I have a Heil oil furnace. It is about 5 years old. There is 24 inches of pipe between the furnace and the chimney. The pipe rises about 4 inches in this 2 feet. The furnace started smoking around the "inspection port". When the furnace fires, you can see flame around this port. There is not excessive smoke or soot around this port. It almost acts like the chimney or the pipe is stopped up. I used a shop vac and cleaned up all the soot i could access. I removed the plate where the "pipe" goes out to the chimney and cleaned that area. Inside there was probably 1/4 inch of soot.