5715184-BIM-A-0519
Johnson Controls Ducted Systems
5
SECTION II: DUCTWORK
DUCTWORK GENERAL INFORMATION
The duct system’s design and installation must:
•
Handle an air volume appropriate for the served space and within
the operating parameters of the furnace specifications.
•
Be installed in accordance of National Fire Protection Association
as outlined in NFPA standard 90B (latest editions) or applicable
national, provincial, state, and local fire and safety codes.
•
For Manufactured (Mobile) Home and Modular Home Return
Duct System Installations:
The return air duct and the return air
plenum are not required by the furnace manufacturer provided
the combustion air and vent system have been installed and
maintained as a Two Pipe Sealed Combustion Air System and
provided a return air duct and plenum are not required by state,
local or regional codes.
•
Complete a path for heated or cooled air to circulate through the
air conditioning and heating equipment and to and from the con-
ditioned space.
When the furnace is used with a indoor coil, the coil must be installed
parallel with, or in the supply air side of the furnace to avoid condensa-
tion in the primary heat exchanger. When a parallel flow arrangement is
used, dampers or other means used to control airflow must be ade-
quate to prevent chilled air from entering the furnace. If manually oper-
ated, the damper must be equipped with means to prevent the furnace
or the air conditioner from operating unless the damper is in full heat or
cool position.
When replacing an existing furnace, if the existing plenum is not the
same size as the new furnace then the existing plenum must be
removed and a new plenum installed that is the proper size for the new
furnace. If the plenum is shorter than 12 in. (30.5 cm) the turbulent air
flow may cause the limit controls not to operate as designed, or the limit
controls may not operate at all.
The duct system is a very important part of the installation.
If the duct
system is improperly sized, the furnace does not operate properly.
The ducts attached to the furnace plenum must be of sufficient size so
the furnace operates at the specified external static pressure and within
the air temperature rise specified on the nameplate.
If a matching indoor coil is used, it may be placed directly on the fur-
nace outlet and sealed to prevent leakage. If an uncased indoor coil
with a thermoplastic drain pan is to be installed in the upflow/horizontal
configuration, then extra 2 in. minimum spacing may be needed to
ensure against drain pan distortion.
On all installations without a coil, a removable access panel is recom-
mended in the outlet duct such that smoke or reflected light would be
observable inside the casing to indicate the presence of leaks in the
heat exchanger. This access cover shall be attached in such a manner
as to prevent leaks.
DUCT FLANGES
Four flanges are provided to attach ductwork to the furnace. These
flanges are rotated down for shipment. In order to use the flanges,
remove the screw holding an individual flange, rotate the flange so it is
in the upward position and reinstall the screw then repeat this for all 4
flanges.
If the flanges are not used, they must remain in the rotated down posi-
tion as shipped.
CAUTION
The indoor coil must be installed in the supply air duct, downstream
of the furnace. Cooled air may not be passed over the heat
exchanger.
IMPORTANT:
The minimum plenum height is 12 in. (30.5 cm). The
furnace does not operate properly on a shorter plenum height. The
minimum recommended rectangular duct height is 4 in. (10 cm)
attached to the plenum.
!
WARNING
The duct system must be properly sized to obtain the correct airflow
for the furnace size that is being installed.
Refer to the furnace rating plate for the correct rise range and static
pressures or to Table 5 for the correct rise range.
If the ducts are undersized, the result will be high duct static pres-
sures and/or high temperature rises which can result in a heat
exchanger OVERHEATING CONDITION. This condition can result
in premature heat exchanger failure, which can result in personal
injury, property damage, or death.
FIGURE 1:
Duct Attachment
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