23
openings are used in Fig. 28, each opening used will require an
optional filter.
The optional media cabinet (or field supplied accessory air cleaner)
can also be installed in the common return duct prior to entering
the return air opening in any orientation.
Refer to the instructions supplied with optional media cabinet or
accessory air filter for assembly and other details.
Filter and Return Duct Sizing
Pressure drop must be taken into account when sizing filters, filter
racks, IAQ devices, and associated system ductwork. See Table 5
for a comparison of Pressure Drop (initial/clean resistance to
airflow) versus Airflow for a variety of filter media types and sizes.
These are representative numbers. Consult the filter or IAQ device
manufacturers’ specification sheet for performance data for a
particular filter media or IAQ device.
Design the filter and associated ductwork for the best match of
pressure drop versus filter size. Best practice usually chooses filter
systems with pressure drops under 0.2 in. W.C. (50 Pa), with the
best blower electrical efficiency and system airflow performance
occurring with filter pressure drops under 0.1 in. W.C. (25 Pa).
Design the duct system FIRST to determine how much
pressure drop may be allowed in the filter system. See the Air
Ducts section. Excessive filter pressure drop often
compromises system airflow and duct performance, causes
inadequate airflow to the furthest ends of the duct system, as
well as causes excess noise and higher than anticipated
electrical consumption.
NOTICE
Provide duct transitions, as required, to smoothly transition airflow
from the return duct system to the filter (or IAQ device) to the
furnace when the dimensions of the ductwork or furnace return air
opening do not match the required filter or IAQ device dimensions.
See the instructions supplied with factory--accessory duct adapters.
Table 5 – Filter Media Pressure Drop (Clean) Versus Airflow -- In. W.C. (Pa)
14 x 25 Filter
(356 x 635 mm)
Factory-Accessory
Washable
Factory-Accessory
Media*
Representative After-Market Filter Media*
Fiberglass*
Pleated*
CFM
L/s
(1-in. / 2.5 cm)
(4-in. / 10 cm)
(1-in. / 2.5 cm)
(2-in. / 5 cm)
(1-in. / 2.5 cm)
(2-in. / 5 cm)
600
(283)
0.04
(12)
0.05
(12)
0.07
(17)
0.10
(26)
0.24
(60)
0.16
(40)
800
(378)
0.06
(15)
0.07
(19)
0.10
(25)
0.15
(39)
0.34
(85)
0.23
(59)
1000
(472)
0.07
(18)
0.10
(27)
0.13
(34)
0.21
(52)
-
-
0.32
(81)
1200
(566)
0.08
(20)
0.14
(36)
0.17
(43)
0.27
(68)
-
-
-
-
16 x 25 Filter
(406 x 635 mm)
Factory-Accessory
Washable
Factory-Accessory
Media*
Representative After-Market Filter Media*
Fiberglass*
Pleated*
CFM
L/s
(1-in. / 2.5 cm)
(4-in. / 10 cm)
(1-in. / 2.5 cm)
(2-in. / 5 cm)
(1-in. / 2.5 cm)
(2-in. / 5 cm)
600
(283)
0.04
(10)
0.05
(13)
0.06
(15)
0.09
(22)
0.20
(51)
0.13
(34)
800
(378)
0.05
(13)
0.07
(18)
0.08
(21)
0.13
(32)
0.29
(72)
0.20
(49)
1000
(472)
0.06
(16)
0.11
(28)
0.11
(28)
0.17
(43)
-
-
0.27
(67)
1200
(566)
0.07
(18)
0.15
(37)
0.14
(36)
0.22
(56)
-
-
-
-
1400
(661)
0.08
(21)
0.19
(48)
0.18
(45)
0.28
(70)
-
-
-
-
1600
(755)
0.09
(23)
0.24
(60)
0.21
(54)
-
-
-
-
-
-
1800
(850)
0.10
(25)
-
-
0.26
(64)
-
-
-
-
-
-
20 x 25 Filter
(508 x 635 mm)
Factory-Accessory
Washable
Factory-Accessory
Media*
Representative After-Market Filter Media*
Fiberglass*
Pleated*
CFM
(L/s)
(1-in. / 2.5 cm)
(4-in. / 10 cm)
(1-in. / 2.5 cm)
(2-in. / 5 cm)
(1-in. / 2.5 cm)
(2-in. / 5 cm)
800
(378)
0.04
(11)
0.05
(12)
0.06
(16)
0.09
(24)
0.22
(55)
0.15
(37)
1000
(472)
0.05
(13)
0.07
(18)
0.08
(21)
0.13
(32)
0.29
(72)
0.20
(49)
1200
(566)
0.06
(15)
0.09
(22)
0.11
(27)
0.16
(41)
-
-
0.25
(63)
1400
(661)
0.07
(17)
0.12
(31)
0.13
(33)
0.20
(51)
-
-
0.31
(79)
1600
(755)
0.08
(19)
0.15
(38)
0.16
(40)
0.24
(61)
-
-
-
-
1800
(850)
0.08
(21)
0.18
(47)
0.18
(47)
0.29
(73)
-
-
-
-
2000
(944)
0.09
(23)
0.22
(56)
0.21
(54)
-
-
-
-
-
-
2200
(1038)
0.09
(24)
0.26
(66)
0.25
(62)
-
-
-
-
-
-
25 x 25 Filter
(635 x 635 mm)
Factory-Accessory
Washable
Factory-Accessory
Media*
Representative After-Market Filter Media*
Fiberglass*
Pleated*
CFM
L/s
(1-in. / 2.5 cm)
(4-in. / 10 cm)
(1-in. / 2.5 cm)
(2-in. / 5 cm)
(1-in. / 2.5 cm)
(2-in. / 5 cm)
800
(378)
0.03
(9)
0.03
(8)
0.05
(12)
0.07
(18)
0.17
(43)
0.11
(28)
1000
(472)
0.04
(11)
0.05
(12)
0.06
(16)
0.09
(24)
0.22
(55)
0.15
(37)
1200
(566)
0.05
(13)
0.07
(17)
0.08
(20)
0.12
(31)
0.27
(68)
0.18
(47)
1400
(661)
0.06
(15)
0.09
(23)
0.10
(24)
0.15
(38)
-
-
0.23
(58)
1600
(755)
0.06
(16)
0.12
(31)
0.11
(29)
0.18
(45)
-
-
0.28
(69)
1800
(850)
0.07
(18)
0.14
(35)
0.13
(34)
0.21
(53)
-
-
-
-
2000
(944)
0.08
(19)
0.16
(41)
0.16
(39)
0.24
(61)
-
-
-
-
2200
(1038)
0.08
(21)
0.19
(49)
0.18
(45)
0.28
(70)
-
-
-
-
If the filter size that you are looking for is not contained in Table 5, refer to Table 6 for a comparison of Pressure Drop (initial/clean resistance
to airflow) versus Face Velocity for a variety of filter media types.
The following equations relate Face Velocity (FPM), Filter Area and Airflow (CFM):
Filter Face Velocity = Airflow / Filter Area
Minimum Filter Area = Rated System Airflow / Maximum Filter Face Velocity
922SA
