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Temperature Reset —

The control system is capable of

providing leaving fluid temperature reset based on return fluid
temperature. Because the temperature difference between leav-
ing 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 leav-
ing 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 typi-
cal 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 ef-
ficiency of the chiller increases. Amount of reset can be de-
fined 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 28.
NOTE: Reset set points are not accessible unless the reset
function is enabled first. The Control Point will be recalculated
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 Temperature Reset,
or 4 to 20 mA Signal Reset. See Table 28.

If dual set point control is enabled, the amount of reset is ap-

plied to whichever set point is in effect at the time.

Tables 29-31 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 max-
imum 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 29. A field-sup-
plied outdoor air thermistor must be connected to PSIO-2 as
shown on page 74 or 75 of Field Wiring.

To activate this function for space temperature reset leave

both the External Reset Sensor Select and the OAT Sensor Se-
lect at 0, and change the Select/Enable Reset Type to 2, follow-
ing the procedure in Table 29.
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 30.

RETURN FLUID TEMPERATURE RESET — In this exam-
ple, 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 dif-
ference across the cooler and no reset would be at a 10

F dif-

ference across the cooler. See Fig. 10 and Table 31.

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 con-
figured. The first type is through 2 switch inputs, which will
reduce the maximum capacity to 2 user-configurable percent-
ages. The second type is by a 4 to 20 mA signal input which
will reduce the maximum capacity linearly between 100% at a
4 mA input 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 cur-
rent operating capacity to maximum and further reduce 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%. Capacity
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 32.

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 capacity en-
tered as Demand Switch 1 set point. Closing contacts on the
second demand limit switch prevents the unit from exceeding
the capacity entered as Demand Switch 2 set point. The de-
mand limit stage that is set to the lowest demand takes priority
if both demand limit inputs are closed. If the demand limit per-
centage does not match unit staging, the unit will limit capacity
to the closest capacity stage.

To Disable Demand Limit: Enter

as shown in Ta-

ble 32. Scroll down to Demand Limit Select and press

to select no Demand Limit control.

Table 28 — Temperature Reset Set Point Limits

ENTER

INPUT DATA DESCRIPTION

MEASUREMENT METHOD

Type 1 — 4-20 mA

(With 500-Ohm resistor)

Type 2 — OAT/Occupied Space

(External Sensor)