Installation, Operation & Maintenance Manual
E
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Gas Furnace
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07-2021
Data subject to change
7.5 Air Ducts
General requirements
The duct system should be designed and sized according to accepted national
standards such as those published by: Air Conditioning Contractors Association
(ACCA), Sheet Metal and Air Conditioning Contractors National Association
(SMACNA) or American Society of Heating, Refrigerating and Air Conditioning
Engineers (ASHRAE) or consult The Air Systems Design Guidelines reference tables
available from your local distributor. The duct system should be sized to handle the
required system design CFM at the design external static pressure. The furnace
air
fl
ow rates are provided in Tables 3 & 4 "Air Delivery - CFM (Without Filter)".
When a 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 duct(s) sealed to the furnace cabinet and terminating outside the
space containing the furnace.
Secure ductwork with the proper fasteners for type of ductwork used. Seal supply
and return-duct connections to furnace with code approved tape or duct sealer.
Flexible connections should be used between duct work and furnace
to prevent transmission of vibration. Duct work passing through
unconditioned space should be insulated and sealed to enhance
system performance. When air conditioning is used, a vapor barrier is
recommended.
Maintain a 1-inch clearance betweem combustible materials and the supply air
ductwork for a distance of 36 inch horizontally from the furnace. See NFPA 90B or
local code for further requirements.
Ductwork acoustical treatment
Metal duct systems that do not have a 90 degree elbow and 10 ft.
of main duct to the
fi
rst branch take-off may require internal acoustical
lining. As an alternative,
fi
brous duct work may be used if constructed
and installed in accordance with the latest edition of SMACNA
construction standard on
fi
brous glass ducts. Both acoustical lining and
fi
brous duct work shall comply with NFPA 90B as tested by UL Standard
181 for Class 1 Rigid air ducts.
Supply air connections
For a furnace not equipped with a cased coil, the outlet duct shall be provided with
a removable access panel. This opening shall be accessible when the furnace is
installed and shall be of such a size that the heat exchanger can be visually checked
for possible damage using light assistance or a probe that can be inserted for
sampling the airstream. The cover attachment shall prevent leaks.
Up
fl
ow and horizontal furnaces
Connect supply-air duct to
fl
anges on furnace supply-air outlet. Bend
fl
ange upward
to 90° with wide duct pliers. The supply-air duct must be connected to ONLY the
furnace supply-outlet-air duct
fl
anges or cased coil cabinet (when used).
NOTICE:
DO NOT cut main furnace cabinet side to attach supply
air duct, humidifier, or other accessories. All accessories
MUST be connected to duct external to main furnace
cabinet.
For horizontal applications, the top-most
fl
ange may be bent past 90
degrees to allow the evaporator coil to hang on the
fl
ange temporarily
while the remaining attachment and sealing of the coil are performed.
