Sheet Metal Duct

Sheet Metal Duct

DIY sheet metal duct is the most common type of ductwork system that is installed for heating and cooling purposes. Like other materials used for duct, such as fiberglass panels and rigid foam boards, a sheet metal duct system offers a means of distributing treated air into spaces in residential, commercial, industrial and institutional buildings. Choose between radial or extended plenum sheet metal duct systems. Radial ducts are round ducts. Extended plenums are comprised of radial and rectangular shaped ducts.

Function
All sheet metal duct systems require three basic components in order to work properly: return air, air handling and treated-air distribution. Return air ducts are employed to draw air into the furnace. This air is pulled into the air handling function of the system, where it is heated or cooled. Pulling air from the space also allows the treated air to circulate more freely, during the distribution phase, without having to push aside excess air in the room.
Materials
Radial sheet metal ducts are available in diameters ranging from three to 14 inches. The lengths can be purchased in lengths up to 10 feet. Rectangular ducts have depths of eight inches and diameters from 10 to 28 inches. The available length is 60 inches. Flexible round duct is also available for limited uses. The ducts are connected to a plenum, which is a rectangular shaped sheet metal box that must be attached to the furnace outlet. The plenum functions as an air mixing chamber from which air is distributed. Typically, the thickness of the sheet metal duct and accessories is 30 gauge.
Installation Guidelines
Install rectangular ducts primarily as extended plenums or trunk lines. Use radial ducts for the primary trunk line or for supply lines. These ducts can be employed for drawing return air or delivering supply air. Generally, use six inch ducts for heating only. Install eight inch ducts for heating and cooling. The radial ducts are easy to assemble. Simply snap the ducts together. Insert the factory-fabricated "crimped" end of one pipe into the end of another length of pipe to create long runs. Use a starter collar to attach radial pipes to the plenum. Connect the ends of the radial ducts to register boots and install floor, wall or ceiling registers in the rooms.

Check the local building code, usually sheet metal ducts must have a minimum clearance of one inch from combustible materials. Secure seams and joints with a combination of sheet metal screws, mastic and aluminum foil tape. Mount polypropylene or perforated metal straps to support the sheet metal ducts every two to five feet and at the joints. Insulate sheet metal duct that is installed in unconditioned spaces, such as garages, attics and crawl spaces.

Wrap all connections with aluminum foil duct tape. Support ducts at two to five foot intervals with polypropylene or perforated metal straps. Insulate all ducts installed in unconditioned spaces, including attics, garages basements and crawl space. The ducts must have at least one inch of clearance from combustible materials.