Below is an example of duct sizing
The load calculation indicates a heat gain of 28,000 BTUH, Therefore a 2-1/2 ton A/C unit is
needed. The total system CFM is 1000 (for heating only, use the furnace cfm). The cfm of each
branch is determined using the example shown for room C.
The return trunk must handle 1000 cfm. The first section of the supply trunk must handle 1000 cfm
while the second section will have to handle the remaining 517 cfm
Room C
3000 btuh Heat Gain (obtain from room-by-room load calculation)
3000/28,000 X 1000 = 107 CFM
C
107 cfm
125 cfm
88 cfm
1000 cfm
AHU
1000 cfm
1000 cfm
A
return
517 cfm
68 cfm 110 cfm
110 cfm
External static pressure of air handler @ 100 cfmwet coil
Air filter
registers
Grille
Total Available Static Pressure
50 cfm 123 cfm
84 cfm
Measured length of duct (A to B ) =
Equivalent lengths of turns and fittings between A and B =
Total effective length =
135 cfm
B
65 feet
190 feet
255 feet
.5” w.c
-.10” w.c.
-.15” w.c.
-.03” w.c.
-.03” w.c.
.19’ w.c.
Friction Rate= .19 X 100
255
= .075” w.c. (Size all ducts at this friction rate)
Use the Friction Rate (.075) to size all trunks and branches
Return and first section of
supply trunk will be 20”
diameter (1000 cfm
@ .075” w.c.
Second section of supply
trunk will be 16” diameter
(517 cfm @.075’ w.c.)
Outlet C will be 8”
diameter (107 cfm
@ .075” w.c.)
Size the remaining outlets
according to their CFM
@ .075” w.c.