Instructions / Assembly

Chapter 28 Ductwork
evaporator is located inside the plenum, Figure 28-9B,
and the supply air duct attaches to the plenum. The
furnace manufacturer offers a plenum kit along with
the necessary evaporator. For adding air conditioning to
a previously installed furnace, some sheet metal work
may be required. After all parts are assembled and
screwed together, the seams must be anchored with
.sheet metal screws and sealed with mastic (a nontoxic
adhesive material used for permanently sealing joints
and seams). See Figure 28-9C.
Ductboard made from compressed fiberglass is
becoming increasingly popular for plenums and
rectangular duct. Ductboard is inexpensive, easy to
work with, cuts with a knife, and is already insulated.
Applying insulation by hand to metal ducts is often
necessary, but is time-consuming and costly. See
Figure 28-10.
Ductboard
Insulation
Mastic
28.4.2 Round Metal Pipe
Round pipe can be used as a main trunk line or a
branch line, in both supply air and return air situations.
Round metal pipe has the most efficient charac tens tics,
and is commonly used. For heating only, the round pipe
supplying air to a register outlet need only be 6" in
diameter. For air conditioning, or combined heating and
cooling, the pipe should be 8"in diameter. Additional
6"- runs are often used to obtain required capacity.
Standard round pipe comes in 30-gauge, 3"-14" in
diameter, and in 2', 3', 5', and 10' lengths. All round
pipe is factory crimped on one end for easy insertion
into another length of pipe, See Figure 28-11.
Seam
Figure 28-10. A plenum can be fabricated from ductboard,
which is less expensive than installing insulation by hand on
a metal plenum.
Crimped
end
Figure 28-11. Hound pipe for branch lines can easily be
snapped together at the seam. The crimped area at one end
allows the pipe to be slipped inside the next section of pipe,
forming a joint
28.4.3 Rectangular Duct
Rectangular duct is 30-gaugc, galvanized sheet
metal, and mainly used as a trunk line, or "extended
plenum." Rectangular duct is widely used in the north-
eastern and northwestern parts of the United States. It
is used as a spacesaver and can be easily routed in
overhead space between floor joists, or in an attic.
Branch lines consisting of round pipe come off the
extended plenum to transport air to a register. A branch
line begins with a starting collar or takeoff installed on
the extended plenum, (main trunk line). The end of the
extended plenum must be blocked wilh an end cap that
is anchored with sheet metal screws and sealed with
mastic. See Figure 28-12.
'Takeoff
Rectangular duct
Plenum
End cap
Figure 28-12. An extended plenum, formed from rectangular duct, has a takeoff fitting at one end and a cap at the other.