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18-CD31D1-5
Installer’s Guide
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TABLE 3
MINIMUM FREE AREA IN SQUARE INCHES
EACH OPENING (FURNACE ONLY)
Furnace
Maximum
BTUH/INPUT
Rating
Air
From
Inside
Air From Outside
Vertical
Duct
Horizontal
Duct
60,000
80,000
100,000
120,000
140,000
100
100
100
120
140
15
20
25
30
35
30
40
50
60
70
DUCT CONNECTIONS
Air duct systems should be installed in accordance with
standards for air conditioning systems, National Fire Pro-
tection Association Pamphlet No. 90. They should be sized
in accordance with ACCA Manual D or whichever is appli-
cable. Check on controls to make certain they are correct
for the electrical supply.
Central furnaces, when used in connection with cooling
units, shall be installed in parallel or on the upstream side
of the cooling units to avoid condensation in the heating
element, unless the furnace has been specifically approved
for downstream installation. With a parallel flow arrange-
ment, the dampers or other means used to control flow of
air shall be adequate to prevent chilled air from entering
the furnace, and if manually operated, must be equipped
with means to prevent operation of either unit unless the
damper is in full heat or cool position.
On any job, flexible connections of nonflammable mate-
rial may be used for return air and discharge connections
to prevent transmission of vibration. Though these units
have been specifically designed for quiet, vibration free
operation, air ducts can act as sounding boards and could,
if poorly installed, amplify the slightest vibration to the
annoyance level.
When the furnace is located in a utility room adjacent to
the living area, the system should be carefully designed
with returns which minimize noise transmission through
the return air grille. Although these winter air condition-
ers are designed with large blowers operating at moderate
speeds, any blower moving a high volume of air will pro-
duce audible noise which could be objectionable when the
unit is located very close to a living area. It is often advis-
able to route the return air ducts under the floor or through
the attic. Such design permits the installation of air return
remote from the living area (i.e. central hall).
When the furnace is installed so that the supply ducts carry
air circulated by the furnace to areas outside the space
containing the furnace, the return air shall also be handled
by a duct(s) sealed to the furnace and terminating outside
the space containing the furnace.
Minimum return air/“air entering” temperature for
the furnace is 55° F.
Where there is no complete return duct system, the return
connection must be run full size from the furnace to a
location outside the utility room, basement, attic, or crawl
space.
DO NOT install return air through the back of the
furnace cabinet.
RETURN AIR DUCT CONNECTION
NOTE:
On upflow 5 or 6 ton airflow models, if the airflow require-
ment exceeds 1800 CFM, these models will require return
air openings and filters on both sides; OR 1 side and the
bottom; OR just the bottom.
All return air duct systems should provide for installation
of return air filters.
PREPARATION FOR UPFLOW BOTTOM AND SIDE RE-
TURN AIR FILTER INSTALLATION
All return air duct systems should provide for installation
of return air filters.
1. Determine the appropriate position to set the furnace
in order to connect to existing supply and return duct-
work.
