8" galvanized flue pipe, 8" RC barometric damper. Black stove pipe works fine also. The first 90 off the boiler should be a tee with a cap, not an elbow. Flyash will collect there and the tee/cap allows you to clean it without shutting the boiler down. The second 90 can be a tee, or an elbow. I have never found much flyash in the upper section of pipe, the first 90 gets most of it.