Instructions / Assembly

Chapter 28 Ductwork
used for extended plenums is 30-gauge, galvanized
sheet metal, 8" deep, 60" long, and comes in standard
widths of 10", 14", 18", 24" and 28". One of these
standard sizes should provide the needed capacity, but
any length of duct can be cut to a shorter length.
28.5.2 Rule of Thumb for Determining
Plenum Size
To determine the width required for an extended
plenum, counl the number of round pipes it will serve,
Figure 28-16. For 6" round pipes, multiply this number
by 2, then add 2 — the total is the required duct width
in inches. For 8* round pipes, multiply by 3, then add 2.
If the desired size is not available, choose the next
larger size. See
EXAMPLES:
A duct serving four 6" round pipes will have a
widthof4x2 = 8 + 2 = 10.
A duct serving five 8" round pipes should have a
width of 5 X 3 = 15 + 2 = 17. Since rectangular
duct is not available in a 17" width, use the next
larger standard size, which is 18".
28.5.3 Rule of Thumb for Determining Size
of Branch Lines
1. Determine square footage of the room.
Length of room, times width of room, equals
square footage.
10'X 12'= 120 sq.ft.
2. Determine cfm of air needed to supply the room,
cfm means "Cubic Feet per Minute" and defines
the amount of air flowing.
One cfm of air is required to heat or cool
1.00-1.25 sq. ft. of floor area.
EXAMPLE:
A 120 sq. ft. room will require between 96 and
120 cfm.
6" pipes
Figure 28-16. The number and size of round pipe branch
lines determines the width of the extended plenum.
3. Select pipe(s) size according to cfm needed.
How much air can a given diameter of pipe carry?
A 120 sq. ft. room requiring 96-120 cfm will need
one 6" pipe (110 cfm), or use a combination to
total about 96-120 cfm.
120 -f 1.00= 120 120- 1.25 = 96
Square footage
Room size
cfm required
64
(8X8)
51-64
80
(8X10)
64-80
100
(10 X 10)
80-100
120
(10 X 12)
96-120
150
(10 X 15)
120-150
200
(12.5 X 16}
160-200
256
(16X 16)
205-256