SCXF-SVX01T-EN
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If the switch is open at start, no compressors on that circuit
will operate. They are locked out and a manual reset
diagnostic initiates. If the LP switch opens after a
compressor start, all compressors on that circuit will stop
and remain off a minimum 3 minutes before restarting. If
the LP cutout trips four times in the first three minutes of
operation, all compressors on that circuit lockout and a
manual reset diagnostic initiates.
LP switches close at 74 ± 6 psig.
Evaporator Temperature Sensor Frostat
The evaporator temperature sensor is an analog input
device used to monitor refrigerant temperature inside the
evaporator coil to prevent coil freezing. It is attached to the
suction line near the evaporator coil with circuits 1 and 2
connected to the SCM/MCM and circuits 3 and 4
connected to the WSM. The coil frost cutout temperature is
factory set at 30°F. It is adjustable at the HI from 25-35°F.
The compressors stage off as necessary to prevent icing.
After the last compressor stages off, the compressors will
restart when the evaporator temperature rises 10°F above
the coil frost cutout temperature and the minimum three
minute “off” time elapses.
Saturated Condenser Temperature Sensors
The saturated condenser temperature sensors are analog
input devices. They are mounted in the condenser shell on
water-cooled units. The sensors monitor the saturated
refrigerant temperature inside the condenser coil and are
connected to the SCM/MCM for circuits 1 and 2, and WSM
for circuits 3 and 4 .
Head Pressure Control
Head pressure control is accomplished using up to four
saturated refrigerant temperature sensors on water-cooled
units.
Water-cooled: Units without WSE, the condenser valve
modulates to maintain an average saturated condenser
temperature. Units with WSE, if economizing and
mechanical cooling is necessary the economize valve will
sacrifice free cooling and modulate to maintain condensing
saturated temperature. If not economizing, the condenser
valve will modulate to maintain condensing saturated
temperature. Water-cooled units without head pressure
control will lock out mechanical cooling at entering
condenser water temperatures below 54°F. Mechanical
cooling will resume when the entering condenser water
temperature exceeds 58°F.
Low Ambient Compressor Lockout
Utilizes an analog input device. When the system is
configured for low ambient compressor lockout, the
compressors are not allowed to operate if the temperature
of the outside air falls below the lockout setpoint. When the
temperature rises 5°F above the lockout setpoint, the
compressors are allowed to operate. The factory preset is
50°F.
Return Air Temperature Sensor
The return air temperature sensor is an analog input device
used with a return humidity sensor on units with the
comparative enthalpy option. The sensor monitors the
return air temperature and compares it to the outdoor
temperature to establish which temperature is best suited
to maintain cooling requirements. It is mounted in the return
air path and connected to the ECEM.
Supply Fan Circuit Breaker, Fuses, and
Overloads
The supply fan motor is protected by either circuit breakers
fuses or a combination of fuses and overloads, dependent
upon unit configuration. Circuit breakers are used on units
without a VFD. They will trip and interrupt the motor power
supply if the current exceeds the breaker trip value. The
RTM shuts all system functions off when detecting an open
fan proving switch. Units with a VFD have fuses to protect
the VFD and motor. Units with a VFD w/bypass have fuses
to protect VFD circuit and overloads to protect the motor
when in bypass.
Supply Air Temperature Low Limit
Uses the supply air temperature sensor input to modulate
the economizer damper to minimum position in the event
the supply air temperature falls below the occupied heating
setpoint temperature.
Supply Air Temperature Sensor
The supply air temperature sensor is an analog input
device. It monitors the supply air temperature for supply air
temperature control, supply air temperature reset, supply
air temperature low limiting, and supply air tempering. It is
mounted in the supply air discharge section of the unit and
connected to the RTM.
Supply Airflow Proving Switches
This is binary input device used on units to signal the RTM
when the supply fan is operating. It is mounted in the
supply fan section and is connected to the RTM. During a
request for fan operation and if the differential switch opens
for 40 consecutive seconds, compressor operation turns
off, heat operation turns off, the request for supply fan
operation turns off and locks out, economizer damper
option closes, and a manual reset diagnostic initiates.
Low Entering Air Protection Device
(LEATPD)
The low entering air protection device (LEATPD) is a binary
input on units with hydronic heat or a waterside
economizer. It is optional on water-cooled units.
If the LEATPD is on a unit with factory-installed heat, it is
mounted in the heat section and connected to the heat
module. If the entering air temperature to the heating coil
falls to 40°F, the normally open contacts on the LEATPD
close and cause the following events:
• The hydronic heat actuator fully opens.
• The supply fan turns off
Controls
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