Why would a 50-gallon propane water heater just stop working?

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Why would a 50-gallon propane water heater just stop?
We don't know how old but it looks perfectly white and it is labeled as a 50 gallon water heater with clean readable labels.
Here is what we know. 1. The water heater has been working for more than 5 years. 2. Last night and this morning there was no hot water. 3. The propane tank reads 30% & the kitchen stove lights fine. 4. The outside temperature is 17C which is quite normal. 5. The 15cm hot-air flue is at room temperature (it should be hot). 6. The 1cm hot water cupric line was room temperature (should be hot). 7. All the gas lines appear to be intact (no horse urine egg smell). 8. All the water lines appear to be intact (no ponds of water). 9. Black thermostat dial was on the "B" setting (which is normal). 10. Red push-button off/pilot/on dial was in the "on" position. 11. No flame was visible at the sighting glass when we removed the plate. 12. Sparks were visible when pressing the pushbutton green "igniter".
Following directions, we A. Turned the black temperature dial to pilot B. Pressed and turned the red off/pilot/on dial to pilot C. Repeatedly pressed & released the green piezo igniter D. The pilot candle lit after a few presses of the igniter E. We held the red pilot dial down for more than 1 minute F. We stopped pressing the red pilot dial (it popped up) G. The pilot candle remained steady H. We slowly turned the black temperature dial to B I. The flames circled the bottom of the water heater
And that's where we are at this time. At this junction, nothing seems wrong. The heater is in its own closet so there are no rain or drafts.
Why would a 50-gallon propane water heater just stop working?
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On Monday, August 1, 2016 at 11:46:14 AM UTC-4, Tatsuki Takahashi wrote:

The thermocouple could be going bad and is marginal. It's function is to generate enough electricity from the heat of the pilot flame to keep the gas valve open. Or the pilot cam sometimes get blown out if there is some unusual high wind event that effects it. I assume that when lit, the pilot flame looks normal? If not, if it's too small, uneven, etc then there could be some debris in the orifice.
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On Mon, 1 Aug 2016 08:55:26 -0700 (PDT), trader_4 wrote:

I don't know what "normal" looks like, but what you said about a bad thermocouple is understandable.
Do you know if it's easy to replace?
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On 08/01/2016 02:06 PM, Tatsuki Takahashi wrote:

long it took. Maybe a couple of hours -- or less. One of local "home improvement" stores (a regional chain) had the thermocouple in stock; I'm almost certain it was less than $20.
Perce
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On Mon, 1 Aug 2016 15:17:58 -0400, Percival P. Cassidy wrote:

That is good to know.
How did you know it was the thermocouple?
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On Monday, August 1, 2016 at 5:01:29 PM UTC-4, Tatsuki Takahashi wrote:

I think you can measure the output with a VOM meter while it's hot. You're supposed to get X millivolts. It's been many years since I did it, but I'm sure there are youtube videos.
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On 08/01/2016 05:18 PM, trader_4 wrote:

That's what I did.
Perce
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On Mon, 1 Aug 2016 23:27:04 -0400, Percival P. Cassidy wrote:

The candle went off again this morning. The burner may have stopped heating overnight.
How do I know if it's not the igniter? http://www.searspartsdirect.com/part-number/9003410/0042/153.html
The water heater label says "State Select" made in Mexico, model GS640HBRSG (also known I think as GS-640-HBRSG) which is a 40-gallon 37,000BTU heater made in 2006, of part number 9200455U? (the last digit(s) are obscured).
I'm looking for the manual here: http://www.statewaterheaters.com/literature/
But I can only find manuals for "similar" models here: http://www.statewaterheaters.com/literature/instruction-manuals/residential-gas/
However, I think this is the thermocouple part number 9000056015: http://www.searspartsdirect.com/part-number/9000056015/0042/153.html
And I found this video for replacing "a" thermocouple:
https://www.youtube.com/watch?v=XruoUVwJowo

It's for a different model heater but I hope it's similar.
I happen to have an older water heater that leaked right next to it, so I wonder if I can transfer the thermocouple?
The older disconnected water heater was built in 2004 and started leaking about a year ago so I disconnected it (the two were in series). It's a Rheem Guardian Fury model number 42V0S-36PF and is also 40 gallons.
Do people think a thermocouple will fit different heaters?
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On 08/02/2016 11:04 AM, Tatsuki Takahashi wrote:

Our "home improvement" store had a few different thermocouples -- certainly different lengths and perhaps even different fittings.
Perce
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On Tue, 2 Aug 2016 11:17:58 -0400, Percival P. Cassidy wrote:

