14
CIRCULATING AIR REQUIREMENTS
WARNING:
• Plenumsandairductsmustbeinstalledinaccordance
with the Standard for the Installation of Air Conditioning
and Ventilating Systems (NFPA No. 90A) or the
Standard for the Installation of Warm Air Heating and
Air Conditioning Systems (NFPA No. 90B).
• Table 6 (page 34), Table 7 (page 35), Table 8 (page
36)
, Table 9 (page 38), & Table 10 (page 39)
contain the maximum airflow and temperature rise
data for each furnace input rate. If the maximum airflow
is 1,600 CFM or more, it is recommended that two
openings be used for return air on upflow furnaces.
Downflow furnaces can only use one return opening.
• It is recommended that the outlet duct contain a
removable access panel. The opening should be
accessible when the furnace is installed in service and
shall be of a size that smoke or reflected light may be
observed inside the casing to indicate the presence of
leaks in the heat exchanger. The cover for the opening
shall be attached in a way that prevent leaks.
• If outsideairisusedas return airtothefurnace for
ventilation or to improve indoor air quality, the system
must be designed so that the return air is not less than
60° F (15° C) during operation. If a combination of
indoor and outdoor air is used, the ducts and damper
system must be designed so that the return air supply
to the furnace is equal to the return air supply under
normal, indoor return air applications.
• When a cooling system is installed which uses the
furnace blower to provide airflow over the indoor coil,
the coil must be installed downstream (on the outlet
side) of the furnace or in parallel with the furnace.
• If a cooling system is installed in parallel with the
furnace, a damper must be installed to prevent chilled
air from entering the furnace and condensing on the
heat exchanger. If a manually operated damper is
installed, it must be designed so that operation of
the furnace is prevented when the damper is in the
cooling position and operation of the cooling system is
prevented when the damper is in the heating position.
• Seal all connections and joints withindustrial grade
sealing tape or liquid sealant. Requirements for sealing
ductwork vary from region to region. Consult with local
codes for requirements specific to your area.
Return Air Connections
• In applications where the supply ducts carry heated
air to areas outside the space where the furnace is
installed, the return air must be delivered to the furnace
by duct(s) secured to the furnace casing, running full
size and without interruption.
• Position the furnace with the return air ductwork
ensuring even alignment of furnace (or coil casing)
air opening and return air duct. NOTE: The ductwork
must have an opening equal to that of the return air
opening of the furnace (or coil casing). See Figure
31 (page 32)
or Figure 32 (page 33) for return air
opening size.
Upflow & Horizontal Furnaces
• The return air ductwork may be connected to the
left side, right side, or bottom of the furnace. NOTE:
If using the left or right side of the furnace for return
air, the bottom panel (Figure 32) must not be removed
from the bottom of the furnace.
WARNING:
• Side Return Installations: To attach the return air
duct to the left or right side of the furnace, punch out
the 4 knockouts from the side of the furnace (Figure 32).
Using sharp metal cutters, cut an opening between all
4 knockouts to expose the blower assembly. Position
the return air duct over the opening and secure to the
side with sheet metal screws.
• If using the bottom of
the furnace for return air, the bottom panel (Figure 32)
must be removed from the bottom of the furnace. See
page 21 for removal instructions. Position the furnace
over the return air duct and secure together with sheet
metal screws. Make sure the screws penetrate the
duct and furnace casing.
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