A flue is a pipe or passage that carries smoke and hot gases out of a building from a fireplace, furnace, or water heater. When you look at a house with a chimney, the flue is the hollow space inside that chimney that lets the smoke escape safely into the air above the rooftop.

Without a properly working flue, smoke would fill up the rooms of a house instead of going outside. That's why chimney sweeps used to climb inside flues to clean out soot and creosote (a tarry buildup from smoke). Today, people still need to have their flues inspected and cleaned to prevent dangerous fires or carbon monoxide buildup.

The flue works because hot air rises. As the fire burns, the heat makes the smoke and gases flow upward through the flue and out into the open air. Most flues have a damper, which is like a little door inside the chimney that you can open when you want to use the fireplace and close when you don't, helping keep cold air from coming down into your house.