There is an old saying about having too much of a good thing and I had that experience recently with my furnace. It has been a particularly rough winter and we have used the furnace quite a bit to keep warm. There have more than a few nights where the temperature dipped below zero and the heat that has consistently poured out of the furnace has been like a godsend. During the last heavy snow storm, we had to crank up the furnace during the evening because of the extremely cold temperatures we were once again dealing with. Everything seemed fine and through the next morning, nobody noticed anything wrong. It was not until the midday hour that I started to notice that it was excessively warm inside I went to the thermostat and turned down the temperature thinking nothing of it, but about an hour later I realized that it was not any cooler inside. In fact, it actually felt warmer! This time I turned the thermostat to the off position thinking that would take care of the problem, but after a minute the furnace was still running. I went down into the basement to confirm the furnace was still on and it was. I hit the kill switch on it and it shut off. Perplexed I reset the kill switch only this time it would not turn on at all. I immediately called the HVAC technician we use to come out and fix it. He located the problem in the control board on the furnace. A transistor had blown out. Crisis averted.