Specifications

IM 1049-4 / Page 7 of 36
Horizontal Unit 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exible
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-
brousinsulationforsoundattenuation.Glassberductboard
materialismoreabsorbingandmaypermitomissionofthe
canvas connector.
Asageneralrecommendation,theacousticbrousinsulation
shouldbeatleast1/2-inchthickovertheentireductrun(Fig-
ure 8).Forbettersoundattenuation,linethelastvediam-
etersofductbeforeeachregisterwithaone-inchthicksound
blanket.Elbows,teesanddamperscancreateturbulenceor
distortionintheairow.Placeastraightlengthofduct,5to
10timestheductwidth,beforethenextttingtosmoothout
airow.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up-
stream 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lter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transmis-
sion 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 Airow)
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 Airow)
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