508144-01
Page 12 of 42
Issue 2128
If a high-efficiency filter is being installed as part of this
system to ensure better indoor air quality, the filter must
be properly sized. High-efficiency filters have a higher
static pressure drop than standardefficiency glass/foam
filters. If the pressure drop is too great, system capacity
and performance may be reduced. The pressure drop
may also cause the limit to trip more frequently during
the winter and the indoor coil to freeze in the summer,
resulting in an increase in the number of service calls.
Before using any filter with this system, check the
specifications provided by the filter manufacturer
against the data given in the appropriate Technical
Specification.
WARNING
Furnace Cabinet
Width
Filter Size
Side Return
Bottom Return
A - 14-1/2”
16 x 25 x 1
14 x 25 x 1
B - 17-1/2”
16 x 25 x 1
C - 21”
20 x 25 x 1
Table 1.
Duct System
Use industry approved standards (such as those published
by Air Conditioning Contractors of America or American
Society of Heating, Refrigerating and Air Conditioning
Engineers) to size and install the supply and return air duct
system. This will result in a quiet and low static system that
has uniform air distribution.
NOTE:
Do not operate the furnace in the heating mode
with an external static pressure that exceeds 0.8 inches
w.c. Higher external static pressures may cause erratic
limit operation.
Supply Air Plenum
If the furnace is installed without a cooling coil, a removable
access panel must be installed in the supply air duct. The
access panel should be large enough to permit inspection
(either by smoke or reflected light) of the heat exchanger
for leaks after the furnace is installed. The furnace access
panel must always be in place when the furnace is operating
and it must not allow leaks into the supply air duct system.
Install self tapping screws in the three evaporator coil
screw holes made for horizontal applications to seal the
top cap to the vestibule panel.
Return Air Plenum
NOTE:
Return air must not be drawn from a room where
this furnace, or any other gas fueled appliance (i.e., water
heater), or carbon monoxide producing device (i.e., wood
fireplace) is installed.
When return air is drawn from a room, a negative pressure
is created in the room. If a gas appliance is operating in
a room with negative pressure, the flue products can be
pulled back down the vent pipe and into the room. This
reverse flow of the flue gas may result in incomplete
combustion and the formation of carbon monoxide gas.
This toxic gas might then be distributed throughout the
house by the furnace duct system.
In upflow applications, the return air can be brought
in through the bottom or either side of the furnace. If a
furnace with bottom return air is installed on a platform,
make an airtight seal between the bottom of the furnace
and the platform to ensure that the unit operates properly
and safely. Use fiberglass sealing strips, caulking, or
equivalent sealing method between the plenum and the
furnace cabinet to ensure a tight seal. If a filter is installed,
size the return air duct to fit the filter frame.
HORIZONTAL UNIT
SUPPLY
AIR
SUPPLY
AIR
Install self tapping screws
to seal any potential air
leaks
UPFLOW UNIT
Figure 15.
