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4.0 DUCTWORK
Field ductwork must comply with the National Fire Protection Association NFPA 90A,
NFPA 90B and any applicable local ordinance.
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. Ductwork must comply
with National Fire Protection Association as tested by U/L Standard 181 for Class I Air
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 is important that the system airflow be adequate. Make
sure supply and return ductwork, grills, special filters, accessories, etc. are accounted
for in total resistance. See airflow performance tables in this manual.
• Design the duct system in accordance with “ACCA” Manual “D” Design for Residential
Winter and Summer Air Conditioning and Equipment Selection. Latest editions are
available from: “ACCA” Air Conditioning Contractors of America, 1513 16th Street,
N.W., Washington, D.C. 20036. If duct system incorporates flexible air duct, be sure
pressure drop information (straight length plus all turns) shown in “ACCA” Manual
“D” is accounted for in system.
• Supply plenum is attached to the 3/4” duct flanges supplied on the unit around the
blower outlet. Flanges are flat for shipping purposes and must be bent up along perfo-
rated edge around blower opening. Be sure to bend flanges completely up so they do
not interfere with air being discharged from blower.
IMPORTANT: Flanges around blower opening for attaching supply duct must be
bent up out of blower discharge even if not used so they do not restrict airflow
from blower.
• Supply plenum should be the same size as the flanges provided around the blower
outlet. Ideally, it should extend 3 feet from the unit before turning or branching off
plenum into duct runs. The plenum forms an extension of the blower housing and
minimizes air expansion losses from the blower. Changing the size, shape or length
will degrade blower performance. If supply discharges directly into a larger duct or
plenum as much as .1” W.C., static pressure will be lost. If 3 feet is not possible, even
6, 12 or 18 inches will help.
IMPORTANT: If an elbow is included in the plenum close to the unit, it must not be
smaller than the dimensions of the supply duct flange on the unit.
• Some units with electric heaters require 1 in. clearance to supply plenum and branch
ducts to combustible material for the first 3 feet from the unit. See CLEARANCES.
• A 3/4” return duct flange is supplied on all sides of the air inlet opening of the unit coil
casing. If the unit is to be installed without a coil casing (no indoor coil), a 3/4” flange
is supplied on the back and sides of the air inlet opening of the blower casing. No
flange is provided on the front of the opening to the blower casing. If return duct is
attached to the inlet of the blower casing, the front flange of the duct should be run up
into the opening or 90° brake made on the front flange to tape to the front of the blow-
er casing.
• IMPORTANT: The front flange on the return duct if connected to the blower cas-
ing must not be screwed into the area where the power wiring is located. Drills
or sharp screw points can damage insulation on wires located inside unit.
• Return duct flanges on blower or coil casing are flat for shipping purposes and must
be bent out along perforated edge around opening.
• Secure the supply and return ductwork to the unit flanges, using proper fasteners for
the type of duct used and tape the duct-to-unit joint as required to prevent air leaks.
!
WARNING
UNITS ARE FOR DUCTED APPLICATIONS ONLY. A MINIMUM OF 36 INCHES OF
SUPPLY AIR PLENUM AND DUCTWORK IS REQUIRED. NO SUPPLY AIR OPEN-
INGS, REGISTERS OR FLEXIBLE AIR DUCTS MAY BE LOCATED WITHIN THE
FIRST 36 INCHES OF SUPPLY PLENUM AND DUCTWORK ON UNITS WITH
ELECTRIC HEATERS. FAILURE TO OBSERVE SUPPLY PLENUM/DUCT WARN-
INGS CAN CAUSE A FIRE RESULTING IN PROPERTY DAMAGE, PERSONAL
INJURY OR DEATH.
(SEE SPECIFIC AIRFLOW POSITION FOR ADDITIONAL
WARNINGS)
