Unit installation

Operating
54 SCXF-SVX01K-EN
cooling, thus savings in compressor kW, but an increase in
supply fan kW may occur.
Outdoor air heating reset is the inverse of cooling, with the
same principles applied.
For both outdoor air cooling reset and heating reset, there
are three user defined parameters that are adjustable
through the human interface panel.
• Beginning reset temperature
• Ending reset temperature
• Maximum amount of temperature reset
Reset based on zone temperature
Zone reset is applied to the zone(s) in a building that tends
to overcool or overheat.The supply air temperature
setpoint is adjusted based on the temperature of the
critical zone(s).This can have the effect of improving
comfort and/or lowering energy usage.The user-defined
parameters are the same as for outdoor air reset.
Supply Air Tempering (Hot Water and Steam
VAV Units Only)
When supply air temperature falls below the supply air
temperature deadband low end, the heating valve
modulates open to maintain the minimum supply air
temperature setpoint.
Daytime Warm-up (Units with Supply Air
Temperature Control Only)
During occupied mode, if the zone temperature falls to a
preset, user-defined zone low limit temperature setpoint,
the unit is put into daytime warm-up.The system changes
over to CV heating, the VAV boxes drive full open.
However, unit airflow modulation control operates to
maintain duct static setpoint, and full heating capacity is
provided until the daytime warm-up setpoint is reached.
The unit is then returned to normal occupied mode.
Supply Air Tempering
Supply air tempering is available on units without volume
control and with hot water, steam, or electric heat or units
with supply air temperature control with steam or electric
heat. When the unit is in heat mode but not actively
heating, if the supply air temperature drops to 10°F (5.5°C)
below the occupied zone heating temperature setpoint,
electric heat will stage on or the hydronic valve will
modulate to maintain a minimum supply air temperature.
The unit transitions out of heat mode if the supply air
temperature rises to 10°F (5.5°C) above the occupied zone
heating temperature setpoint.
Changeover
This mode only functions on units with supply air
temperature control with hydronic heat. When the
changeover binary input is closed the unit will control to a
discharge air heating setpoint.This setpoint is entered
from the HI, and can be a higher temperature than the
supply air cooling setpoint.This function maintains duct
static pressure.
Thermostatic Expansion Valve
Refrigerant system reliability and performance is heavily
dependent upon proper superheat.The importance of
maintaining the proper superheat cannot be
overemphasized. Accurate measurements of superheat
will provide the following information:
• How well the expansion valve is controlling the
refrigerant
flow.
• The efficiency of the evaporator coil.
• The amount of protection the compressor is receiving
ag
ainst flooding.
The expected range for superheat is 14-20°F at full load
conditions. At part load, expect a properly adjusted
expansion valve to control to 8-12°F superheat. Systems
operating with lower superheat could cause serious
compressor damage due to refrigerant floodback.
Compressors
Units use two sizes of hermetic scroll compressors, 10 and
15 hp, and have from two to six compressors. When
viewing the front of the unit, compressors are identified A
through F from left to right.The second compressor from
the left, or B compressor, is always the first to come on,
unless locked out for a malfunction or shut off on frost
protection. See Table 35, p. 55 for compressor cycling
stages and Table 1, p. 11 and Table 2, p. 12 for percent
cooling capacity by stage.
R-410 compressors have belly band heaters that must be
energized 24 hours before starting compressor. Power to
the unit will energize the heaters. Heaters will be energized
during the off-cycle as long as the unit has power. Failure
to perform these pre-start instructions could result in
compressor damage.
The control system logic permits compressor operation
only after the supply fan is on. If the supply fan shuts down,
compressors will not operate. Units without head
pressure control (units with intermediate piping packages)
will lock out mechanical cooling when the entering
condenser water temperature falls below 54°F. Mechanical
cooling will resume when the entering condenser water
temperature exceeds 58°F.
When there are more than two compressors in an air
cooled unit, the first two compressors are manifolded
together. If there are four compressors, the second two are
manifolded.
NOTICE:
Compressor Damage!
Systems operating with lower superheat than
recommended could cause serious damage to the
compressor.