Page 22 of 84
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 packaged 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. 2017
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. Ductwork for curb-mounted units
should be attached to the curb prior to setting the unit,
and may not be attached to the bottom of the unit.
Units ordered in a horizontal discharge are supplied
with flanges to attach 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 Inlet Ductwork
Inlet ductwork height and width must be no smaller than
the packaged air conditioning unit inlet height and width
and supply only uncontaminated air to the unit.
8.2 Return Air Ductwork
Return air ductwork height and width must be no smaller
than the unit return air opening height and width.
8.3 Discharge Ductwork
Discharge air ductwork height and width must be no
smaller that the unit discharge air opening height
and width.
PR Series Installation, Operation, and Maintenance Manual
