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Installer’s Information Manual Page 19
Follow these instructions and the filter manufacturer's
instructions for minimum size and quantity
recommendations. 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 washable filters. Undersized filters reduce airflow
and can adversely affect furnace and cooling system
operation.
See Table 6 for recommended filter size for bottom return
installations. Side return filters must be at least 16" X 25" X
1", high-velocity washable filters.
Table 6. Recommended Filter Size--Bottom Return.
GAS
INPUT*
(Btu/hr)
MOTOR*
HORSE-
POWER
DISPOSABLE
AIR FILTER
- two required
(inches)
HIGH-
VELOCITY
WASHABLE
AIR FILTER
54,000 1/3 16 X 25 X 1 16 X 25 X 1
72,000
90,000
1/2
16 X 25 X 1 16 X 25 X 1
72,000
90,000
108,000
126,000
3/4 20 X 25 X 1 20 X 25 X 1
* GAS INPUT and MOTOR HP can be found on furnace
rating plate.
FILTER CABINETS
Use a bottom filter cabinet, side filter cabinet, or return-air
filter grille. Field fabricated filter cabinets should allow 1
inch spacing between filter and furnace. Placing a field
supplied filter directly against furnace bottom or side will
decrease airflow and adversely affect furnace operation.
Filter cabinets that provide correct filter spacing to ensure
designed airflow are available from the manufacturer.
Figure 16. Bottom Filter Cabinet.
Manufacturer's bottom filter cabinet and upflow side filter
cabinets are shown in Figures 16 and 17 respectively. See
Figure 18 for floor cutout dimensions for bottom filter
cabinet.
Air delivery above 1,800 CFM requires returns on both
sides, a combination of side and bottom returns, or bottom
return only. Use appropriate filter cabinets with these
combinations.
Figure 17. Side Filter Cabinet.
Figure 18. Floor Cutout Dimensions.