Performed a heating pm on an oil furnace. Brushed and vacuumed. Replaced the air filter. 20x20x1 Replaced the oil filter, nozzle and strainer. Started and checked the safeties. Performed a combustion test. 7.0% o2, 0 ppm, 84.0% efficient System is operating properly. The 6” Tee flue pipe, the metal is starting to get thin and soft in a couple of spots. Recommend replacing before a hole starts to form.