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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, filters, accessories, etc. are accounted for in
total resistance. Refer to the airflow performance tables in this manual to determine the
available external static pressure for the particular air-handler model being installed.
• 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 duct flanges supplied with the unit. See Section 3.5.
•
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 or caulk the duct-to-unit joint as required to prevent air leaks.
3.7 RETURN AIR FILTER
An internal filter rack is not provided with this air-handler. Therefore, an external means
of filtering the return air is required. 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 filter 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 the unit.
Important:
Do not install a return air filter grille
and
a filter rack at the unit and 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 of filters into the external static pressure of
the duct system when selecting blower speeds and designing ductwork to assure the
system 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 tempera-
ture rise (heating mode) and/or temperature drop (cooling mode) with all filters in place.
Important:
High efficiency pleated filters and electronic air cleaners typically have sig-
nificantly higher pressure drop than standard efficiency fiberglass filters, especially when
they get dirty. Do not use high efficiency filters or electronic air cleaners unless ade-
quate filter area is provided to lower the filter pressure drop to an acceptable level.
!
WARNING
Do not operate the system without filters. A portion of the dust entrained in the
air may temporarily lodge in the duct runs and at the supply registers. Any cir-
culated dust particles could be heated and charred by contact with the heating
elements. This residue could soil ceilings, walls, drapes, carpets and other
articles in the house. Operating the system without a filter will also allow lint
and dirt particles to accumulate on the indoor coil fin and restrict air-flow
through the coil. Soot damage may occur even with filters in place when cer-
tain types of candles, oil lamps or standing pilots are burned.
!
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.
