Flue
The pipe or channel that carries combustion gases safely outside.
A flue is the duct that vents exhaust from a furnace, water heater, fireplace, or boiler to the outdoors. A blocked, corroded, or disconnected flue can let carbon monoxide spill into the home — one reason CO alarms and annual heating inspections matter. Never block a flue, and have a pro check it if you see soot, rust, or back-drafting.