Flex duct, also known as flexible duct, is a type of ductwork that is commonly used in heating, ventilation, and air conditioning (HVAC) systems to distribute air throughout a building or structure. Flex duct is made of a flexible, lightweight material, such as PVC, metalized polyester, or fiberglass, and it is designed to be easily bent or shaped to fit around obstacles and through tight spaces.

Flex duct is often used in residential and commercial HVAC systems because it is easy to install and can be easily routed through attics, crawl spaces, and other tight spaces that may be difficult to access with traditional, rigid ductwork. In addition, flex duct is relatively inexpensive and can be easily cut to the desired length, making it a popular choice for many contractors and homeowners.

Flex duct is typically used to distribute cooled or heated air from the air conditioning system or furnace to the various rooms or zones of a building. It is usually connected to the air handling unit or the furnace using a round connector, and it is attached to the registers or diffusers in the ceiling, walls, or floors to distribute the air throughout the space. Flex duct is also commonly used to return air to the air handling unit or furnace, and it is typically connected to a return grille or other opening in the ceiling or wall.

It is important to note that flex duct can be more prone to leaks and air loss than traditional, rigid ductwork, and it may not be as efficient at distributing air. As a result, it is important to properly install and maintain flex duct to ensure that it is functioning properly and efficiently.