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86 SCXG-SVX01B-EN
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Unit Airside Components
The unit’s air delivery system consists of
condensers, dampers, enthalpy switch
option, airside economizer option, filters,
low ambient sensors, and factory
mounted single or double wall plenums.
Supply Air Fan
The unit has a single supply fan that runs
at a constant speed. However, the fan
may have the IGV or VFD option that
modulates airflow based on supply air
temperature control. Pressing the stop
key on the HI will turn the supply fan off.
The fan is on continuously when a CV unit
is in occupied mode and except when a
unit is in the night heat/morning warmup
mode. During the night heat and setback
mode the fan cycles on and off in
response to a call for heat. See Table O-
GI-9 for available fan horsepower.
Low Entering Air Temperature Sensor
This is standard on all units with a
hydronic coil or waterside economizer. It
can also be ordered as an option.
A thermostat limit switch is factory
mounted on the unit’s entering air side
with a capillary tube serpentine across
the coil face. If the temperature falls
below 35 F (2 C), the fan shuts down and
the waterside economizer and/or
hydronic heat valve options open to allow
full water flow. The heat output also
energizes. A manual reset is required.
The low entering air temperature
setpoint is adjustable.
High Duct Temperature Thermostat
A factory-supplied temperature limit
switch with reset element detects the
supply air duct temperature. This sensor
should be field-installed downstream
from the unit’s discharge in the supply air
duct. If the supply air duct temperature
exceeds 240 F (115.6 C), the unit shuts
down and displays a diagnostic. A
manual reset is required at the unit. The
high duct temperature can be adjusted at
the thermostat.
Dirty Filter Sensor Option
A factory installed pressure switch
senses the pressure differential across
the filters. When the differential pressure
exceeds 0.9-inches (23 mm) WG, contact
closure occurs and the HI will display a
diagnostic. The unit will continue to run
until you replace the air filters.
A field installed indicator device may be
wired to relay terminals to indicate when
filter service is required. Contacts are
rated at 115 VAC and are powered by a
field supplied transformer.
Low Ambient Sensor (Air-Cooled Units)
The low ambient sensor is field-installed
on air-cooled units. Position it in a location
subject to ambient temperatures only
and not exposed to direct sunlight or
exhaust fans.
The low pressure cutout initiates based
on the ambient temperature. A time
delay on the low pressure cutout initiates
for ambient temperatures between 50
(zero minutes) and 0 F (10 minutes). This
helps to prevent nuisance low pressure
cutout trips.
Inlet Guide Vane Option
Inlet guide vanes (IGV) are driven by a
modulating 0-10 vdc signal from the RTM
module. A pressure transducer
measures duct static pressure, and the
IGV modulates to maintain the supply air
static pressure within an adjustable user-
defined range. The range is determined
by the supply air pressure setpoint and
supply air pressure deadband, which are
set through the HI panel.
IGV assemblies installed on the supply
fan inlet regulate fan capacity and limit
horsepower at lower system air require-
ments. When in any position other than
full open, the vanes pre-spin the air in the
same direction as the supply fan rotation.
As the vanes approach the full-closed
position, the amount of “spin” induced by
the vanes increases at the same time
that intake airflow and fan horsepower
diminish. The IGV’s will close when the
supply fan is off.
Supply Air Static Pressure Limit
The opening of the IGV’s and VAV boxes
coordinate during unit startup and
transition to/from occupied/unoccupied
modes to prevent supply air duct
overpressurization. However, if for any
reason the supply air pressure exceeds
the user-defined supply air static
pressure limit set at the HI panel, the
supply fan/VFD shuts down and the IGV’s
close. The unit will attempt to restart, up
to three times. If the overpressurization
condition still occurs on the third restart,
the unit shuts down and a manual reset
diagnostic sets and displays at the HI.
Variable Frequency Drive Option
The variable frequency drive (VFD) is
driven by a modulating 0-10 vdc signal
from the RTM module. A pressure
transducer measures duct static
pressure, and the VFD adjusts the fan
speed to maintain the supply air static
pressure within an adjustable user-
defined range. The range is determined
by the supply air pressure setpoint and
supply air pressure deadband, which are
set at the HI panel.
VFD’s provide supply fan motor speed
modulation. The drives will accelerate or
decelerate as required to maintain the
supply air static pressure setpoint.
VFD with Bypass
Bypass control is an option that provides
full nominal airflow in the event of drive
failure. The user must initiate the bypass
Table O-GI-9. Supply Fan Horsepower Selections.
Unit Model HP
5 7.5 10 15 20 25
SXWG 20 X X X X X
SXWG 25, 30, 35 X X X X X X
SXRG 20 X X X X X
SXRG 25, 32 X X X X X
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