5
economizer coil or heating coil, is a factory-installed averag-
ing air temperature sensor positioned in the unit inlet airstream.
This may be either a thermistor sensor (automatic reset) or a
switch with capillary tube (manual reset). The unit activates
freeze protection when the sensed air temperature falls below
37 F. In freeze conditions, the supply fan will be stopped, all
compressors will stop, the economizer valve will open to
100%, the pump request output will remain on, and the alarm
lamp will light. This will maintain condenser water flow
through the coil to prevent freezing the coil while stopping all
other operations that could have contributed or will be affected
by the freeze condition. Unit reset is automatic when the con-
tacts on the freeze protection switch close again OR when the
thermistor sensed air temperature exceeds 42 F.
TOWER SUMP TEMPERATURE SENSOR (PCB2) —
This field sensor is used for monitoring (only) the tower sump
temperature.
LEAVING WATER TEMPERATURE SENSOR
(PCB2) — This sensor is used only for monitoring the leaving
water temperature.
BUILDING STATIC PRESSURE SENSOR (PCB1) —
This sensor is used to control the speed of the building exhaust
fan to maintain the building static pressure.
BUILDING EXHAUST FAN SPEED CONTROL
(PCB1) — This output controls building exhaust fan speed.
INDOOR AIR QUALITY (CO
2
) SENSOR (PCB1) — This
sensor monitors CO
2
levels and is used to provide demand ven-
tilation override with an air economizer.
OUTDOOR AIR TEMPERATURE SENSOR (PCB2) —
This sensor is used to monitor outdoor-air temperature. When
an air economizer is installed, this is used for changeover and
lockout.
INDOOR RELATIVE HUMIDITY SENSOR (PCB1) —
This sensor monitors the indoor relative humidity and controls
the humidity control relay.
HUMIDITY CONTROL RELAY (PCB2, DEHUMID-
IFY) — This relay controls a humidifier or dehumidification
device.
Wiring Control Devices —
Standard controls for the
50XJ unit require no field wiring. Standard 50XJ controls in-
clude: supply-air temperature (SAT), duct static pressure
(DSP), duct high static limit switch (DHS), compressor status
relays (CRx), supply fan control (BPS_S and/or VFD), and
mixed-air/return-air sensor (MA_RA).
NOTE: The MA_RA sensor will be located in the return
airsteam if the unit does not have a water economizer, and in
the mixed airstream if the unit is equipped with a water
economizer.
EXTERNAL 0 TO 10 VOLT DC RESET SIGNAL (RESET,
PCB1) — This field-supplied 0 to 10 vdc signal is used to reset
the supply-air temperature. The controller will scale the signal
to provide 0 reset at 0 volts and 20 degrees of reset at 10 volts.
Wire the positive of the signal to controller terminal block J4,
terminal 16 and the negative to terminal 17.
SUPPLY AIR RESET BY SPACE TEMPERATURE — Use
a 10K-type II thermistor sensor installed at the entering water
temperature sensor location on PCB2 and configure the unit to
control reset by the entering water temperature sensor OR use
the normal space temperature sensor as described below. These
sensors may be connected in a series-parallel network in the
same manner as the space temperature sensor.
SPACE TEMPERATURE SENSOR (PCB1) — The space
temperature sensor (33ZCT55SPT) is used in the following
cases:
• When using the optional water economizer
• To determine the average temperature of the space
served
• To determine supply-air temperature reset, occupied
heating, unoccupied heating and cooling (refer to
Sequence of Operation in Operation section).
To wire the sensor, perform the following (see Fig. 1).
1. Identify which cable is for the sensor wiring.
2. Strip back the jacket from the cable for at least 3 inches.
Strip
1
/
4
-in. of insulation from each conductor. Cut the
shield and drain wire from the sensor end of the cable.
3. Wire the sensor to controller terminal block J4, terminals
9 and 10. A typical 10K thermistor such as the
33ZCT55SPT sensor may be used. If the SPT sensor is
not installed and the MA_RA (mixed air/return air) sen-
sor is configured for return air, the 50XJ unit will use this
sensor to control supply air reset, occupied heat, and un-
occupied heating and cooling. See Fig. 2 for space tem-
perature sensor averaging.
HOT WATER OR STEAM VALVE (HWV, PCB1) — The
controller supplies a 0 to 10 vdc control signal for hot water or
steam valve control or electric heat control in occupied and un-
occupied heat modes. A field-installed step sequencer is re-
quired to convert this signal into 2, 4 or other stages of electric
heat. The positive signal is on terminal block J9, terminal 7,
with the signal common (negative) on terminal 8.
AIR TERMINALS AND FRESH AIR DAMPER
(PCB2) — The VAV terminal open (TRMOP), ventilation out-
put (VENTR), VAV terminal control (TRMCT), and heat inter-
lock relay (HIR) terminals provide dry contacts to command
the VAV terminals open; a ventilation damper open; VAV ter-
minals to control to their cooling set points; and VAV terminals
to control to their heat set points, respectively. These contacts
are located on terminal block J5: TRMOP terminals 7 and 8,
VENTR terminals 5 and 6, TRMCT terminals 3 and 4, and
HIR terminals 1 and 2.
REMOTE OCCUPANCY (PCB1) — The unit may be com-
manded by a remote control system or a twist timer to become
occupied and run when a set of dry contacts close. To enable
remote occupancy control, configure the occupancy point in
BACview to "remote (DI) on/off" and wire the remote contacts
to terminal block J4, terminals 17 and 18.
2
3
4
5
6
1
SW1
SEN
BRN (GND)
BLU (SPT)
RED(+)
WHT(GND)
BLK(-)
CCN COM
SENSOR WIRING
Fig. 1 — Space Temperature Sensor
Typical Wiring (33ZCT55SPT)
a33-675ef
