Came home after a whole day away to a CO alarm with the level at 184 in a room adjacent to the stove. Problem was immediately obvious, the loading door was open. Appears to be an operator error (me) in that the door was not latched properly. That plus it was fully dampered and in base-burner mode with it not so cold out so that some exhaust was being pulled into the house. No smell. Opened up windows on opposite sides of the house to clear. A CO detector is a must have even if you think you don't need it. My house is old and "leaky" so I think it would be difficult to ever get an extremely high level. In this case I did not need it to detect a problem which was obvious but it was essential to know the extent of the CO level and to later be assured that the house was cleared. I am thinking of possibly getting a second one as a backup although I feel that one good one is probably adequate if it is tested regularly.