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Operating Principles
Table 30. Supply fan horsepower selections (continued)
Unit Model
SXRF
SXWF
72, 80
60
90, 100, 110
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 entering
air side of the unit 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 at
the HI.
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
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. High duct temperature can be adjusted at 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
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.
Supply Air Static Pressure Limit
The opening of the VAV boxes coordinate during unit
startup and transition to/from occupied/unoccupied modes
to prevent supply air duct over pressurization. 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. The unit will attempt to
58
5
7.5
10
15
X
HP
20
25
X
X
X
X
X
X
restart, up to three times. If the over pressurization
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
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 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
dead band, which are set at the HI panel.
VFDs 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 mode at the HI panel. When in bypass mode,
VAV boxes need to be fully open. The self-contained unit
will control heating and cooling functions to maintain
setpoint from a user-defined zone sensor. Supply air static
pressure limit is active in this mode.
For more detailed information on VFD operation, reference
VFD technical manual that ships with the unit.
Airside Economizer Option
Units with the airside economizer option are equipped with
the necessary control sequences to use outside air for the
first stage of cooling, in occupied or unoccupied mode and
when ambient conditions are favorable for economizing.
Inherent in the unit controller is the ability to suppress the
setpoint below the normal unit setpoint. This allows the
building to improve comfort levels when possible, and at
the same time, optimize building mechanical cooling
operation for peak operating efficiency. An outside air
temperature and relative humidity sensor are provided to
allow monitoring of reference enthalpy and are field
installed.
If the unit has the ECEM board, economizer operation
enables when the outside air enthalpy is less than 25
BTUs/lb. default (adjustable 19-28 BTUs/lb). During
occupied mode, the outside air damper opens to 15%
(adjustable 0-100% at the HI) for ventilation purposes.
Also, the ability to alter the outside air damper position to
compensate for VAV supply air modulation is inherent in
the unit controls, and can be enabled by the operator.
30
40
50
X
X
X
X
X
X
SCXF-SVX01T-EN
60
X
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