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Maintain all required clearances to combustible surfaces
as stated on the unit rating plate.
Supply and return ductwork must be adequately sized to
meet the system’s air requirements and static pressure
capabilities. Ductwork should be insulated with a minimum
of 1” thick insulation with a vapor barrier in conditioned
areas or 2” minimum in unconditioned areas. Connect
Sheet metal ductwork run in unconditioned spaces must
be insulated and covered with a vapor barrier. Fibrous
ductwork may be used if constructed and installed in
accordance with SMACNA Construction Standard on
Fibrous Glass Ducts. Check local codes for requirements
on ductwork and insulation.
Duct system must be designed within the range of external
static pressure the unit is designed to operate against. It
accessories, etc. are accounted for in total resistance.
the unit around the blower outlet.
opening provided around the blower outlet and should be
installed as recommended in ASHRAE or ACCA Manual D.
The plenum forms an extension of the blower housing and
minimizes air expansion losses from the blower.
If an elbow is included in the plenum close to the unit, it
must not be smaller than the dimensions of the supply duct
Non-Ducted Return Closet Installation
The cabinet can be installed in a closet with a false bottom
to form a return air plenum or be installed with a return
air plenum under the unit. Louvers or return air grilles are
without a ducted return to single story buildings.
• Install louvers in a closet. Use the free area of louver or
grille to determine the size opening required to provide
the free area for metal louvers or grilles.
• If the free area is not known, assume a 25% free area
for wood or a 75% free area for metal louvers or grilles.
• If the return air plenum is used, the return air grille
should be immediately in front of the opening in the
When not installed in front of the opening, there must be
adequate clearance around the unit to allow for the free
Nominal Tons Air
Conditioning & Nominal Air
Flow
Square Inch Surface Area & Nominal Size
Minimum Return Air Free
Area
Disposable Filters Washable Filters
Up through 2 ton
800 - 900 CFM
432 sq. in.
20 in x 25 in
260 sq. in.
15 in x 20 in
260 sq. in.
2-1/2 ton
900 - 1000 CFM
480 sq. in.
20 in x 30 in
288 sq. in.
14 in x 25 in
288 sq. in.
3 tons
1300 - 1500 CFM
576 sq. in.
14 in x 25 in (2)
346 sq. in.
16 in x 25 in
346 sq. in.
3-1/2 tons
1300 - 1500 CFM
672 sq. in.
16 in x 25 in (2)
404 sq. in.
20 in x 25 in
404 sq. in.
4 tons
1500 - 1700 CFM
768 sq. in.
20 in x 25 in (2)
461 sq. in.
20 in x 25 in
461 sq. in.
5 tons
1900 - 2100 CFM
960 sq. in.
20 in x 30 in (2)
576 sq. in.
24 in x 25 in
576 sq. in.
Table 1. Recommended Remote Filter Sizes
Page 3
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