I smelled one whiff of smoke as I walked through the house at 2:19 a.m. No smoke detectors went off and I did not smell it again. Searching the house I finally found one Lux thermostat that was not working. It runs the # 2 furnace in this big house. After finally figuring out how to get the cover off (I'm an older lady living with 2 cats), I discovered the batteries ....dated 2011...were moderately corroded. It takes two batteries and of that size I had only one new one. I finally robbed one out of the wireless computer mice...