About two weeks ago. The furnace had been sending a thin film of black, sooty, oily junk through the air and one night while preparing for a class the smoke/carbon monoxide detector went off. In the process of calling emergency repairmen one of them told me to call the fire dept. to have the monoxide level checked. I had already turned off the furnace, so I put the cat outside so he get some fresh air and then I opened the windows and turned on the attic fan to pull fresh air into the house. By the time the fire dept. guys got here there wasn't any carbon monoxide left to find. The friends I called were worried though; they said my speech was slurred. Anyway, the furnace was shot and $4,500 later we have a new furnace, cleaned out the air ducts, and got the cat bathed.