Choosing the ducting for a heating, ventilation, and air conditioning (HVAC) job depends on the desired effects of the duct work. Insulation is an important factor as heat loss or gain through the duct work can impact the efficiency of the system. Noise reduction can also be a consideration in jobs where the sound of the HVAC system must be reduced. Finally, moisture and condensation buildup on the ducts impacts the durability of the duct work and respiratory health as molds and bacteria can grow on the moist surfaces.
Pre-insulated ducts are generally made from mild galvanized steel and insulated with polyurethane or phenol. Ducts made from these materials conform to UnderWriter Laboratory's standard UL 181, which specifies that the insulation have a flame-spread index of less than 25. These ducts are square and the panels are cut to fit and sealed with glue and tape. This type of air duct panel is typically used for building air conditioning and heating systems.
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Fiberglass duct boards are square, like the polyurethane boards, but can be cut with a special knife to form hinges in the boards for easier installation. Fiberglass duct boards offer moisture resistance and buffer sounds better than polyurethane ducts. This reduces the noises produced by the heating and cooling system for quieter operation. Fiberglass boards tend to be more expensive than the polyurethane boards.
Flexible ducts can curve to fit a variety of applications, although curves in the ducts will increase the pressure on the air conditioning or heating system. The design of the ducts increases resistance to air flow and it is best to use as little flexible duct work as possible. Insulated models provide better sound and moisture resistance than the non-insulated models.