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Return Air - Atwood 79 Series Service Manual

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#2 - Voltage
See 4 Always previous page.
#3 - Minimum Ducting Requirements
The various BTU sizes of 85, 89 and 2-stage series furnaces require a minimum number of square inches of heat ducting.
The most common size of soft ducting is 4˝ diameter. Therefore, since a 4˝ duct is equal to 12 square inches, we can call
out the minimum number of ducts needed for the models of furnaces noted below.
Furnace Model
8516, 8520
8525. 8531
8535, 8935, 8940, 1522, 2334, 2540
Ducting Locations
85, 89 and 2-Stage
Series Furnaces
CAUTION - Ducting Installation
4˝ Flexible Hose
each 90° bend adds the equivalent of 10 feet of ducting
ducting should be securely attached to furnace
each run should be as straight and short as possible
2˝ Flexible Hose
2 - 2˝ duct runs do not equal one 4˝ duct. A 2˝ duct is only 3 sq/inches
2˝ duct adapters are available
2˝ ducts are ideally suited for bathroom and holding tank compartments
Closeable Outlets
a closeable outlet does not contribute to the minimum of total outlets recommended for a furnace.
Bottom Discharge
furnace must be completely sealed to floor and plenum with a bottom discharge gasket, with no air gaps.
if furnace is installed in middle of run, the main duct run must be a minimum of 24 sq/inches.
if furnace is installed at the end of the run, the main duct must be 48 sq/inches.

#4 - Return Air

This return air requirement can be met in a couple of ways.
3 3 in
2
7 9 S e ri e s
8 5 , 1 5 2 2 a
8 0 in
2
8 9 a n d 2 5 4
0 S e ri e s
Minimum
Ducting
24 sq. in.
36 sq. in.
48 sq. in.
D u
D u
c t
# 6
D u
c t
# 5
D u
c t
# 4
The return air grille mounted on an inside wall of the trailer, exposed to
the cabinet area of the furnace is the most common mounting used on
the 85, 89 and 2-Stage Series furnaces.
Another option is to provide openings at various locations in the furnace
cabinet area capable of drawing air from inside the trailer (ie. rowtered
6 5 in
2
holes at bases of sofas or walls, etc.).
The total square inches of openings must meet minimum requirements.
n d 2 3 3 4 S
e ri e s
Do not place any types of air filters in front of or behind the return air door.
Blocking this area will substantially decrease the return air causing - less
air delivery to the heat registers - short cycle of the furnace - limiting of
the furnace.
We recommend electrical air filters that can be placed anywhere in the
open living area of the recreation vehicle. They can be purchased in most
hardware stores.
Minimum
Ducts
2
3
4
O u
t s i
c t
# 7
D u
D u
c t
c t
# 2
# 3
27
d e
o f
R V
B o
t t o
o p
m
e n
d i s
( 1 2
i n g
c h
7 m
5 "
m
x 1
D u
x 2
c t
5 4
# 1
a r
g e
0 "
m
m
)

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