Last night at 3 AM, a carbon monoxide alarm went off in my house. As soon as I opened the bedroom door, it stopped. I thought maybe it was a fluke, but I figured I'd check things out. I went into the basement and saw lot of soot around the boiler (side note: never saw a boiler before living in New England; the combined boiler/oil tank deal strikes me as very primitive). I'd heard the oil-based boilers aren't typically a CO threat, but I shut it off anyway (turned off the circuit breaker so it couldn't turn on). I opened the windows in the basement, opened windows in the house, brought our daughter into our bedroom, closed the door and put towels under it, cracked the windows, and turned on an electric heater. I didn't think it was necessary - I thought the alarm had misfired - but my pregnant wife was worried so I did all that to appease her. Even if there was a CO problem, the open windows and the now-quiet but still functional CO alarm meant to me that we were OK, esp. in the sealed-from-the-rest-of-the-house bedroom. Called the service tech at 8 AM. The house at this time, outside the bedroom, was at 40ºF (4ºC). Fun. I thought I'd been paranoid for no reason and had just frozen us without cause (plus caused everyone except my daughter, who could sleep through a war, to lose half a night's sleep). The tech arrived and said that there had been a failure in the boiler that had slowly coated the exit until it was finally blocked, that a fail-safe had, ironically, failed, and that, in fact, there likely was a big CO buildup which had just started to reach the upstairs (we sleep on the 2nd floor) - he said the alarm probably stopped when fresh air from the bedroom lowered the CO to an acceptable level and that opening the windows kept it safe. If we did not have the CO alarm, I have no idea what might have happened. Typically, our daughter gets up before us, goes downstairs, and turns on the TV, then I follow groggily a while later. Had we not had the alarm, there is a very real chance that the CO buildup might have been fatal, esp. downstairs and possibly upstairs, and that at least one of us might not be here any more. I will be buying more CO alarms today. The one we had was really a fluke - a combined smoke/CO alarm cost only a few dollars more, so I got one and put it by the bedrooms. I didn't think we really needed one. Fortunately, our house is so tiny that it was sufficient - a larger house might have filled the downstairs with a deadly amount of CO without triggering an upstairs alarm. If you've been putting off buying one, and there is any flame-based heat (including ovens or ranges) in your home, ever (even if your heat isn't typically needed), I strongly recommend buying CO detectors. There is a very good chance that ours spared my family from, at the very least, becoming sick. It might very well have saved our lives.