Bryant 912SB Installation, Start-Up, Operating And Service And Maintenance Instructions page 22

There are no provisions for an internal filter rack in these furnaces.
An external filter is required and is purchased separately. A field
supplied accessory air cleaner may also be used in place of the filter
rack.
For upflow applications, the filter can be installed on either side of
the furnace, the bottom of the furnace or any combination of side
and bottom of the furnace. See Fig. 18, 19, and 25.
For downflow applications, the filter rack (or field supplied
accessory air cleaner) must only be connected to the bottom
opening on the furnace. See Fig. 19 and 26.
For horizontal applications, the filter rack (or field supplied
accessory air cleaner) can be connected to the bottom opening on
the furnace. For side return use in the horizontal position, refer to
Fig. 27. If both side and bottom openings are used in Fig. 27, each
opening used will require a filter.
A filter rack or any field supplied accessory air cleaner can also be
installed in the common return duct prior to entering the return air
opening in any orientation.
Refer to the instructions supplied with filter rack or accessory air
filter for additional assembly and installation options.
See Table 7 for filter size details.
Filter and Return Duct Sizing
Pressure drop must be taken into account when sizing filters, filter
racks, IAQ devices, and associated system ductwork. See Table 5
for a comparison of Pressure Drop (initial/clean resistance to
airflow) versus Airflow for a variety of filter media types and sizes.
These are representative numbers. Consult the filter or IAQ device
manufacturers' specification sheet for performance data for a
particular filter media or IAQ device.
Design the filter and associated ductwork for the best match of
pressure drop versus filter size. Best practice usually chooses filter
systems with pressure drops under 0.2 in. W.C. (50 Pa), with the
best blower electrical efficiency and system airflow performance
occurring with filter pressure drops under 0.1 in. W.C. (25 Pa).
Due to the relatively high pressure drops of 1- -in. (25 mm) thick
after- -market filter media, it is recommended that the filtration
system be designed for at least 2- -in. (51 mm) thick media.
TIPS FROM CONTRACTORS: Install a media cabinet capable
of incorporating a 4- -in. (102 mm) thick media filter. This allows
room for future upgrades to other IAQ devices.
NOTICE
Design the duct system FIRST to determine how much
pressure drop may be allowed in the filter system. See the Air
Ducts section. Excessive
compromises system airflow and duct performance, causes
inadequate airflow to the furthest ends of the duct system, as
well as causes excess noise and higher than anticipated
electrical consumption.
Provide duct transitions, as required, to smoothly transition airflow
from the return duct system to the filter (or IAQ device) to the
furnace when the dimensions of the ductwork or furnace return air
opening do not match the required filter or IAQ device dimensions.
See the instructions supplied with factory- -accessory duct adapters.
filter pressure drop often
22