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3.6 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 with the unit. Attach
flanges around the blower outlet.
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.
•
IMPORTANT:
The front flange on the return duct if connected to the blower casing
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.
• 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.
3.7 RETURN AIR FILTER
An internal filter rack is provided with this unit and is sufficient for most applications
using this type of air-handler. If an external means of filtering the return air is required,
the external filters should be sized for a maximum of 300 feet/min air velocity or the
maximum velocity recommended by the type of filter installed. One or more return air fil
-
ter grilles, a filter rack attached to unit return air intake, or a filter rack installed between
a sealed return air platform and the return duct are all acceptable means of filtration. All
return ducts must be filtered, either at each return grille or at a common filter near or
inside the unit.
Important:
Do not install a return air filter grille if a filter rack is installed at the unit or
the internal filter is used. Do not install a filter in the supply duct system.
Filter type, sizing, and placement are critical to heating and cooling system performance.
Reduced air-flow can shorten the life of system components such as the compressor,
indoor coil, heater elements, over temperature limits, and relays. As filters near the end
of their useful life, the pressure drop through them increases. Therefore, it is important
to factor the “end of life” (dirty) pressure drop filters the external static pressure of the
duct system when selecting blower speeds and designing ductwork to assure the sys-
tem is operating at the design CFM and system reliability is not compromised. Always
verify that the system’s air-flow is within specifications by performing a temperature rise
(heating mode) and temperature drop (cooling mode) with all filters in place.
Important:
High efficiency pleated filters and electronic air cleaners typically have
significantly higher pressure drop than standard efficiency fiberglass filters, especially
when they get dirty. Do not use high efficiency filters or electronic air cleaners unless
adequate filter area is provided to lower the filter pressure drop to an acceptable level.
WARNING
Do not, under any circumstances, connect return ductwork to any other
heat producing device such as fireplace insert, stove, etc. Unauthorized
use of such devices may result in fire, carbon monoxide poisoning, explo-
sion, personal injury or property damage.
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