Most HVAC systems feature a plenum, which is often called a bonnet. Plenums are directly attached to the furnace in an HVAC system and are usually constructed out of galvanized sheet metal. This is the main trunk line of the system where all branches of ductwork stem from. Plenums have to be custom-sized to meet the air distribution needs of the pipes branching out from them. A general rule states that when planning for plenum construction, count the number of needed pipes branching out of the system, multiply the number by 2 and add 2, if six-inch pipes are being used. This number indicates the ideal width in inches to construct the plenum to provide proper airflow for the pipes. If eight-inch pipes are being used, multiply the number of pipes branching from the plenum by 3 and then add 2. Plenums can be constructed to any length but all are usually designed to only eight inches in depth.
Round metal pipe is often used as ductwork. Round metal pipe ductwork that is constructed to six inches in diameter is ideal for heating applications. However, eight-inch diameter round metal pipe should be used to transport cool air. Eight-inch diameter round metal pipe should also be employed when ductwork is used to transport both heated and cooled air supplies. Round metal pipe is available in diameters that range between three and 14 inches. Typically, this type of pipe is available in two-, three-, five- and 10-foot lengths. Round metal pipe ductwork is very long lasting but offers little in the way of insulation.
Rectangular ductwork is made out of galvanized sheet metal. This type of ductwork is often employed as space-saving ductwork. It can easily be fitted between floorboard studs. This type of ducting is designed to last but must be fitted with a separate layer of insulation if required. Rectangular ductwork is widely used in the construction of plenums. The sheet metal used to create rectangular ductwork is usually 60 inches long and 10-, 14-, 18-, 24- or 28-inches wide. Sheet metal is cut to length and bend to create rectangular ductwork.
Several layers of material make up flexible ducting. This type of ducting features a layer of insulation that can vary in thickness. Many states require certain performance levels out of flexible ductwork insulation. In Florida, flexible ductwork must feature insulation that offers a minimum r-value of 6. Consumers that wish to install flexible insulated ducting in states like Washington must employ a variety of flexible ducting that offers a minimum r-value of 8. This type of ducting is pre-insulated, relatively easy to install and eliminates the use of certain fittings. However, this ducting is not as long lasting as round metal pipe ducting and contributes to a general reduction in airflow. In general, single lengths of flexible ducting should not be run in lengths exceeding 25 feet.