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Flexible Duct

Overview and Guide to Ductwork

What is the role of the duct system?

Duct systems are used to deliver and remove conditioned and unconditioned air from a building or home.

One of the most common problems we find in homes with comfort and energy problems is with the duct system. Often times the original ducts were not sized correctly, have damage and leaks, and are deteriorating. We can improve overall heating and cooling performance by addressing problems with existing duct systems. A proper duct installation can improve the indoor quality of a home and increase thermal comfort. To install ductwork properly and effectively, it must be properly designed.

How do you right size the ducts?

To calculate the amount of air needed to cool or heat each room, Sierra Pacific uses information from ACCA protocols Manual D to right size the duct and Manual J to identify the size of the load system.

What is the best design for your duct system?

Using Manual D, the Sierra Pacific balances the delivery of air against the friction rate of the ducts and the static pressure in the system. To do so, the we examine the location of the air handler, the distance to the duct, the number of turns the duct makes, and the amount of air needed. The duct material greatly impacts the results of this calculation (for example, sheet metal ducts have a lower friction rate than flex ducts or rigid fiberglass duct board).

Where are ducts usually located?

Ductwork is usually installed in unconditioned spaces (for instance, attic or crawlspace). Exposure to the temperature fluxuations in uncondition spaces can significantly increase heating/cooling losses because of heat transfer, in addition to air leaks in the ductwork. Energy losses in the ducts can range from 10% to 45%.

What is the best way to improve performance when ducts are located in an unconditioned space?

Sierra Pacific only uses UV-resistant R‐8 duct insulation and highly recommends locating ducts on the attic floor and deep burying them under loose‐fill insulation. Deep burying is a cost‐effective way to ensure air moving through the ducts remains at the desired temperature. Deep burying ducts is considered best practice in home performance.

For example, during the summer, Sacramento attics routinely reach 140 degrees. The upper portion of an attic can be 20°F hotter than the attic floor, so ducts located on the attic floor are exposed to less ambient heat, and ducts buried in loose‐fill insulation are further protected from temperature changes in the attic.

What is the best way to deep bury ducts?

The ducts should be positioned on the attic floor. The top of the duct should be at least 3.5” below the surface of the loose‐fill insulation level. Each individual section of ductwork must be buried at the same depth. During installation, vertical indicators (rulers) must be placed on buried trunk and branch ducts at a minimum spacing of every eight feet.

The rulers serve two purposes: (1) to indicate the insulation level above the attic floor, and (2) to indicate the location of the deep buried ducts. When ducts are found hanging from the attic ceiling, Sierra Pacific highly recommends relocating the ducts to the attic floor and covering the ducts with insulation until they meet the deep buried protocols. When installing flex duct, you need to pull the inner liner tight to reduce friction and provide optimum airflow.

Download the free PG&E Duct Testing brochure

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Contractors License #453302
Sierra Pacific Home & Comfort, Inc.
2550 Mercantile Drive Suite D
Rancho Cordova, CA 95742
(800) 551-3040

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