19
• MODE changes to heat.
• OK-FAN turns off during normal shut down.
During normal compressor operation the minimum on time
is 5 minutes and the minimum off time is 5 minutes.
ECONOMIZER COOLING — The unit diverts condenser
inlet water flow through an optional economizer coil to precool
evaporator entering airflow. This occurs when there is demand
for the cooling, and the temperature at an entering water tem-
perature (EWT) thermistor is colder than the economizer start
set point. Water flow is controlled via two electronic water
flow valves. This option also incorporates an economizer
freeze sensor or switch (economizer temperature/freeze stat),
located at the inlet of the economizer coil.
Economizer water flow is in series with the condensers
allowing compressor operation while the economizer is
operating.
The equipment mode will change to "Free Cooling" if there
is no call for compressor operation, the fan is on, and there is
no demand for heat. If the unit is configured for variable flow
the reverse head pressure control valve will open (otherwise it
will already be open), and if there is condenser water flow, then
economizer cooling is enabled.
If the entering-water temperature is below the economizer
set point then the Mod. Econ Enabled point (ECONOK) will
change to enable and the economizer valve will modulate open
to lower the economizer control temperature to the supply air
set point temperature. The economizer modulation is con-
trolled by a PID loop and the reverse/head pressure control
valve will modulate in reverse of the economizer valve using
the formula MVLV = 100 - ECONO.
COOLING RESET — The controller can reset the supply air
set point using these two methods:
• An external 0 to 10 volt input reset
• The value of the space control point
The external 0 to 10 volt input reset is configured to produce
a 0 to 20 degree supply air reset over the 2 to 10 volt range. If
more than 1.8 volts is sensed on the input, this method of reset
takes priority over other methods.
NOTE: The reset from all methods may be limited from 0 to
20 F by the Maximum Reset set point.
The return-air sensor, space temperature sensor, or entering
water temperature input will be used as the space control point.
If this control input goes below the Occupied Cooling set point,
then for each degree that the point is below the set point value
the supply air set point will be reset by the value configured in
the reset ratio set point.
HEATING — The controller is configured to control either a
hot water/steam valve or electric heat through a modulating
0 to 10 vdc output. A field-supplied and installed step sequenc-
er can convert the modulating signal into two, four, or more
discrete stages of control for non-SCR electric heat control.
For either method of heat to function, a space control point
must be configured in the custom configuration. This control
point comes from a return-air sensor or space sensor, or the en-
tering water temperature sensor.
Whenever the space control point is below the occupied or
unoccupied heat set point the mode will change to heat and if
unoccupied the fan will be started. The heat control routine
uses a PID to control the heating output to maintain the supply
air set point.
When unoccupied heat is enabled the fan will be stopped
and the heat turned off when the space control temperature is
more than halfway above the difference between the occupied
heat set point and the unoccupied heat set point. For example,
if the occupied heat set point is 70 and the unoccupied heat set
point is 60 the unit will come on for unoccupied heating below
60 F and turn off again above 65 F.
HEAD PRESSURE CONTROL (HPC) — In installations
where entering water temperature can fall below 55 F, where a
water economizer (described above) is not installed, the HPC
provides an electronic water flow control valves to vary flow to
the condensers. Controlling the water flow maintains
compressor discharge pressure above a minimum value, ensur-
ing sufficient refrigerant flow out of the condenser and
throughout the refrigerant circuit. Refrigerant pressure is mea-
sured at compressor circuit no. 1 by a discharge pressure sensor
(PRES).
The 50XJ units not equipped with a water economizer can
be ordered with the reverse/head pressure control valve factory
installed and a pressure transducer located in the discharge line
of compressor no. 1.
When the unit is operating, cooling is enabled, a compressor
is engaged, and the entering water temperature falls below
60 F, head pressure control will be enabled. The head pressure
control valve will modulate to keep the head pressure at the Re-
frigerant Head Pressure set point. The default set point is
225 psig and may be set from 200 to 300 psig. The minimum
output value for the head pressure control algorithm is 40% in
order to maintain a minimum flow through the condensers. The
valve will modulate between 40 and 100%.
VENTILATION REQUEST — The ventilation request out-
put will close a set of relay contacts to activate a ventilation
damper whenever the supply fan and supply fan status are both
true and the unit status is occupied or override.
VAV TERMINAL OPERATION OUTPUT — The VAV
terminal control output (TRMCT) closes a set of relay contacts
to indicate to non-Carrier air terminals that the fan is on and op-
eration has been verified. This signals the terminals to open and
start controlling to the desired cfm and temperature set points.
VAV TERMINAL OPEN OUTPUT — The VAV terminal
open output (TRMOP) closes a set of relay contacts to com-
mand the air terminals to open to maximum cfm at times when
the fan is operating on the VFD bypass. This contact will close
30 seconds before the supply starts in bypass mode.
PUMP OUTPUT — The pump output closes a set of relay
contacts to indicate that the 50XJ unit is in operation and may
require condenser water flow through the unit. This contact is
closed when the unit is not in heating mode and the supply fan
is operating. This contact is also close if the economizer freeze
controls (described above) detects unacceptable conditions
with the unit configured without an air economizer or head
pressure control.
TOWER OUTPUT — The tower output closes a set of relay
contacts to start/stop tower controls. This contact closes when
the unit is not in heating mode and the supply fan is operating.
BUILDING PRESSURE CONTROL — The building pres-
sure control output provides an analog 0 to 10 vdc signal to
control return fan or exhaust fan speed. Fan speed is modulated
to maintain the building static pressure set point.
The control parameters for the building pressure set point
and building pressure are read and controlled in voltage but are
converted to inches of water for ease of setting and display. The
range and low start values of the sensor selected should be con-
figured in the setpoint screen, after the use of space static pres-
sure sensor has been enabled on the configuration screen.
Diagnostic Features —
The controller and BACview
display provide a number of features to help protect the unit
and allow problem diagnosis.
CRITICAL FAULT — The controller provides an output (for
field connection) to signal an external building systems moni-
tor or control that the unit has a critical alarm that may be im-
pacting unit operation. The alarm lamp on the BACview dis-
play provides visual indication of this condition.
The BACview display allows the user to monitor alarms
and determine sensor input values.
