On Tue, 04 Mar 2014 15:12:16 -0600, dpb wrote:

On 3/4/2014 2:24 PM, dgk wrote:
...

The last time this occurred was a few years back and the culprit was
the thermocouple, ...

... 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.

...

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.

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...
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.