Furnace Installation and Owners Manual
40
LDJ620_E01_R03_2006.03
Models LDJ620-9 and LDJ620-10
inexpensive to install. However, they should be avoided
because they are difficult to seal and cannot always be
insulated. In addition, because they tend to be rough and have
many twists and turns, it is difficult to build them so as to
ensure good air distribution. Even return plenums built under a
stairway or in a closet, for example, should be avoided if a
completely ducted system is possible.
S
H E E T
M
E T A L
D
U C T S
Sheet metal is the most common duct material and can be used
on most all supply and return duct applications (for plenums,
trunks, branches, and runouts). Sheet metal ducts have a
smooth interior surface that offers the least resistance to air
flow. They must be carefully and completely sealed during
construction/installation, using approved tapes or preferably
mastic, because each connection, joint, and seam has potential
leakage. Screws should be used to mechanically fasten all
joints. When located in an unconditioned space, sheet metal
ducts must be insulated with either an interior duct liner or
exterior insulation.
DUCT BOARD
BOOT AND DUCT CONNECTION HIGHLIGHTS
F
I B E R G L A S S
D
U C T
B
O A R D
D
U C T S
Fiberglass duct board is insulated and sealed as part of its
construction. It is usually used to form rectangular supply and
return trunks, branches, and plenums, although it can be used
for runouts as well. Fiberglass duct board provides excellent
sound attenuation, but its longevity is highly dependent on its
closure and fastening systems. Connections should be
mechanically fastened using shiplap or V-groove joints and
stapling and sealed with approved pressure-sensitive tapes
and mastic.
F
L E X I B L E
N
O N M E T A L L I C
D
U C T S
Flexible nonmetallic duct (or flex duct) consists of a duct inner
liner supported on the inside by a helix wire coil and covered
by blanket insulation with a flexible vapor-barrier jacket on the
outside. Flexible duct is often used for runouts, with metal
collars used to connect the flexible duct to supply plenums,
trunks, and branches constructed from sheet metal or duct
board. Flexible duct is also commonly used as a return duct.
Flexible duct is factory-insulated and has fewer duct
connections and joints. However, flexible duct is easily torn,
crushed, pinched, or damaged during installation. It has the
highest resistance to air flow. Consequently, if used, it must be
properly installed.
