| Unit Control Features (cont.) They are:
- Supply air fan off due to running current overload, short circuit, or loss of
duct static pressure, or high duct pressure.
- Input voltage phase reversal, or phase loss.
- Unit control module failure.
- Sensor failure; supply air temperature out of range (low temp.).
- Entering air high temperature (o p t i o n a l ).
The following malfunctions require manual reset to re-enable the affected
compressor circuit and clear the diagnostic code:
- Compressor(s) off due to running current overload or short circuit.
- Compressor(s) off via high or low pressure cutouts.
- Compressor(s) off due to high motor or high refrigerant discharge temperature,
or low evaporator suction temperature.
The above are the only diagnostics that require manual reset. All other diagnostic code conditions do not normally
result in unit or circuit shut down and the diagnostic
resets automatically when the condition is corrected.
Supply Fan
The supply fan is off in the Stop/Reset mode. The fan is on continuously
when the Fan On Ventilate function is used and in
either Auto switch night heat morning warmup position except on units with
the Programmable Night Heat Morning Warmup and Time
Clock control options. Refer to page 15 for a description of these options.
Step Control
The steps of mechanical cooling are controlled based on supply air temperature
on VAV units and on zone temperature on constant
volume units. Refer to the data section, pages 28 through 83 for number of
capacity control steps for each unit.
The control system logic will permit compressor operation only after supply
fan operation is called for. Compressor operation will
not be permitted if the supply fan is off. Also, compressor operation will not be permitted when condenser entering water
temperature is below 55 degrees F on units without
condensing pressure control, and below 35 degrees F on units with condensing
pressure control valve.
Remote Supply Air Setpoint Adjustment (Optional)
On VAV units, the supply air setpoint can be set or adjusted remotely. The unit contains a pair of terminals that will
accept a 4-20 milliamp or 0-10 volt electronic signal.
This signal can be provided by a Tracer system or
other building automation systems.
Alarm Contact Closure
The alarm contact closure output is energized in the event of any malfunction
that normally initiates a diagnostic code display.
Refer to Figure 13-1 for a list of the diagnostics that will cause a code to be displayed. Also, the following additional
diagnostics are included:
- Operation of the entering air temperature sensor
- Loss of condenser water flow; if flow switch is present
Setpoints
COOLING — The cooling setpoint is adjustable from 50 to 90 degrees F. On VAV units the setpoint is adjusted at the
unit control module. On constant volume units the
setpoint is adjusted at the unit control module,
or, can be remotely adjusted with the addition of an optional remote potentiometer.
HEATING — The heating setpoint is adjustable from 40 to 90 degrees F. On both VAV and constant volume units, the
setpoint is adjustable only at the unit control module.
Fan On/Ventilate
A pair of field accessible terminals is provided which can energize ventilation
functions. When the connection between them is closed,
the fan will start, the airside economizer fresh air damper (if provided) will
be driven full open, and the ventilate output will be
energized. The inlet guide vanes will be active.
This input will override the external interlock auto/stop input. It will also override Tracer computer inputs when
energizing the ventilation functions. However, both the
Tracer computer input and the Fan On\ Ventilate input must be "off" to terminate the Fan On\ Ventilate mode.
During the Fan On\ Ventilate mode all cooling and heating functions will be
locked out. Manual reset is not required to restore
normal operation.
This input will be overridden by the Unit Switch when it is in the Stop/Reset
position.
External Interlock Auto/Stop
A pair of field accessible terminals is provided which will cause shutdown of the unit when the connection between them
is open. Unit operation will be restored when the
connection is remade. The shutdown is similar to placing the Unit Switch in the
Stop/Reset position. This feature will override any
Tracer® computer
inputs. Manual reset is not required to restore
operation.
Any malfunction that requires manual reset is not affected by this input.
Manual reset is still required
Setpoint Reset (Optional)
On VAV units, supply air temperature setpoint can be reset upward as the space temperature drops below a preset point
called the reset reference setpoint. This setpoint can
be set in a range from 60 to 80 degrees F. The amount that the supply air
temperature is reset upward is based on a setting
called the reset gain. This setting can be adjusted to allow the supply air temperature
to be reset upward from 1 degree F for every 1 degree
drop below the reset reference setpoint to 8 degrees F for every 1 degree drop below the reset reference setpoint.
A reset limit adjustment is also provided to limit the maximum supply air
setpoint. This limit can be set in a range from 65 to
90 degrees F.
A zone temperature sensor to sense when the space temperature falls below
the reset reference setpoint is shipped with the unit
for field installation in the remote space.
Frost Protection Control Due to the unique design of the unit control
module and the 3-D™ Scroll compressor, frost
protection is provided without the use of inefficient hot gas bypass. Frosting is controlled by measuring suction temperature on
each circuit. If the suction temperature falls below 28
degrees for the required time, that circuit is turned off and another circuit is
brought on line to handle the load. The first circuit
remains off until the temperature rises above 34
degrees for the required time, and can then be brought back on line as required. |
