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18-CD20D3-18
Installer’s Guide
DUCT CONNECTIONS
Air duct systems should be installed in accordance with
standards for air conditioning systems, National Fire Protec-
tion Association Pamphlet No. 90. They should be sized in
accordance with ACCA Manual D or whichever is applicable.
Central furnaces, when used in connection with cooling units,
shall be installed in parallel or on the upstream side of the
cooling coil to avoid condensation in the heat exchanger. With
a parallel flow arrangement, 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 material
may be used for return air and discharge connections to pre-
vent 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 to minimize noise transmission through the return
air grille. Although these furnaces are designed with large
blowers operating at moderate speeds, any blower moving a
high volume of air will produce audible noise which could be
objectionable when the unit is located very close to a living
area. It is often advisable 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.
RETURN AIR DUCT SYSTEMS
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, base-
ment, attic, or crawl space.
Do Not
install return air through the back of the furnace
cabinet.
Do Not
install return air through the side of the furnace
cabinet on horizontal applications.
NOTE:
Minimum return air temperature is 50° F.
All return air duct systems should provide for installation of
return air filters.
1. Set the furnace in place.
2. For upflow side return installations, remove the insulation
around the opening in the blower compartment.
3. The side panels of the upflow furnace include locating
notches that are used as guides for cutting an opening
for return air, refer to Figure 11 and the outline drawing
on page 4 for duct connection dimensions for various
furnaces.
4. If a 3/4" flange is to be used for attaching the air inlet
duct, add to cut where indicated by dotted lines in
Figure 11. Cut corners diagonally and bend outward to
form flange.
5. If flanges are not required, and a filter frame is installed,
cut between locating notches (See Figure 11).
6. The bottom panel of the upflow furnace must be re-
moved for bottom return air.
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