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nstallation
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onsiderations
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9
IM 1049-9
Ductwork & Attenuation
Discharge ductwork is normally used with these
conditioners. Return air ductwork may also be required.
All ductwork should conform to industry standards of good
practice as described in the ASHRAE Systems Guide.
The discharge duct system will normally consist of a
flexible connector at the unit, a transition piece to the full
duct size, a short run of duct, an elbow without vanes,
and a trunk duct teeing into a branch duct with discharge
diffusers as shown in
Figure 7
. The transition piece must
not have angles totaling more than 30 degrees or severe
loss of air performance can result.
Do not connect the full duct size to the unit without using
a transition piece down to the size of the discharge collar
on the unit. With metal duct material, the sides only of the
elbow and entire branch duct should be internally lined
with acoustic fibrous insulation for sound attenuation.
Glass fiber duct board material is more absorbing and
may permit omission of the canvas connector.
As a general recommendation, the acoustic fibrous
insulation should be at least 1/2-inch thick over the
entire duct run (
Figure 8
). For better sound attenuation,
line the last five diameters of duct before each register
with a one-inch thick sound blanket. Elbows, tees and
dampers can create turbulence or distortion in the
airflow. Place a straight length of duct, 5 to 10 times the
duct width, before the next fitting to smooth out airflow.
Diffusers that are located in the bottom of a trunk duct
can also produce noise. For this same reason, volume
control dampers should be located several duct widths
upstream from an air outlet.
For Hotel, Motel, Dormitory or Nursing Home
applications that use a single duct discharge, a velocity
of 500 to 600 fpm is suggested. These applications
typically have static pressures as low as 0.05 inches of
water and duct lengths approximately six feet in length.
The discharge duct must be fully lined and have a
square elbow without turning vanes. Return air for these
applications should enter through a “low” sidewall filter
grille and route up the stud space to a ceiling plenum.
For horizontal heat pumps mounted from the ceiling,
an insulated return plenum is sometimes placed at the
return air opening to further attenuate line-of-sight sound
transmission through return openings.
Figure 7: Suggested supply ducting per ASHRAE and SMACNA publications
Flexible Connector
Acoustic/Thermal Lining
Two 90° Turns
(Ductwork Sized Based on Airflow)
Diffuser
Diffuser
Acoustic/Thermal Lining
3ft. (.9m) to 5ft. (1.5m)
Ductwork Supported
Independent of Unit
Figure 8: Suggested return ducting per ASHRAE and SMACNA publications
Two 90° Turns Prior to the Intake
(Ductwork Sized Based on Airflow)
Flexible Connector
Acoustic/Thermal Lining
10ft. (3 meters)
Ductwork Supported
Independent of Unit
Acoustic/Thermal Lining
Acoustic/Thermal Lining
Return Air Intake Located
Away from the Unit Blower
Flexible Connector