I read up on this, and now that I'm a "youtube expert" on thermocouples, there seem to be a score of different sizes but the main difference seems to be length (according to the repair people who make the youtube videos).
Someone mentioned to *test* the thermocouple, so tomorrow in the daylight I will disconnect the thermocouple at the thermostat and clip the positive DMM lead on it with the other DMM lead connected to ground.
When the pilot light is lit, I am expecting something like 20 millivolts.
At this point, I don't *think* it's the thermocouple, only because the pilot light stays lit while I watch it - but the flame eventually goes out.
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On Tuesday, August 2, 2016 at 11:04:39 AM UTC-4, Tatsuki Takahashi wrote:

I don't know what that igniter does. I'm guessing that maybe you're WH has a push button to light the pilot light and that that's what it is? Otherwise the igniters I've seen on WH have been ones where there is no pilot and a hot surface igniter is used to directly light the burner. If it's a push button to light the pilot, then that isn't your problem.

That's a thermocouple, that's for sure. Whether it's the right one, IDK. There are many online parts stores, if they all come up with the same part # for your model, then that's it. Once you have the part #, people sell them on Ebay too.

It likely is.

If it's the same, sure.

Sure, many WHs use the same thermocouple.
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On Tue, 2 Aug 2016 08:23:54 -0700 (PDT), trader_4 wrote:

I don't know what the igniter does either other than when I press the green button, the igniter clicks and throws sparks which ignite the pilot candle.
I'm wondering if the problem is the thermocouple though, since the pilot light stays lit when I manually light it with the piezoelectric igniter.
That must imply the thermocouple has the millivolts to keep the pilot light lit, at least in the short term. The problem could be something else since the pilot light will stay lit, at least while I'm watching it.
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On Wednesday, August 3, 2016 at 2:47:28 AM UTC-4, Tatsuki Takahashi wrote:

Well then that's what it does, it lights the pilot light instead of having to use a lighter.

If the TC is on the way out, I would think it could be marginal and later go out. It's the most likely suspect.
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On Mon, 1 Aug 2016 14:18:42 -0700 (PDT), trader_4 wrote:

I found the test on youtube which I'll run tomorrow looking for 20mv from the heated up thermocouple.
It could be that the thermocouple is out of alignment with the pilot flame but I think the thermocouple is working because the pilot light stays lit when I watch it.
But, overnight, the flame goes out for whatever reason.
I see from the youtube videos that it's pretty easy to remove the entire burner assembly so I will try that tomorrow when it's daylight also.
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On Monday, August 1, 2016 at 2:06:50 PM UTC-4, Tatsuki Takahashi wrote:

On the typical older ones it's easy. Newer ones from the last 5 year or so, IDK, because those have some kind of flame arrestor now and IDK how much that might complicate getting at it. The thermocouple fastens to the burner assembly and has what looks like a small tube that runs to the gas valve. To replace it, you pull the whole burner assembly out, vacuum up any rust/debris inside, clean up the burner if needed, put the new one on and put it back together. 30 min job. I'm sure there are videos.
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On 2016-08-01 9:46 AM, Tatsuki Takahashi wrote:

I created a vacuum attachment from 1/2" pvc to vacuum the underside of my HW tank, it seems the reduced air flow overheats the the burner chamber and the thermal fuse opens and shuts down pilot.
Once the thermal fuse had reset the tank lit normally and then a few hours later the pilot light would be out again. It's a simple fix.
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On Mon, 1 Aug 2016 16:12:20 -0600, Idlehands wrote:

This may be my real problem because the pilot light stays lit when I light it and watch for a few minutes, so, the thermocouple is at least working for those few minutes.
Where is this "thermal fuse"? That's what I need to check I think.
Also I will follow the advice to remove the burner assembly, which, after watching a few youtube videos, seems super easy.
I guess this is stuff I should have done long ago as preventive maintenance.
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On 2016-08-03 12:47 AM, Tatsuki Takahashi wrote:

The thermal fuse on my tank is a round silver disk on the side of the valve body. It self resets when the system cools down and then I have to light the pilot again. By vacuuming the base of the tank I have yet to remove the whole burner assembly and takes 5 minutes.
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On Wednesday, August 3, 2016 at 8:31:01 AM UTC-4, Idlehands wrote:

So, if his tank has one and that's the problem, he should know it, because he'd be resetting it. So far, no mention of that, only the pilot going out.
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if you have a pilot light and it goes out overnight,
see if there is an adjustment to increase the gas flow to the pilot light and make it a bit larger...
m
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