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
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.