Installation Manual
11
Plenums & Air Ducts
This unit is designed only for use with a bottom supply
duct and must be installed in accordance with the
standards of the National Fire Protection Association
Standard for Installation of Air Conditioning Systems
(NFPA 90A), Standard for Installation of Residence Type
Warm Air Heating and Air Conditioning Systems (NFPA
90B), and all applicable local codes. NFPA publications
are available by writing to: National Fire Protection
Association, Batterymarch Park, Quincy, ME 02269 or go to
www.NFPA.org on the web.
• Design the air ducts according to methods described by
the Air Conditioning Contractors of America (ACCA).
• Air ducts must be aluminum, tin plate, galvanized sheet
steel, or other approved materials for outlet or return
air ducts. Snap-Lock or Pittsburgh-Lock seams are
preferred. All other types of seams must be made tight
to prevent leakage.
• It is good practice to seal all connections and joints
with industrial grade sealing tape or liquid sealant.
Requirements for sealing ducts vary from region to
region. Consult with local codes for requirements specific
to your area.
• Gas piping must not run in or through any of the air duct
system.
• Hollow spaces used as ducts or plenums for
environmental air may contain mineral-insulated metal
sheathed cable, aluminum sheathed cable, electrical
metallic tubing, rigid metal conduit, flexible metal
conduit (not to exceed 4 ft), or metal-clad cables. Wiring
materials, fixtures, are to be suitable for the expected
ambient temperatures to which they will be subjected.
Wiring materials located in the return duct system shall
conform to Articles 300-22 of the National Electrical
Code (NFPA-70).
• All duct work passing through unconditioned space
must be properly insulated to minimize duct losses
and prevent condensation. Use insulation with an outer
vapor barrier. Refer to local codes for insulation material
requirements
• Air conditioning systems may require more duct register
and open louver area to obtain necessary airflow..
• Noncombustible pans having one inch upturned flanges
are located beneath openings in a floor return duct
system.
Supply Air Connections
• For proper air distribution, the supply duct system must
be designed so that the static pressure measured
external to the furnace does not exceed the listed static
pressure shown on the furnace rating plate. The supply
air must be delivered to the heated space by duct(s)
secured to the furnace casing, running full length and
without interruption.
• Duct system must be designed so that no supply registers
are located in duct system directly below the furnace.
• Location, size, and number of registers should be
selected on the basis of best air distribution and floor
plan of the home. Three typical distribution systems are
shown in Figure 6.
A Single trunk duct
B
Dual trunk duct
w/crossover connector
C
Transition duct
w/branches
Figure 6. Typical Supply Duct Systems
Return Air Connections
• MG2R furnaces with an “F” following the input rate in the
model number are factory configured for the return air to
flow through the front louvered door. The return air may
also be attached to either side of the furnace cabinet
using a field installed kit (PN 904003). The location and
size of the side and top return air connections are shown
in Figure 24 (page 29) & Figure 25 (page 30). The
filter size for the side return air is 20” x 20” x 1”.
• Model numbers with a “T” following the input rate are
factory configured for the return air to enter the top of
the furnace. See Figure 26 (page 31) & Figure 27
(page 32).
NOTE: For top return installations, an external or in-
wall filter mount can be used. The factory installed filter
should be removed and the return duct should be sized
to provide adequate airflow.
• For floor return systems, the manufactured housing
manufacturer or installer shall affix a prominent marking
on or near the appliance where it is easily read when
the closet door is open. The marking shall read:
CAUTION, HAZARD OF ASPHYXIATION. DO NOT
COVER OR RESTRICT FLOOR RETURN AIR
OPENING.
• Return air openings should not be located to draw air
directly from a bathroom.
• Materials located in the return duct system shall have
a flame spread classification of 200 or less.
• The total free area of the openings in the floor or the
ceiling registers serving the return air duct system must
not be not less than 352 in
2
(2,270 cm
2
). At least one
register should be located where it is not likely to be
covered by carpeting, boxes and other objects.
Closet & Alcove Installations
WARNING:
Failure to comply with the the following
instructions may result in fire, asphyxiation or
carbon monoxide poisoning.
For proper air circulation, closet installations require
a return air grill installed in the door or side wall that