Startup
RT-SVX34F-EN
51
Low Pressure Control
This input incorporates the low pressure cutout of each
refrigeration circuit and can be activated by opening a field
supplied contact.
If this circuit is open before a compressor(s) is started,
neither compressor in that circuit will be allowed to
operate.
Anytime this circuit is opened for 5 continuous seconds,
the compressor(s) in that circuit are turned off
immediately. The compressor(s) will not be allowed to
restart for a minimum of 3 minutes.
If four consecutive open conditions occur during the first
three minutes of operation, the compressor(s) in that
circuit will be locked out, a diagnostic communicated to
Tracer, and a manual reset will be required to restart the
compressor(s).
The dehumidification option has one reheat low pressure
cutout (RLP). The RLP is located on the reheat circuit.
Dehumidification Low Pressure Control
The RLP has been added to insure proper refrigerant
management during active modulating hot gas reheat
operation.
The RLP will be ignored for the first 10 minutes of
compressor run time during active hot gas reheat
operation. Anytime this circuit is opened for 5 continuous
seconds, the compressor(s) in that circuit are turned off
immediately. The compressor(s) will not be allowed to
restart for a minimum of 3 minutes. If four consecutive
open conditions occur during active dehumidification, the
compressor(s) in that circuit will be locked out.
High Pressure Cutout and Temperature
Discharge Limit
The high pressure controls and temperature discharge
limit are wired in series between the compressor outputs
on the RTRM and the compressor contactors. On 27.5, 30,
and 35 Ton units, if the high pressure safety switch or
temperature discharge limit opens, the RTRM senses a lack
of current while calling for cooling and locks both
compressors out with an auto reset. On 40 and 50 Ton
units, if the high pressure safety or temperature discharge
limit opens, the compressor(s) on the affected circuit is
locked out. If the compressor output circuit is opened four
consecutive times during compressor operation, the
RTRM will generate a manual reset lockout.
Power Exhaust Control (Standard)
The power exhaust fan is started whenever the position of
the economizer dampers meets or exceed the power
exhaust setpoint when the supply fan is on.
The setpoint potentiometer is on the RTOM and is factory
set at 25% for traditional constant volume and variable air
volume units.
For SZ VAV units the default power exhaust enable
setpoint will be 25% as on non-SZ VAV units. However, for
SZ VAV the Exhaust Enable Setpoint will need to be
adjusted for the proper setpoint during the maximum Fan
Speed Command for the unit. Once selected, the
difference between the Exhaust Enable Setpoint and
Design OA Damper Minimum Position at Full Fan Speed
Command will be calculated. The difference calculated will
be used as an offset to be added to the Active Building
Design OA Minimum Position Target to calculate the
dynamic Exhaust Enable Target to be used throughout the
Supply Fan Speed/OA Damper Position range:
Exhaust Enable Target = Active Bldg Design OA Min
Position (Active Exhaust Enable Setpoint – Active
Bldg Design OA Min Position @ Full Fan Speed Command)
The Exhaust Enable Target could be above or below the
Active Bldg Design OA Min Position Target Setpoint based
on the Active Exhaust Enable Setpoint being set above or
below the Bldg Design Min Position at Full Fan Speed
Command. Note that an Exhaust Enable Setpoint of 0%
will result in the same effect on Exhaust Fan control as on
non-Single Zone VAV applications with and without
Statitrac; Exhaust Fan ON. See
Figure 44, p. 51
for how the
exhaust enable setpoint is modified throughout the OA
damper operating range.
Space Pressure Control - Statitrac
A pressure transducer is used to measure and report direct
space (building) static pressure. The user-defined control
parameters used in this control scheme are Space
Pressure Setpoint and Space Pressure Deadband. As the
Economizer opens, the building pressure rises and
enables the Exhaust Fan. The Exhaust dampers will be
modulated to maintain Space Pressure within the Space
Pressure Deadband.
Note:
The Exhaust Enable setpoint will need to be
selected as on units with standard power exhaust
control.
Figure 44.
SZ VAV exhaust
Maximum Fan Speed
Minimum Fan Speed
Middle Fan Speed
OAD
Posion
Setpoints
Design @ Full
Fan Speed
Exhaust Enable
Offset
Exhaust Enable @ Full
Fan Speed
Exhaust Enable Target
