19
Minimum Recommended Filter Sizes
Qty. - Nominal Size, Inches (Sq. In. Surface Area)
Size_Air Flow
Disposable
Permanent
045_30
1 - 20 X 25 (500)
1 - 15 X 20 (300)
070_30
1 - 20 X 25 (500)
1 - 15 X 20 (300)
070_40
2 - 14 X 25 (350)
1 - 16 X 25 (400)
090_30
1 - 24 X 24 (576)
1 - 16 X 25 (400)
090_50
2 - 18 X 25 (450)
1 - 20 X 25 (500)
115_40
2 - 14 X 25 (350)
1 - 18 X 25 (450)
115_50
2 - 18 X 25 (450)
1 - 20 X 25 (500)
140_50
2 - 18 X 25 (450)
1 - 20 X 25 (500)
Figure 25
Upflow/Horizontal Furnaces
UPFLOW INSTALLATIONS
This furnace contains rails for installing 16 x 25 x 1 filters on
each side of the interior furnace cabinet. A retainer is also
included for bottom return. Refer to the chart below for filter
sizes when using bottom return.
Bottom Return Air Filters
Size
Filter Size, Inches
Type
045, 070
14 X 25 X 1
P
90
16 X 25 X 1
P
115, 140
20 X 25 X 1
P
Figure 26
Bottom Return
Filter Sizes
The illustration below shows how the filter is retained over
the bottom return air opening.
Figure 27
Filter Retainer
One inch throwaway filters should be sized for a face
velocity of 300 feet per minute or less (14 x 25 x 1
throwaway = 730 CFM max.; 16 x 25 x 1 throwaway = 830
CFM max.; 18 x 25 x 1 throwaway = 940 CFM max.; 20 x
25 x 1 throwaway = 1040 CFM max.) All other filters should
be sized according to their manufacturer’s instructions.
For air delivery of less than 1800 CFM; use one side return
or bottom return.
For air delivery of 1800 CFM or higher; use two side returns,
or one side return plus bottom return.
Guide dimples locate the side and bottom return cutout
locations. Use a straight edge to scribe lines connecting the
dimples. Cut out the opening on these lines. An undersized
opening will cause reduced airflow. For bottom return
connection, remove the bottom of the cabinet before setting
the furnace on the raised platform or return air duct.
HORIZONTAL INSTALLATIONS
Filter(s) must be installed external to the furnace casing.
Using a central return with filters installed in the duct behind
the return air grille allows filters to be replaced by just
removing the grille. This prevents having to go into the attic
or crawl space when a filter has to be changed.
Refer to the preceding chart on Recommended Minimum
Filter Sizes. (Figure 25)
COUNTERFLOW INSTALLATIONS
A filter rack is shipped with the furnace. To use this rack,
proceed as follows:
Figure 28
Furnace Mounted Filter Rack
1. Center the filter rack over the plenum flange and push
down firmly (Figure 28). Since the return air plenum will
need to be attached later, do not screw the filter rack
down.
2. Install the return air plenum on the furnace. For proper
filter performance, the sides of the plenum must be at
least as tall as dimension “A” shown in Figures 29 and
30.
"A"
Min
Return Air
Optional
Access
Door
Figure 29
Return Air Plenum Installation
NOTE: For easier filter inspection and replacement, the
installer may wish to provide a removable panel in the front
of the return air plenum.
