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Ductwork and attenuation
Discharge ductwork is normally used with these condition-
ers. Return air ductwork may also be required, but will require
field installation of a return air duct collar/2" (51mm) filter rack
kit.
All ductwork should conform to industry standards of
good practice as described in ASHRAE Systems Guide.
The discharge duct system will normally consist of a
flexible connector at the unit, a noninsulated transition piece
to the full duct size, a short run of duct, an elbow without
vanes, and a trunk duct teeing into a branch circuit with
discharge diffusers as shown in Figure 2. The transition piece
must not have an angle greater than 30
o
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 insulation for sound attenua-
tion. Glass fiber duct board material is more absorbing and
may permit omission of the flexible connector.
The ductwork should be laid out so that there is no line of
sight between the conditioner discharge and the distribution
diffusers.
Return air ducts can be brought in through a wall grille and
then to the unit. The return duct system will normally consist
of a flexible connector at the unit and a trunk duct to the
return air grille. With metal duct material, the return air duct
should be internally lined with acoustic insulation for sound
attenuation. Glass fiber duct board material is more absorb-
ing and may permit omission of the flexible connector.
Return air ductwork to the unit requires the optional return
air duct collar/2" (51mm) filter rack kit. See Figure 3. The kit
can be installed for face side or bottom filter removal. The
flexible connector can then be attached to the 1" (25mm)
duct collar.
Do not use sheet metal screws directly into the unit
cabinet for connection of supply or return air ductwork,
especially return air ductwork which can puncture the drain
pan or the air coil.
Figure 2.
Ventilation air
Outside air may be required for ventilation. The temperature
of the ventilation air must be controlled so that mixture of
outside air and return air entering the conditioner does not
exceed application limits. It is also general practice to close
off the ventilation air system during unoccupied periods
(night setback).
The ventilation air system is generally a separate building
subsystem with distribution ductwork. Simple introduction
of the outside air into each return air plenum chamber
reasonably close to the conditioner air inlet is not only
adequate, but recommended. Do not duct outside air directly
to the conditioner inlet. Provide sufficient distance for thor-
ough mixing of outside and return air. See “Operating limits”
on page 7.
Figure 3.
Sizes 007 thru 042
Sizes 048 & 060
Heat Pump
Duct
Transition
Discharge Collar
(on Heat Pump)
Square Elbow
(Both Sides
Internally Lined
With Acoustic
Insulation)
Trunk Duct
Branch Duct
(Internally
Lined)
2 ft. x 2 ft
Diffuser
Canvas
Collar