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.
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.
Return Air
Figure 27
Filter Retainer
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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.
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.
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