forth in their Manual D or Manual CS as applicable.
Connecting two pieces of Duct
• Before installation the ends of the ducts should be
trimmed so they're square
• Collars need to be a minimum of 2 inches long
• Collars and sleeves need to be inserted at least an inch
into the duct before you fasten them
• The sleeves used for joining two sections of duct should
be at least 4 inches wide
• When you're joining lengths of flexible duct or
attaching duct to air supply or terminal equipment,
make sure the adhesive is chemically compatible with
the materials you're using
• To secure nonmetallic flexible duct to a sleeve or collar,
use a draw band. If the duct is larger than 12 in. in
diameter, the metal collar needs to have a bead put in
it, and the draw band should be located behind that
bead
• Tapes and sealants used on connections should be listed
for UL 181B, Closure Systems for Use with Flexible Air
Ducts and Connectors, a standard out in first edition in
1995
DUCTWORK
• A closed return duct system must be used
• Ductwork should be flexible to prevent noise
transmission from the unit to the structure.
• Ductwork should be insulated and weatherproofed in
accordance with local codes. A minimum of two (2)
inches of insulation is recommended to minimize
heating and cooling losses.
• Ductwork should have a watertight vapor barrier to
prevent moisture damage to the insulating material.
• Exposed ductwork must be weatherproofed between
the unit and the building.
• Ductwork should conform to the methods of the Air
Conditioning Contractors of America (ACCA), as set
• Flexible duct connectors must be of a flame retardant
material.
Flexible Duct Bends
• Use a minimum length of flexible duct to maximize
efficiency and minimize energy loss.
• Any necessary bends should be made with a minimum
diameter of one duct centerline radius.
• The duct should extend a few inches beyond the end of
a sheet metal connection before you bend it.
• Avoid compressing ductwork when bending.
Flexible Duct Support
Flexible duct work should be supported by hangers or
supports adequately attached to the structure.
Hangers must be wide enough to support the section of
duct resting on the hanger without reducing the internal
diameter of the duct.
The part of the hanger touching the duct should never be
less than one inch wide. Narrower hanger material may be
used in conjunction with a sheet metal saddle that meets
this specification. This saddle must cover one-half the
circumference of the outside of the flexible duct and fit
neatly around the lower half of the duct.
Attaching Ductwork
Use sheet metal screws to attach flexible metal duct. These
screws should be equally spaced around the duct's
perimeter.
When fastening duct work to the side duct flanges on the
unit, insert the screws through the duct flanges only. Do not
insert the screws through the casing.
When installing ducts bigger than 12 inches use five evenly
spaced #8 screws. The screws should be located at least 1/2
in. from the end of the duct.
Insulation and vapor barriers on factory-fabricated ducts
should be fitted over the core connection and then also
secured with a draw band.
CAUTION
The duct's vapor barrier can be subject to damage by
sharp edges. Do not support the entire weight of the
flexible duct on any one hanger during installation.
Avoid contacting the flexible duct with sharp edges of
the hanger material. You can repair damage to the
vapor barrier using an approved tape. If you penetrate
the interior of the duct, you should either replace that
section, or treat the torn area as a connection.
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