What is a Furnace Duct?

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  • Written By: T. Briseno
  • Edited By: Jacob Harkins
  • Last Modified Date: 12 December 2019
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A furnace duct is used to distribute and deliver heat to various parts of a building or home. It is a component of heating, ventilating, and air conditioning design that develops in shape and placement with the need of circulating air throughout a structure. Ducts are generally made of metal and plastics, and they may be placed below the floor substructure, above the ceiling drywall, or suspended with brackets from the ceiling itself. Most often a furnace duct, or ductwork, is connected from a furnace, which is the central heating element providing the heat.

In simple terms, ductwork is a delivery system. Also known as HVAC ductwork, it is not generally operational mechanically so much as it is a static structural element in place to reach different areas of a residential or commercial space. Internally, a furnace duct is typically hollow and open throughout so air can flow most efficiently from place to place.

Furnaces are often the beginning of connecting ductwork, as the opening of the duct itself is connected to the part of the furnace that expels the flow of heat. Molding connectors are screwed in or otherwise adhered around the furnace duct opening to prevent loss of heat between the exiting furnace air and the inside of the duct. Air that escapes through an incomplete or faulty seal may disappear into the air around the furnace without reaching its needed destination, causing heat loss and likely higher energy costs.


It is typically important to maintain seals at each segment of ducting. Pieces of ductwork often are joined together from furnace to vent and can be of different shapes to accommodate the corners, curves, and levels, or floors, of a structure. Even a straight path from furnace to vent is often too long for one duct alone, although in smaller spaces this can be accomplished with support bracketing along key points.

At the end opening of a furnace duct, there is usually a wall, ceiling, or floor cutout covered by a grate. There are usually several of these exit points where heat is blown into rooms of a building in the system coming from the furnace. Ventilation attached and sealed at ends of ductwork may then be controlled by opening and closing the grate. Controlling heat flow in this way, and by ensuring that the insides of the ducts remain clean and unobstructed, completes the delivery of air from source to rooms via the furnace ducts.


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