Specifications

Temperature Reset — The control system is capable
of providing leaving fluid temperature reset based on return
fluid temperature. Because the temperature difference be-
tween leaving and return temperature is a measure of the
building load, return fluid temperature reset is essentially an
average building load reset method.
Under normal operation, the chiller maintains a constant
leaving fluid temperature approximately equal to the chilled
fluid set point. As building load drops from 100% down to
0%, entering cooler fluid temperature drops in proportion to
load. Thus, the temperature drop across the cooler drops from
a typical 10° F (5.5° C) at full load to a theoretical 0° F
(0° C) at no load. See Fig. 8.
At partial load, leaving chilled fluid temperature may be
lower than required. If this is allowed to increase (reset), the
efficiency of the chiller increases. Amount of reset can be
defined as a function of cooler temperature drop. This is a
simple linear function that requires 3 pieces of input data for
the set function that will vary depending on measurement
method used. See the following sections and Table 25.
NOTE: Reset set points are not accessible unless the reset
function is enabled first. The Control Point will be recalcu-
lated taking into account the set point plus the amount of
reset. This is done as a field configuration. Select one of the
3 choices for type of reset: Return Fluid Reset, External Tem-
perature Reset, or 4 to 20 mA Signal Reset. See Table 25.
If dual set point control is enabled, the amount of reset is
applied to whichever set point is in effect at the time.
Tables 26-28 demonstrate how to activate reset.
EXTERNAL TEMPERATURE RESET — In this example,
the unit set point is reset from full load at 90 F (32 C) to a
maximum reset value of 10° F (5.5° C) at 25 F (−6.7 C)
outdoor ambient. This means that if the chilled fluid set point
is 44 F (6.7 C), there is no reset if the temperature is 90 F
(32 C). At a temperature of 25 F (−6.7 C), the chilled fluid
set point would be reset to 54 F (12.2 C). See Fig. 8 and
Table 26. Afield-supplied outdoor air thermistor must be con-
nected to PSIO-2 as shown in Fig. 37.
To activate this function for space temperature reset leave
both the External Reset Sensor Select and the OAT Sensor
Select at 0, and change the Select/Enable Reset Type to 2,
following the procedure in Table 26.
EXTERNALLY POWERED RESET (4 to 20 mA) — In this
example, the unit set point is reset from full load at 4 mA to
a maximum reset value of 10° F (5.5° C) at 20 mA. See
Fig. 9 and Table 27.
RETURN FLUID TEMPERATURE RESET — In this ex-
ample, the unit set point is reset from full load based on the
chilled fluid return temperature. The example uses a reset
value of 10 degrees at full reset. Full reset is at a 2-degree
temperature difference across the cooler and no reset would
be at a 10° F difference across the cooler. See Fig. 10 and
Table 28.
Demand Limit — Demand Limit is a feature that allows
the unit capacity to be limited during periods of peak energy
usage. There are 3 types of demand limiting which can be
configured. The first type is through 2 switch inputs, which
will reduce the maximum capacity to 2 user-configurable per-
centages. The second type is bya4to20mAsignal input
which will reduce the maximum capacity linearly between
100% ata4mAinput signal (no reduction) down to the user-
configurable level at a 20 mA input signal. The third type
uses the CCN Loadshed module and has the ability to limit
the current operating capacity to maximum and further re-
duce the capacity if required.
To use Demand Limit, select the type of demand limiting
to use. Then configure the Demand Limit set points based on
the type selected.
DEMAND LIMIT (Switch Controlled, 30GX only) — In this
example, demand limit by switch control will be configured
and the switch set points will be set at 75% and 40%. Ca-
pacity steps are controlled by 2 relay switch inputs field wired
to PSIO-2, terminal 28 for switch 1 and terminal 25 for switch
2. See Table 29.
For Demand Limit by switch control, closing the first stage
demand limit contact will put the unit on the first demand
limit level. The unit will not exceed the percentage of ca-
pacity entered as Demand Switch 1 set point. Closing con-
tacts on the second demand limit switch prevents the unit
from exceeding the capacity entered as Demand Switch 2
set point. The demand limit stage that is set to the lowest
demand takes priority if both demand limit inputs are closed.
If the demand limit percentage does not match unit staging,
the unit will limit capacity to the closest capacity stage.
To Disable Demand Limit: Enter as shown
in Table 29. Scroll down to Demand Limit Select and press
to select no Demand Limit control.
Table 25 — Temperature Reset Set Point Limits
INPUT DATA DESCRIPTION
MEASUREMENT METHOD
Type 1 — 4-20 mA
(With 500-Ohm Resistor)
Type2—OAT/Occupied Space
(External Sensor)
Type 3 — Return Fluid
Variable Limits Variable Limits Variable Limits
Maximum Reset Amount —
Allowable range for maximum
amount which LWT set point
is reset.
Degrees Reset
at 20 mA
(−30 to 30 F)
−34 to −1 C
Degrees Reset (−30 to 30 F)
−34 to −1 C
Degrees Reset (−30 to 30 F)
−34 to −1 C
Maximum Reset Reference —
Temperature at which
maximum reset occurs.
— — Remote temp =
Full Reset
(20 to 125 F)
−7 to 52 C
CHW Delta T =
Full Reset
(0 to 15 degrees F)
0to8°C
cooler temperature rise
Minimum Reset Reference —
Temperature at which
no reset occurs.
— — Remote temp =
No Reset
(20 to 125 F)
−7 to 52 C
CHW Delta T =
No Reset
(0 to 15 degrees F)
0to8°C
cooler temperature rise
LEGEND
CHW — Chilled Water
OAT — Outdoor-Air Temperature
LWT — Leaving Fluid Temperature
39