SECTION 4: RETURN AIR REQUIREMENTS AND SUPPLY AIR DISTRIBUTION SYSTEM
MORTEX PRODUCTS INC. 501 TERMINAL RD FORT WORTH, TX 76106 Page 5
Furnaces With 3/4 HP Blower Motor
• Minimum 390 in² (251,612 mm²) free area opening
• Use Return Grille, or A/C Coil Cabinet, or any return grille with
a minimum 390 in² (251,612 mm²) free area opening
Top Return Only – Recommended Return Grille Size
CFM / L/s in x in (mm x mm) in² / mm²
800 / 377 20 x 20 (500 x 500) 324 / 209,032
1000 / 472 20 X 25 (500 x 630) 414 / 267,096
1200 / 566 25 X 25 ( 635 x 635) 414 / 267,096
1400 / 661 25 X 30 ( 635 x 760) 644 / 415,483
1600 / 755 25 X 30 (635 x 760) 644 / 415,483
1800 / 843 30 X 30 ( 762 x 762) 784 / 505,805
Louvered Door & Top Return – Recommended Return Grille Size
CFM / L/s in x in (mm x mm) in² / mm²
800 / 377 10 x 20 (250 x 500) 144 / 92,903
1000 / 472 12 X 20 (300 x 500) 180 / 116,129
1200 / 566 14 X 20 (360 x 500) 216 / 139,355
1400 / 661 18 X 20 (460 x 500) 288 / 185,806
1600 / 755 18 X 20 (460 x 500) 288 / 185,806
1800 / 843 20 X 20 (500 x 500) 324 / 209,032
The return air opening can be located in a closet door, side wall
above the furnace cabinet, or in a louvered door on the furnace.
If the opening for the return air is located in the side wall of a
closet or the closet door below the top of the furnace cabinet, a
6 inch (152.4 mm) minimum clearance must be provided on the
side where the return is located to provide for proper air flow
to the furnace return opening. The 6 inch (152.4 mm) minimum
clearance is not required if the return grille is installed above the
top of the furnace cabinet.
Supply Air Distribution System
The furnace with a cooling coil installed is designed to operate
against a maximum of 0.30-inch WC (0.074 kPa) external static
pressure. In order to assure proper air-flow through the furnace
and coil, the air distribution system (duct system) must not
exceed a total static pressure of 0.30-inch WC (0.074 kPa). It is
recommended that the manual “Manufactured Housing Duct
Systems Guide to Best Practices” by Manufactured Housing
Research Alliance (MHI-MHRA) be reviewed before selecting the
air distribution system for use in the application.
Figure 5: Typical Alcove Installation
Figure 4: Typical Closet Installation
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