Specifications

JOHNSON CONTROLS
2-64
FORM 102.20-N1 (1109)
Installation
The performance ratings of coils will be met only if
the airow is uniform over the face of the coils. High
air velocity spots on the coil may cause the carry-over
of moisture from the coil. High or low air velocity
areas of the coil will not deliver the published ratings.
The duct connections must be designed to provide for
uniform ow of air across the face of the coil. The
entering duct must provide a smooth transition from
any high velocity effects. Stratications of outside and
return air, especially where below freezing outside air
enters, must be avoided to prevent coil freeze-up or
nuisance low limit trips.
Sound and Vibration Transmission
All roof mounted air-handling units generate some
sound and vibration that may or may not require some
special treatment of the air-conditioned space. The
noise generated by the air-handling unit is dependent
on the speed of the fan, the amount of air the fan is
moving, the fan type and the static efciency of
the fan. In applications where sound and vibration
transmissions may be objectionable, good acoustical
engineering practices must be incorporated in the
system design.
On units with return fans, it is espe-
cially important to consider the effects
of sound transmission into the condi-
tioned space.
When a unit is used with a ceiling plenum return
air system, sound may be transmitted from the unit
through the ceiling to the conditioned space. For
such applications, there should be a sound absorption
chamber installed near the unit return air inlet. Various
reference sources are available regarding acoustic
design.
Front and Rear Discharge Outdoor Unit Duct
Installation
• Roof penetrations by ducts should utilize counter
ashed curbs. (Typical arrangements are shown
in Fig. 2-96.)
• All penetrations into ducts should be sealed
watertight. Attachment of supports should use a
minimum number of duct penetrations.
• Duct systems should not be pressurized without
sufcient time for curing of sealant systems. Fol-
low sealant manufacturer's recommendations for
application of the sealant.
• Adequate clearances between ducts and roof
penetration openings should be provided.
• Ducts should be supported to avoid transfer of
duct weight across exible connections (see Fig.
2-97).
• Horizontal ducts should be pitched and provided
with drainage outlets as illustrated (by the system
designer).
• Ducts should be installed at a height sufcient to
install roong and ashing.
• See Duct connection guidelines on previous
page.
FIG. 2-97 – ROOF TO DUCT INSTALLATION -
HORIZONTAL DISCHARGE
WATERPROOF
ALL SEAMS AND
JOINTS IN DUCTS
ROOF
CURB
ELEVATE BOTTOM OF DUCT
ROOF SUPPORT
SEAL ALL DUCT WALL
PENETRATIONS AT POINTS OF
SUPPORT ATTACHMENT AND
REINFORCE ATTACHMENT
FRONT OR REAR
DISCHARGE UNIT
MECHANICALLY FASTEN
DUCT TO HVAC UNIT WITH
FLANGE AND WATER PROOF
LD06337B
FIG. 2-96 – DUCT PENETRATION OF ROOF
FLASHING RECEIVER
CURB
UPPER DUCT SECTION NOT
CONTINUOUS THROUGH ROOF
SEALANT
WOOD NAILER
NOMINAL 2 X 4
LOWER DUCT SECTION
COUNTERFLASHING
BASE FLASHING
ROOFING PLIES
DECK
UPPER SECTION
EXTENDS 3" TO 4"
BELOW TOP OF CURB.
CANT STRIP
LD06494A