Looking at my flue cap this evening, it looked darker, but not covered in black. There are areas which look somewhat rusty colored, and shiny black on the grating (what I perceive as more than before). Tomorrow I can post a pic, and I'm going up on the roof to take a closer peek, but what are the tell tale signs? There is no soot or ash anywhere on the roof or gutter from what I can see. Pics of your own would be helpful.