Replaced all flue vent from furnace to what comes out the roof. Roofer sealed flashing and flue coming out. Ensured all connections are sealed. Tested system and let it run for 20 plus minutes with no issues. Replaced 4 inch by 5 foot, 4 inch by 4 foot, two 4 inch elbows, 4 inch by 1 1/2, flashing, and cap. System is heating properly at this time. Tested COin home. No CO in the home.