Temperature Reset - Carrier Air Conditioner Operation And Service Manual

SUB-MODE KEYPAD ENTRY
CRNT
RCRN
HIST
RHIS
KEYPAD
SUB-MODE
ENTRY
CRNT
ENTER
CRNT
ESCAPE
RCRN
ENTER
ENTER
Temperature Reset —
handling leaving-fluid temperature reset based on return cooler
fluid temperature. Because the change in temperature through
the cooler is a measure of the building load, the return tempera-
ture reset is in effect an average building load reset method.
The control system is also capable of temperature reset based
on outdoor-air temperature (OAT), space temperature (SPT), or
from an externally powered 4 to 20 mA signal. Accessory sen-
sors must be used for OAT and SPT reset (HH79NZ073 for
OAT and HH51BX006 for SPT). The Energy Management
Module (EMM) must be used for temperature reset using a 4 to
20 mA signal.
To use the return reset, four variables must be configured. In
the Configuration mode under the sub-mode RSET, items
CRST, CRT1, CRT2, and DGRC must be set properly. See Ta-
ble 26 on page 44 for correct configuration. See Fig. 2-4 for
wiring details.
Under normal operation, the chiller will maintain a constant
leaving fluid temperature approximately equal to the chilled
fluid set point. As the cooler load varies, the entering cooler
fluid will change in proportion to the load as shown in Fig. 21.
Usually the chiller size and leaving-fluid temperature set point
are selected based on a full-load condition. At part load, the flu-
id temperature set point may be colder than required. If the
leaving fluid temperature was allowed to increase at part load,
the efficiency of the machine would increase.
Table 24 — Alarms Mode and Sub-Mode Directory
ITEM
ENTER AXXX or TXXX
YES/NO
ENTER
AXXX or TXXX
ENTER
YES/NO
ENTER
Table 25 — Example of Reading and Clearing Alarms
ITEM
AXXX or TXXX CURRENTLY ACTIVE ALARMS
NO
NO
YES
NO
The control system is capable of
ITEM EXPANSION
CURRENTLY ACTIVE ALARMS
RESET ALL CURRENT ALARMS
ALARM HISTORY
RESET ALARM HISTORY
ITEM EXPANSION
Return temperature reset allows for the leaving temperature
set point to be reset upward as a function of the return fluid
temperature or, in effect, the building load.
LEGEND
EWT — Entering Water (Fluid) Temperature
LWT — Leaving Water (Fluid) Temperature
Fig. 21 — Standard Chilled Fluid
Temperature Control — No Reset
43
COMMENT
Alarms are shown as AXXX.
Alerts are shown as TXXX.
Alarms are shown as AXXX.
Alerts are shown as TXXX.
COMMENT
ACTIVE ALARMS (AXXX) OR
ALERTS (TXXX) DISPLAYED.
Use to clear active alarms/alerts
NO Flashes
Select YES
Alarms/alerts clear, YES changes to NO