The Thermocouple and the Boiler - success

I got home around 6 last night and around 7 noticed that it was pretty cold. 63F to be exact, not really cold, but not the 68 that it should have been. The thermostat was blinking, indicating that heat was on the way, but the boiler was off. I panicked for a second since it was going down to 10F that night, but I took off the front cover and tried to remember what was supposed to be happening.
The last time this occurred was a few years back and the culprit was the thermocouple, a fairly clever device looking like a 36" piece of thick copper wire. After the service guy replaced it, I bought another and had it sitting on top of the boiler all this time. I figured that I'd replace it and see what that did.
One end screws into a slot that holds the end of the thermocouple in the pilot flame, which was off. The other end screws into an electrical box that I assume is the brains of the thing. The job of the thermocouple appears to be telling the brain that the pilot light is indeed lit, so that the gas doesn't just fill up the house.
I turned off the power to the boiler but not the gas. I replaced the thermocouple, turned on the power, pressed down the red switch that controls the pilot gas and lit the pilot light. I know from the gas water heater that you need to hold that switch down for 10 seconds or so while it burns.
The pilot stayed lit, and as I stood there for a few seconds trying to figure out what to do next, there's a click that was apparently the brain getting the news from the thermocouple that all was well, and the thing fires up. Catastrophe averted.
I'll pick up another thermocouple and leave it on top of the boiler. I'll likely sell the house in a year or two and move to Florida, but I'm sure the next folks will need it one day.
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dgk wrote:

Hi, You wanna one Atta boy for that?
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On 3/4/2014 2:24 PM, dgk wrote: ...

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That's most unusual to fail a TC so rapidly -- I've never had the TC itself fail in 40+ yr from the time the previous furnace was installed until upgraded it a couple of years ago. I'd wonder why -- is it routed too close to the main burner, perhaps?
And, yes, the TC generates a thermoelectric voltage that is what's needed for the gas safety valve to open. Before that, there were "wild pilots" that didn't cut off. I have an old small stove in the pump house that dates back to WW II era that was before in the bathroom of the old house before central heat. It worked that way for 70 years w/o it ever causing an issue even when the KS wind occasionally would blow out the pilot light. The pilot assembly itself corroded so much that I had to replace it a couple of years ago so when I did I put in a new safety valve at the time as well...
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dpb wrote:

Hi, Only thermocouples in my house/cabin are in the NG fire places, almost 20 years old. So far they did not fail. Maybe OP is adjusting the pilot flame size too big?
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On 3/4/2014 5:27 PM, Tony Hwang wrote:

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Indeed, I've never heard of one failing at all (altho I'm sure they do on occasion) but twice within just a few years implies to me something's just not right.
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How about Chinese thermocouples?
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wrote:

Thermocouple or thermopile??? Many fireplaces use milivolt systems with thermopiles.
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wrote:

How often do you use the fireplace? This winter the boiler has been on an awful lot. I will take a look at how high the pilot light is, and whether the thermocouple is too close to the main burner.
Since it's the only house I own, I have nothing to compare it to. That's sort of the problem for many of us who try to do some home repairs. It's usually the first time on lots of things, and we learn a lot and get better for the second time, if there is a second time.
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dgk wrote:

Hi, The position of it and flame size is adjustable. Even thermocouple in my camping trailers for many years never failed during my ownership.
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wrote:

I'm going to take another look. I guess if the thermocouple fails the electronics also turn off the pilot light, which would be why it was off.
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I've seen a lot of thermocouples fail. 2 on each of my old water heaters, 2 on the old furnace, 1 on my old friend's water heater, and 2 on his boiler. That's in 32 years in this house, and just over 15 in my old friend's.
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