268882-UIM-A-0407
Unitary Products Group
5
When the furnace is used in conjunction with a cooling coil, the coil
must be installed parallel with, or in the supply air side of the furnace to
avoid condensation in the primary heat exchanger. When a parallel flow
arrangement is used, dampers or other means used to control airflow
must be adequate to prevent chilled air from entering the furnace. If
manually operated, the damper must be equipped with means to pre-
vent the furnace or the air conditioner from operating unless the damper
is in full heat or cool position.
FLOOR BASE AND DUCTWORK INSTALLATION
Downflow Combustible Floor Base
Installations on combustible materials require the use a
combustible floor base shown in Figure 1.
The floor base must be secured to the floor. A supply air
duct plenum with 1" (2.54 cm) flange is installed through the
opening provided. The supply air duct is then secured to the
duct system with screws and sealed to prevent leaks. Do
not shoot screws through the flanges of the supply air duct
into the top of the combustible floor base. Install the furnace on the
combustible floor base so that the corners of the furnace are parallel
with the corner brackets of the floor base. Follow the instructions sup-
plied with the combustible floor base accessory.
This combustible floor base can be replaced with a matching cooling
coil, properly sealed to prevent leaks. Follow the instructions supplied
with the cooling coil cabinet for installing the cabinet to the duct connec-
tor. Refer to the installation instructions for additional information.
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.
IMPORTANT:
If the supply air duct is being connected to the furnace
without the use of an accessory duct connector, then a transition duct
must be installed with flanges or tabs that are securely attach and
sealed to the supply air duct and to the base of the furnace. The transi-
tion duct must have insulation between the transition duct and any com-
bustible material.
The transition duct must be the same dimensional size as the rectangu-
lar opening in the base of the furnace.
The duct system is a very important part of the installation.
If the duct
system is improperly sized the furnace will not operate properly.
The ducts attached to the furnace plenum, should be of sufficient size
so that the furnace operates at the specified external static pressure
and within the air temperature rise specified on the nameplate.
IMPORTANT:
Fabricate and install an inspection door in the plenum
base below the unit to allow an annual inspection of the heat exchang-
ers. The inspection door can be fabricated by the following method.
1.
Cut a rectangular opening in the plenum base.
2.
A sheet metal plate can be made that completely covers the open-
ing in the base.
3.
The plate must be secured with screws.
4.
This plate must be sealed to prevent leaks.
To properly design the ductwork for the building, refer to the ASHRAE
Fundamentals Handbook, Chapter on “DUCT DESIGN” or a company
that specializes in Residential, Modular, and HUD Home duct designs.
The cooling coil must be installed in the supply air duct, down-
stream of the furnace. Cooled air must not flow over the heat
exchanger.
The duct system must be properly sized to obtain the correct airflow
for the furnace size that is being installed.
Refer to Table 3 and the furnace rating plate for the correct rise
range and static pressures
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:
Combustible Floor Base Accessory
The supply air temperature MUST NEVER exceed the Maximum
Supply Air Temperature, specified on the nameplate.
Operating the furnace above the maximum supply air temperature
will cause the heat exchanger to overheat, causing premature heat
exchanger failure. Improper duct sizing, dirty air filters, incorrect
manifold pressure, incorrect gas orifice and/or a faulty limit switch
can cause the furnace to operate above the maximum supply air
temperature. Refer to SECTIONS II, III and XI for additional infor-
mation on correcting the problem.
DOWNFLOW
FURNACE
WARM AIR PLENUM
WITH 1” FLANGES
FIBERGLASS
INSULATION
FIBERGLASS TAPE
UNDER FLANGE
COMBUSTIBLE FLOOR
BASE ACCESSORY
