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A flue is a duct, pipe, or opening in a chimney for conveying exhaust gases from a fireplace, furnace, water heater, boiler, or generator to the outdoors. Historically the term flue meant the chimney itself. [1] In the United States, they are also known as vents for boilers and as breeching for water heaters and modern furnaces.
A centrifugal fan is a mechanical device for moving air or other gases in a direction at an angle to the incoming fluid. Centrifugal fans often contain a ducted housing to direct outgoing air in a specific direction or across a heat sink; such a fan is also called a blower, blower fan, or squirrel-cage fan (because it looks like a hamster wheel).
Large temperature differences between the outside air and the flue gases can create a strong stack effect in chimneys for buildings using a fireplace for heating. Before the development of large volume fans, mines were ventilated using the stack effect. A downcast shaft allowed air into the mine.
Mechanical fans can be used to provide positive pressure ventilation by breaking or removing windows, skylights or heat/smoke vents on the roof; or by cutting new exhaust vents in the building. If there is no suitable existing hole, firefighters may use their equipment to make one, such as specialized saws for cutting a large hole in the roof.
Direct vent fireplaces are extremely efficient compared to a traditional fireplace and can operate at about 85% efficiency. Even a very efficient traditional fireplace only operates at about 15% efficiency. This is because most of the hot air generated by the fire travels up the chimney due to convection.
Some fireplace units incorporate a blower, which transfers more of the fireplace's heat to the air via convection, resulting in a more evenly heated space and a lower heating load. Fireplace efficiency can also be increased with the use of a fireback, a piece of metal that sits behind the fire and reflects heat back into the room.