SUPPLY DUCT CONFIGURATIONS
S U P P L Y D U C T S Y S T E M S
Supply ducts deliver air to the spaces that are to be
conditioned. The two most common supply duct systems for
residences are the trunk and branch system and the radial
system because of their versatility, performance, and economy.
The spider and perimeter loop systems are other options.
T
R U N K
A N D
B
R A N C H
S
Y S T E M
In the trunk and branch system, a large main supply trunk is
connected directly to the air handler or its supply plenum and
serves as a supply plenum or an extension to the supply plenum.
Smaller branch ducts and runouts are connected to the trunk.
The trunk and branch system is adaptable to most houses, but
it has more places where leaks can occur. It provides air flows
that are easily balanced and can be easily designed to be
located inside the conditioned space of the house.
There are several variations of the trunk and branch system. An
extended plenum system uses a main supply trunk that is one
size and is the simplest and most popular design. The length of
the trunk is usually limited to about 24 feet because otherwise
the velocity of the air in the trunk gets too low and air flow into
branches and runouts close to the air handler becomes poor.
Therefore, with a centrally located air handler, this duct system
can be installed in homes up to approximately 50 feet long. A
reducing plenum system uses a trunk reduction periodically to
maintain a more uniform pressure and air velocity in the trunk,
which improves air flow in branches and runouts closer to the
air handler. Similarly, a reducing trunk system reduces the
cross-sectional area of the trunk after every branch duct or
runout, but it is the most complex system to design.
S
P I D E R
S
Y S T E M
A spider system is a more distinct variation of the trunk and
branch system. Large supply trunks (usually large-diameter
flexible ducts) connect remote mixing boxes to a small, central
supply plenum. Smaller branch ducts or runouts take air from the
remote mixing boxes to the individual supply outlets. This system
is difficult to locate within the conditioned space of the house.
R
A D I A L
S
Y S T E M
In a radial system, there is no main supply trunk; branch ducts
or runouts that deliver conditioned air to individual supply
outlets are essentially connected directly to the air handler,
usually using a small supply plenum. The short, direct duct
runs maximize air flow. The radial system is most adaptable to
single-story homes. Traditionally, this system is associated
with an air handler that is centrally located so that ducts are
arranged in a radial pattern. However, symmetry is not
mandatory, and designs using parallel runouts can be
designed so that duct runs remain in the conditioned space
(e.g., installed above a dropped ceiling).
P
E R I M E T E R
L
O O P
S
Y S T E M
A perimeter loop system uses a perimeter duct fed from a
central supply plenum using several feeder ducts. This system
is typically limited to houses built on slab in cold climates and
is more difficult to design and install.
Spider
Radial
Trunk and Branch
Perimeter Loop