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RT-SVX24Q-EN
Filter Switch
A binary input device used on CV and VAV applications
that measures the pressure differential across the unit
filters. It is mounted in the filter section and is
connected to the RTM. A diagnostic SERVICE signal is
sent to the remote panel if the pressure differential
across the filters is at least 0.5" w.c. The contacts will
automatically open when the pressure differential
across the filters decrease to 0.4" w.c. The switch
differential can be field adjusted between 0.17" w.c. to
5.0" w.c. ± 0.05" w.c.
Leaving Recovery Exhaust Temp Sensor
Analog input device used on CV and VAV applications
with Energy Recovery option installed. It is used to
monitor the temperature of the leaving air on the
Exhaust Fan side of the energy recovery wheel. This
temperature is used to determine if the temperature of
the wheel is too cold as compared to the Recovery
Frost Avoidance Setpoint. The result is used to
determine when to enable energy wheel frost
avoidance functions.
Supply, Exhaust and Return Fan and Exhaust
Airflow Proving Switches
Supply Airflow Proving Switch is a binary input device
used on CV and VAV applications to signal the RTM
when the supply fan is operating. It is located in the
supply fan section of the unit and is connected to the
RTM. During a request for fan operation, if the
differential switch is detected to be open for 40
consecutive seconds, the following occurs:
•
compressor operation is turned “Off”
•
heat operation is turned “Off”
•
the request for supply fan operation is turned “Off”
and locked out
•
exhaust dampers (if equipped) are “closed”
•
economizer dampers (if equipped) are “closed”
•
manual reset diagnostic is initiated
Exhaust/Return Airflow Proving Switch is a binary input
device used on all rooftop units equipped with an
exhaust fan. It is located in the exhaust/return fan
section of the unit and is connected to the RTM. During
a request for fan operation, if the differential switch is
detected to be open for 40 consecutive seconds, the
economizer is closed to the minimum position setpoint,
the request for exhaust fan operation is turned “Off”
and locked out, and a manual reset diagnostic is
initiated. The fan failure lockout can be reset at the
Human Interface located in the unit control panel, by
Tracer™, or by cycling the control power to the RTM
Off/On.
Lead-Lag
A selectable mode of operation through the Human
Interface. It alternates the starting between the first
compressor of each refrigeration circuit. Only the
compressor banks will switch, not the order of the
compressors within a bank, providing the first
compressor in each circuit had been activated during
the same request for cooling.
Charge Isolation
During the OFF cycle, most of the charge is isolated
between the compressor (internal) discharge check
valves and liquid line solenoid valve. This reduces the
OFF cycle charge migration, and liquid feedback during
subsequent start-up. The liquid line solenoid is
energized (opened) with the start of the circuit
compressor.
Supply, Exhaust and Return Fan Circuit
Breakers
The supply fan and exhaust fan motors are protected
by circuit breakers or fuses. They will trip and interrupt
the power supply to the motors if the current exceeds
the breaker's “must trip” value. The rooftop module
(RTM) will shut all system functions “Off” when an
open fan proving switch is detected.
Low Pressure Control
Low Pressure Control is accomplished using a binary
input device on CV and VAV applications. LP cutouts
are located on the suction lines near the scroll
compressors. The LPC contacts are designed to close
when the suction pressure exceeds 41 ± 4 psig. If the LP
control is open when a compressor is requested to
start, none of the compressors on that circuit will be
allowed to operate. They are locked out and a manual
reset diagnostic is initiated.
The LP cutouts are designed to open if the suction
pressure approaches 22 ± 4 psig. If the LP cutout opens
after a compressor has started, all compressors
operating on that circuit will be turned off immediately
and will remain off for a minimum of three minutes. If
the LP cutout trips four consecutive times during the
first three minutes of operation, the compressors on
that circuit will be locked out and a manual reset
diagnostic is initiated.
Saturated Condenser Temperature Sensors
Analog input devices used on CV and VAV applications
are mounted inside a temperature well located on a
condenser tube bend. They monitor the saturated
refrigerant temperature inside the condenser coil and
are connected to the MCM. As the saturated refrigerant
temperature varies due to operating conditions, the
condenser fans are cycled “On” or “Off” as required to
maintain acceptable operating pressures.
Head Pressure Control
Accomplished using two saturated refrigerant
temperature sensors on CV and VAV applications.
During a request for compressor operation, when the
condensing temperature rises above the “lower limit”
of the controlband, the Compressor Module (MCM)
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