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Section 8:
Ductwork Consideration:
The unit has been designed to operate at the specific air
volume and external static pressure that was ordered.
This static pressure is generated by any additional
components that are added to the unit (i.e. ductwork, etc).
Additional static pressure beyond that ordered will affect
the performance of the air conditioning unit and lessen
the air volume that can be delivered.
Proper engineering methods need to be employed when
calculating duct and component static pressure (i.e. 2009
ASHRAE Handbook - Fundamentals, Chapter 21).
The system ductwork must comply with Sheet Metal
and Air Conditioning Contractors National Association
(SMACNA) or any other recognized standards.
It is recommended that flexible duct connections be
incorporated into the ductwork design to prevent
the transmission of any vibrations, either mechanical
or harmonic.
As a general rule, all ducts should have a straight run of
at least 3 hydraulic duct diameters immediately before
and after the unit before adding any fittings, elbows,
restrictions, etc.
Hydraulic duct diameter for round ducts (in inches):
Dh = d
Dh: hydraulic diameter
d: round duct inside diameter
Hydraulic duct diameter for rectangular ducts (in inches):
Dh = (2*H*W)/(H+W)
Dh: hydraulic diameter
H: rectangular duct inside height
W: rectangular duct inside width
The unit is not designed to support the weight of
ductwork. Ductwork must be constructed in a fashion that
is self-supporting.
Depending on the options ordered with the unit, flanges
(either external or internal) may be provided to facilitate
connection of ductwork. In cases where flanges are not
provided, flat surfaces on the exterior skin of the unit are
provided to facilitate connection of ductwork.
Neither the flanges nor exterior skin of the unit are
capable of supporting the load of the ductwork. Ductwork
support must come from the structure itself that the unit
is servicing. Ductwork passing through unconditioned
spaces must be insulated (including a vapor barrier) to
prevent unnecessary energy losses and/or condensation.
8.1 Outside or Return Air Ductwork
Return air ductwork height and width must be no smaller
than the unit return air opening height and width.
8.2 Discharge Ductwork
Discharge air ductwork height and width must be no
smaller that the unit discharge air opening height
and width.
RC/FC Series Installation, Operation, and Maintenance Manual
