9
When side return air ducts are used, a solid
metal block-off panel must be used to block the
bottom opening in the furnace. Failure to do so
could cause flue gases to be drawn into the liv-
ing space, which could result in asphyxiation.
The return air ducts to the furnace must have a total
cross sectional area of not less than two square
inches per 1000 BTUH of furnace input rating for
heating operation. If air conditioning is to be installed
with the furnace, or if it may be added at a later time,
larger return air ducts may be required, depending
on the capacity of the air conditioner and the airflow
required. For applications requiring more than 1800
CFM, it will be necessary to use the bottom return,
both side returns or one side plus the bottom return.
Return Air Connection – Downflow
The return air connection to the furnace must be
attached to the top of the furnace. It is not permissi-
ble to cut out the side of the furnace casing for side
return ducts. The furnace is supplied with flanges on
the top of the furnace to which the return air duct
may be fastened. The casing top is embossed to
indicate where to bend the flanges.
The return air ducts to the furnace must have a total
cross sectional area of not less than two square
inches per 1000 BTUH of furnace input rating for
heating operation. If air conditioning is to be installed
with the furnace, or if it may be added at a later time,
larger return air ducts may be required, depending
on the capacity of the air conditioner and the airflow
required. The return air opening in the top of the
furnace is large enough for the largest capacity air
conditioner for which the furnace blower is rated.
Return Air Connection – Horizontal
The return air duct must be connected to the inlet
end of the furnace. It is not permissible to cut out the
side of the furnace casing for side return ducts. The
furnace is supplied with flanges on both ends of the
furnace to which the supply and return air ducts may
be fastened. The casing is embossed on both ends
to indicate where to bend the flanges.
The return air ducts to the furnace must have a total
cross sectional area of not less than two square
inches per 1000 BTUH of furnace input rating for
heating operation. If air conditioning is to be installed
with the furnace, or if it may be added at a later time,
larger return air ducts may be required, depending
on the capacity of the air conditioner and the airflow
required. The return air opening in the top of the
furnace is large enough for the largest capacity air
conditioner for which the furnace blower is rated.
When the furnace is installed so that supply
ducts carry air circulated by the furnace to
areas outside the space containing the fur-
nace, the return air must also be handled by
a duct(s) sealed to the furnace casing and
terminating outside the space containing
the furnace. Failure to do so can result in
asphyxiation.
Filters
Air filters must be used with this furnace. Failure to
do so will cause dirt to accumulate on the furnace
blower motor, blower wheel, heat exchanger and air
conditioning coil, resulting in reduced system
efficiency, erratic control performance and possible
equipment damage.
Air velocity must not exceed 300 feet per minute
through low velocity disposable filters. Air velocity
must not exceed 650 feet per minute through high
velocity cleanable permanent filters. Use of a filter
that is too small will cause static pressure in the duct
system to be too high, which will have an adverse
effect on heating and cooling operation.
Downflow
Filters must be mounted in a location where they are
easily accessible for replacement, either behind a
central return air grille or in a filter box mounted on
top of the furnace as shown in Figure 7. A filter rack
is supplied with the furnace. The table lists the
minimum filter sizes.
FIGURE 7 – DOWNFLOW FILTERS