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.