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Return Air
When sizing the return air ducts, keep in mind that if they are
too small they can create noise, but if they are too large, the fan
coil cannot build up proper pressure. Table 03 has recommended
return air sizes for round and rectangular ducts. A variance of
plus 20% is allowable for sizing return ducts that connect to the
Hi-Velocity Systems unit.
It is recommended to install a grill that is 10 - 20% larger than
specifications require, this will ensure that there is no air velocity
noise at the grill. Where allowed by local codes, a single return
air grill may be used. When using flexible duct for return air, use
one duct size larger due to the higher friction loss.
The Return Air is to be sized on a 0.15 static pressure (37
pa) as compared to 0.10 static pressure (25 pa) for conventional
forced air systems. The maximum length for an individual return
air duct is fifty feet (15.24m).
Duct Sizing
Please note: It is VERY important NOT to
undersize the return air, as this will create
noise, increase motor power consumption,
reduce airflow and increase the possibility of
condensate carry-over.
Table 03 has recommended return air sizes for round and
rectangular ducts. A variance of
+20%
is allowable for sizing
return ducts that connect to the RCM-I or Hi-Velocity Systems
unit.
Where allowed by local codes, a single return air grill may be
used. Note: Return air grill must have equal minimum of free air
area to return air.
Table 03 – Return Air Duct Sizes
Unit
Rigid Ø
Flex Ø
Min Sq.
Inches
(Sq. cm)
50/51/52
12”
(305mm)
14”
(356mm)
120
(774cm)
70/71
12”
(305mm)
14”
(356mm)
120
(774cm)
100/101
14”
(356mm)
16”
(406mm)
168
(1084cm)
Remember: When using flexible duct for return air, use
one duct size larger due to the higher friction loss.
Important: When connecting a round
Return Air duct to the RCM-I coil, a round to
rectangular transition is required.
Important: When using flexible duct for
return air, use one duct size larger due to the
higher friction loss.
Return Cutout
Fig. 07 shows the different locations the return air can
be installed on Hi-Velocity fan coils. Table 03 contains the
dimensions needed for the return. All fan coils are equipped with
return air knockout cuts and can be used as a proper return air
size.
Model
A
B
C
D
50/51 H/BU
70/71 H/BU
100/101 H/BU
3”
3”
3”
17”
17”
17”
10”
15”
22”
14”
15”
15”
Fig. 07 - Return air cutout sizing
Legend
RA - Return air
Notes:
• DO NOT cut past the center plate or electrical
box (Dim A & B).
• 100 and 140 Units CANNOT use return air “C” (RA-C).
• This drawing is NOT to scale.
Table 03 - Return Air Cutouts
Module RCM-I - RCM-I Coil Installation (5/8)
