Key Components of Air Distribution Systems (Ducts)

September 18, 2023

There are two general types of duct systems: single-duct and dual-duct. Each type can be used in both constant- and variable-flow applications.

Ducts are usually made of galvanized steel and are commonly wrapped or lined with fiberglass thermal insulation, both to reduce heat loss or gain through the duct walls and to prevent water vapor from condensing on the exterior of the duct when the duct is carrying cooled air. Insulation, particularly duct liner, also reduces duct-borne noise. Both types of insulation reduce “breakout” noise through the duct walls.

ducts can be manufactured from duct board, a rigid form of fiberglass. The fiberglass provides built-in thermal insulation and the interior surface absorbs sound, helping to quiet the operation of the HVAC system. This type of ducting is typically used in low-pressure systems, 2″ wg or less, due to the structural limitations of the material.

Flexible ducts, also called “flex,” have a variety of material configurations, but are typically flexible plastic over a metal wire coil to make round, flexible duct. Most often, a layer of fiberglass insulation covers the duct, and a thin plastic layer protects the insulation. Flexible duct is convenient for attaching supply-air outlets to the rigid ductwork. However, the pressure loss through flex is higher than for most other types of ducts. As such, duct runs should be short, less than about 15 feet,

The most common use of flex is to connect ducted supply air to terminal units

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