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RPB-IOM (06-22) 131782-B
• Duct sizing: Proper sizing of the supply air ductwork is necessary to ensure a satisfactory heating installation. The
recognized authority for duct size is the Air Conditioning Contractor’s Association, 2800 Shirlington Road, Suite 300,
Arlington, VA 22206 (www.acca.org). A manual covering duct sizing in detail may be purchased directly from them.
⚠ CAUTION ⚠
To prevent possible motor overloading, ensure that the external duct system static pressure is
within the limits shown on the rating plate and that the motor pulley and belt are properly adjusted.
• Access panels: Install removable access panels (see Figure 5) on both the upstream and downstream sides of
the furnace. The access panels must be accessible when the furnace is in service and should be a minimum of
6 × 10 inches (152 × 254 mm) in size so smoke or reflected light may be observed inside the casing to indicate
the presence of leaks in the heat exchanger. Ensure that the access panels are installed in such a manner so as
to prevent leakage.
⚠ CAUTION ⚠
The joint where the supply air duct attaches to the furnace must be sealed securely to prevent
air leakage into draft hood or burner rack area. Leakage can cause poor combustion and pilot
problems, can shorten heat exchanger life, and can cause poor performance.
• Supply air duct/furnace horizontal connection: The seal between the furnace and the duct must be mechanical
using U-type flanges on the top and bottom of the connecting duct to to ensure tight joints and an airtight fit. Refer
to Figure 5 and perform the following steps:
a. Ensure that flanges on the furnace (heat exchanger) turn out as shown.
b. Shape duct connection as shown: U-type on top and bottom and L-type on sides.
c. Slide U-channels over furnace top and bottom flanges making connection.
d. Form U-channels to seal sides (see DETAIL A) and drill and lock with sheet metal screws.
• Bottom duct/furnace connection: On outdoor models, insert ducts from below the roof deck through the roof
opening into the heater. Form 1-inch (25-mm) flanges, fold over, and fasten with sheet metal screws inside the
heater. Gain access to the unit by removing side panels from the blower and downturn plenum sections. Ducts
must be attached and sealed to provide airtight connections.
• Return air duct/furnace connection: All return air ducts should be attached and sealed to the return air flanges
to provide airtight connections.
• Return air duct/grill size: Ensure that return air ducting or grills have a free area equal to the size of the return
duct connection.
Figure 5. Connecting Ductwork to Furnace
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