro come look at it last year he said it's the right thermocouple for this unit but he said the pilot light appeared to be too high. He looked for an adjuster on our gas valve but said our gas control unit has a non-adjustable pilot. He told me the price of a new adjustable one but i forget the price.

Here are some pictures of my unit:

http://img138.imageshack.us/img138/6...ntrolvalve.jpg


http://img695.imageshack.us/img695/8254/pilotlight.jpg

It never did this till about 4 years ago when it's been doing it annually ever since.

I was hoping someone might have some suggestions to this issue. Is there any kind of auto-light pilot feature i can install? or what about that gas control valve in the first picture do you think i should just get a new one and make sure i can tune down the pilot?

Thanks,

Jim
 
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Old 12-25-09, 06:01 AM
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The only thing that kills thermocouples is heat. The pilot flame should cover 3/8" to 1/2" of the thermocouple. It should also be a dull red not cherry red. There should be a pilot adjustment. A few years ago the gas valves came out as unregulated valves as opposed to regulated valves prior. This means what ever gas pressure comes to the gas valve goes to the pilot.
There should be an cap that says pilot. If there is one, remove the cap and see if there is an adjustment screw.
If there is no adjustment cap maybe the gas pressure in the home has increased for some reason. Have someone check the gas pressure inlet on the pump. The minimum should be 4.5 the absolute max is 14" w.c. Most operate around 5 - 7" w.c.
 
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Old 12-25-09, 04:10 PM
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I couldn't find any sort of adjustment regulator for the pilot light. I did look on the side of the boiler and found the specs for the gas pressure:

Min inlet gas pressure 5 w.c.
Max inlet gas pressure 13 w.c.
Manifold gas pressure 3.5 w.c.

If there was some sort of increase in the gas pressure to the house, for some reason, wouldn't that affect all the other gas appliences too? Like the hot water heater, and dryer. Instead it seems as if just the furnace is affected.
 
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Old 12-25-09, 05:24 PM
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Usually, the pilot gas adjustment is underneath a flat-head cover screw. Remove the screw, and there should be an adustment revealed - requiring a fine screwdriver to turn.

Your pilot flame looks too big to me.
 
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Old 12-25-09, 07:25 PM
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Here are some more pictures of my unit: (you should be able to click on them to make them bigger)
http://img194.imageshack.us/img194/839/inletgasregulator.jpg

http://img194.imageshack.us/img194/6516/outletgasregulator.jpg

Is that screw (the one with the red arrow) the one you want me to completely remove and see if there is an adjuster underneath?

I also noticed 2 allen head screws (the green arrows) that line up with the main gas pipe coming from my gas line and going thru that unit and out to the burners. I think those are adusters / regulators but they'll probably adjust the gas to the whole unit, correct?
 
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Old 12-25-09, 09:02 PM
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i had this same problem, and it took about a year each time to burn out. and always in the dead of winter. i finally took the whole pilot mounting unit apart and looked at it and found that it could be mounted two different ways. one was the way i had it, and the other reduced the flame impingement on the thermocouple. either way matched the pre drilled holes. i have not had to replace the thermocouple in 2 years now. just a thought.
 
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Old 12-25-09, 11:27 PM
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The two recessed Allen-head plugs are pressure taps where you connect a manometer for testing the gas pressure. The one on the inlet side is for measuring the supply pressure and the one on the outlet side is for measuring the burner pressure.

On the inlet side you will see two caps with screwdriver slots on the right-hand side. The larger one is the access to the main burner pressure adjustment and the smaller one might be the pilot adjustment, I'm not sure of the latter. You need to remove the caps in order to make the adjustments. There will be some gas leakage when the caps are removed so make the adjustment and then replace the cap snugly. I suggest that you do NOT adjust the main burner pressure unless you use a manometer. The pilot adjustment normally is just a screw that closes the passage to the pilot burner and so turning it clockwise should reduce the amount of gas going to the pilot.
 
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Old 12-26-09, 06:55 AM
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Try the screw with the red arrow. The pilot adjustment is often close to the pilot fuel tube.
 
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Old 12-26-09, 08:25 AM
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My suggestion would be to remove doubt by taking the gas valve model number and looking it up on the Honeywell site and download the PDF file for it... this isn't something anyone should be 'guessing' at!

What is the gas valve model number? V8204 ?

Take a look at this PDF file... pages 7 and 8.

V8204 gas valve
 

Last edited by NJT; 12-26-09 at 08:59 AM.
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Old 12-26-09, 01:48 PM
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That turned out to be great advice Trooper, it's a honeywell model VR8200A gas control. I looked up its pdf on honeywell's site and found instructions to adjust the pilot light.

The adjustment is located under the cover screw directly above to pilot gas tube (the one i marked with a red arrow) I'm going to go down and see if i can tune down the flame a little bit.

They say i want 3/8" to 1/2" flame to cover the thermocouple and that the thermocouple should be a dull red. I'll let you know if i can achieve those results.
 
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Old 12-26-09, 02:00 PM
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well just went down and removed the cover screw, and tried turning the adjuster inside and as it turns out it was already turned as far as it could be turned down.

I was able to turn it counterclockwise which just made my pilot light even bigger but the screw was turned 100% of the way down and my pilot is still the same size. What can i do now?
 
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Old 12-26-09, 04:02 PM
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Hmmmm... interesting...

Two things... MtMtnTop suggested looking to see if you could move the thermocouple somewhat out of the pilot flame by using an alternate set of mounting screw holes... that's worth looking into.

Next, I would probably get someone in who can check the gas pressure. If the pressure is too high, it's likely that this would affect the pilot as well as the main burners.
 
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Old 12-26-09, 04:26 PM
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Originally Posted by kingjim9 View Post
If there was some sort of increase in the gas pressure to the house, for some reason, wouldn't that affect all the other gas appliences too?
Each gas appliance, including your boiler's fuel valve, has a built-in pressure regulator. I'm suspecting there may be something haywire with your boiler's fuel valve.

Since you found your pilot adjustment was turned down all the way, it makes me think that sometime in the past, someone tried to reduce the pilot flame.
 
 

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