My furnace was leaking fuel out on the floor toward the end of the heating season this spring,so I had the service man out this morning before I started it up. He couldn't find a leak anywhere,but when he held his lighter up to the damper with the furnace running,the breeze put it out. We took the pipe down and the chimney was completely plugged at the bottom,just ash and soot that had fallen down. He figured the back pressure was pushing the fuel right back out around the gun while the furnace was actually running. No doubt was filling the house with carbon monoxide too. Guess that explains why I had a headache almost every day last winter. Might have been the best $126 that I've spent in a while.