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Combustion chamber overheating -- why?????  Rate Topic 
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 Posted: Tue Jul 2nd, 2019 02:39 am
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puterboy
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I have a 5 year old direct vent, rapid recovery 50-Gallon State gas water heater.

It worked perfectly until about 10 days ago when the pilot went out. The pilot relights fine and the flame burns fine for 15 to 30 minutes at which point they both suddenly shut off.

The pilot relights but won't stay lit until afterabout 5-10 minutes at which point presumably the thermal switch (in series with the thermocouple) resets.

This suggests there is a problem with overheating in the combustion chamber.

So far:
    1. I have checked the flame - almost pure blue and strong

    2. I have checked the flue - the match test is fine and when I put my put my hand through the trap in the chimney I feel a strong draw upwards

    3. I have tested the thermocouple (it measures 27mv and is fine) and then replaced the thermocouple - still goes out.

    4. I have replaced the gas control valve -- still goes out

    5. I have checked and cleaned out the intake air filters but they were free of dirt anyway and didn't solve anything

    6. I even had the gas company check the pressure to make sure no overpressure (I did so since they swapped out the gas meter about 2 weeks before the problem started)

    7. I don't think there is excessive limestone scaling as our water is municipal (Boston) treated and I didn't see much scaling when I emptied the water heater before changing the gas control. Even if scale build-up is causing over-heating, I would think the problem would come on gradually rather than going from no problems to shutting off every time.

    8. No other appliances are using gas at the same time and the piping is adequately sized. Plus we never had this problem before.

    9. The burner and its positioning relative to the thermocouple, pilot, and thermal shutoff switch seems fine

The ONLY thing I can think of is that perhaps the flame is too large? (though it is a fast recovery model)

Could it be that the burner itself has somehow corroded and is letting out too much gas to burn???

What else could it possibly be???

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 Posted: Tue Jul 2nd, 2019 03:33 am
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eleent
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Hello,  You've done a LOT to get the problem resolved. As it's pretty clearly overheating, three things come to mind. First is that possibly the limit switch is misbehaving and shutting off at a lower temperature than it was designed for. Second is a question of whether the main burner can be adjusted lower. Some gas controls have provision for this. Have you run this all by the manufacturer's tech support? They might have additional info. Also, is there anything depressurizing the room that the heater lives in, like a furnace or clothes dryer? That would slow airflow and contribute to this problem. Please do let us know how it goes.
Yours,  Larry
ps, One more thing. You cleaned the intake filter, but there is a screen that prevents flame from getting out of the heater, normally located under the combustion chamber.  This also can get clogged with fine dust that essentially is invisible. It needs to be cleaned from underneath with a feather duster and or a vacuum with a suction nozzle cut at an angle.

Last edited on Tue Jul 2nd, 2019 03:46 am by eleent

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 Posted: Tue Jul 2nd, 2019 03:50 am
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puterboy
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Thanks for the good suggestions. In response:

The limit switch I believe is inline with the thermocouple assembly. I replaced the whole assembly which includes the switch so that seems unlikely.

Nothing is depressurizing the room... and nothing has changed in the basement... so this seems unlikely.

I don't believe the main burner is adjustable though I will check with the manufacturer.

TRULY STUMPED BY THIS...

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 Posted: Thu Jul 4th, 2019 03:51 am
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eleent
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Hi,  What brand and model is the heater?  I'd like to look it up and see how air is getting to the combustion chamber. I'm suspecting dust has built up in a difficult to clean place. ;)Yours,  Larry

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 Posted: Thu Jul 4th, 2019 04:24 am
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puterboy
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Interesting!
I have a 5-year old State model GS6 50YRR.
It's high-recovery, atmospheric vented, ~60K BTU, 50 gallon.

Since this is overheating, the flame is blue & strong, and the updraft in the chimney is robust, where would you expect the dust to be?

Just trying to understand the causes of overheating that match my symptoms and troubleshooting results...

Thanks!

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 Posted: Thu Jul 4th, 2019 04:27 am
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puterboy
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I spoke at length with State technical support.
They were stumped too.
Their only suggestion was to either pay to have one of their certified consultants come by or to RMA the water heater under warranty.

Still would like to figure this out...

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 Posted: Fri Jul 5th, 2019 03:38 pm
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eleent
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Hello, Have a look at the attached image which I think is for your heater. See the part about cleaning the flame arrestor.  Have you done that? That's the part that often causes trouble. :X

Yours,  Larry

Attachment: Screenshot_1 (216 x 262).jpg (Downloaded 4 times)

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