Bulletin 30-034.009
Copyright © 2022 Unico Inc.
Page 10
DUCT DESIGN
The design of the duct system must account for the added restriction of the furnace. Table 5 lists the static pressure
drop of the furnaces. Usually it will be necessary to add additional outlets to account for the added restriction,
particularly for the systems requiring more than 800 CFM (378 L/s). For these systems it is not unusual to have 7
outlets per ton.
Table 7. Pressure drop, inches of water (Pa)
Air Handler Airflow
Range
Electric Duct Heater Models
CFM (L/s)
2kW
5kW
7.5kW
10kW
15kW
20kW
U
1
2
1
8
150 (71)
0.04 (10.0) 0.06 (14.9) 0.04 (10.0)
200 (94)
0.07 (17.4) 0.11 (27.4) 0.08 (19.9)
250 (118)
0.11 (27.4) 0.17 (42.3) 0.12 (29.9)
300 (142)
0.16 (39.8) 0.24 (59.8) 0.18 (44.8)
350 (165)
0.21 (52.3) 0.32 (79.7) 0.24 (59.8)
M2
4
3
0
B
M3
0
3
6
B
400 (189)
0.14 (34.9) 0.18 (44.8) 0.19 (47.3)
450 (212)
0.17 (42.3) 0.21 (52.3) 0.23 (57.3)
500 (236)
0.20 (49.8) 0.25 (62.3) 0.27 (67.2)
550 (260)
0.23 (57.3) 0.29 (72.2) 0.31 (77.2)
M3
6
4
2
B
600 (283)
0.27 (67.2) 0.33 (82.2) 0.35 (87.2) 0.47 (117.0)
650 (307)
0.30 (74.7) 0.37 (92.1) 0.39 (97.1) 0.53 (132.0)
700 (330)
0.33 (82.2) 0.41 (102.1) 0.44 (109.6) 0.59 (146.9)
M4
8
6
0
B
750 (354)
0.15 (37.4)
-
0.26 (64.7) 0.35 (87.2)
800 (378)
0.16 (39.8)
-
0.28 (69.7) 0.38 (94.6) 0.48 (119.5)
850 (401)
0.17 (42.3)
-
0.30 (74.7) 0.41 (102.1) 0.51 (127.0)
900 (423)
0.19 (47.3)
-
0.32 (79.7) 0.44 (109.6) 0.55 (137.0)
950 (448)
0.20 (49.8)
-
0.35 (87.2) 0.47 (117.0) 0.59 (146.9)
1000 (472)
0.21 (52.3)
-
0.37 (92.1) 0.5 (124.5) 0.63 (156.9)
1050 (496)
0.23 (57.3)
-
0.39 (97.1) 0.53 (132.0) 0.68 (169.3)
1100 (519)
0.24 (59.8)
-
0.42 (104.6) 0.57 (141.9) 0.71 (176.8)
1150 (543)
0.26 (64.7)
-
0.44 (109.6) 0.60 (149.4) 0.75 (186.8)
1200 (566)
0.27 (67.2)
-
0.47 (117.0) 0.63 (156.9) 0.80 (199.2)
1250 (590)
0.28 (69.7)
-
0.49 (122.0) 0.67 (166.8) 0.84 (209.2)
