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Your Guide to the Types of Ducts


The ductwork that's hidden behind your walls and ceilings is an essential element of your forced-air HVAC system. If the duct system is well-designed and in good condition, air gets drawn in through the return registers, heated or cooled by the HVAC equipment, and an equal volume is distributed through the supply ducts. Your home may have rigid or flexible ductwork, or a combination of these two main duct types.

Rigid Air Ducts

Rigid ducts come in sections that are joined together during installation. There are two different rigid duct types commonly used in home HVAC systems:

  • Sheet metal —Round or rectangular ducts made of sheet galvanized steel or aluminum are the most durable and costly available. They have a non-porous surface that's easily cleaned, and the smooth metal makes an inhospitable environment for biological contaminants. The individual sections must be screwed together, then the seams and joints sealed with mastic to stop air leakage. Installing an insulation wrap is necessary as well, to reduce energy losses through the bare metal.
  • Fiberboard —A less expensive alternative to sheet metal, fiberboard ducts are made from compressed fiberglass strands bonded together with resin. The inside is sealed to stop fiberglass fibers from shedding and entering the air supply, and a layer of foil laminate is applied to the outside as a moisture and air barrier.

Flexible Air Ducts

Flexible ducts consist of coiled wire encapsulated in a two-ply plastic with a layer of foil-faced insulation on the outside. This inexpensive ducting is typically installed in a continuous section that's cut to the length needed.

Its flexibility makes this type of ducting ideal for fitting into tight areas where ducts made of rigid materials can't be installed. Extra care is needed during installation to avoid puncturing or tearing the lightweight material, though, and the long runs must be well-secured and supported to keep them from kinking and sagging. Damaged or poorly-installed flexible ducts can leak air or restrict vital system airflow.

Visit us at Valley Heating, Cooling, Electrical for in-depth information on most HVAC topics, or just give us a call at (408) 868-5500.

Our goal is to help educate our customers in the South Bay Peninsula and Santa Cruz areas of California about energy and home comfort issues (specific to HVAC systems).

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