Figure 4-8 Defrost - Carrier 69NT40-511-300 Operation And Service

Manual Defrost/Interval key operation:
Depressing and holding the Defrost Interval key for
five (5) seconds will initiate defrost. If the defrost in-
terval key is released in less than five (5) seconds,
defrost interval (code 27) shall be displayed.
2. The user sends a defrost request by communica-
tions.
3. The defrost interval timer (controller function code
Cd27) reaches the defrost interval set by the user.
4. The controller probe diagnostic logic determines that
a probe check is necessary based on the tempera-
ture values currently reported by the supply and re-
turn probes.
5. The controller Demand Defrost configuration vari-
able (CnF40) is set to "In" and the unit has been in
pull down operation for over 2.5 hours without reach-
ing set point.
ENERGIZED
DE- -ENERGIZED
FOR FULL DIAGRAM AND
LEGEND, SEE SECTION 7
POWER TO
CONTROLLER
SIGNAL TO
CONTROLLER
SIGNAL TO
CONTROLLER

Figure 4-8 Defrost

Processing of a defrost request is controlled by the
Defrost Termination
Termination Thermostat is not a physical component. It
is a software point that acts as a thermostat, allowing
defrost when it is considered "closed" and preventing or
terminating defrost when it is considered "open". The
actual temperatures used to make the "open" or
"closed" determinations are dependent on the type of
defrost request made and the operator setting of
configuration variable CnF41. Configuration variable
CnF41may be factory set at the default value of
25.6°C(78°F) or a lower value of 18°C(64°F).
When a request for defrost is made by the use of the
Manual Defrost Switch or Communications, the unit will
enter defrost when the reading at the Defrost
Temperature Sensor is at or below the CnF41 setting.
Defrost will terminate when the Defrost Sensor
Temperature reading rises above the CnF41 setting.
When a request for defrost is made by probe check, the
unit will enter defrost when the Defrost Temperature
Sensor reading is at or below 25.6°C(78°F). The unit will
terminate defrost when the Defrost Temperature
Sensor reading rises above 25.6°C(78°F)
T-285
CONTROL TRANSFORMER
SIGNAL TO
CONTROLLER
Thermostat. The Defrost
When a request for defrost is made by demand defrost ,
the unit will enter defrost when the reading at the Defrost
Temperature Sensor is at or below 18°C (64.4°F).
Defrost will terminate when the Defrost Sensor
Temperature reading rises above the CnF41 setting.
When a defrost has terminated, the defrost interval
timer will begin counting when the reading at the Defrost
Temperature Sensor is at or below 10°C(50°F). Once
the timer has counted the required time, the unit will
enter defrost if the Defrost Temperature Sensor is at or
below 25.6°C(78°F). Defrost will terminate when the
Defrost Sensor Temperature reading rises above the
CnF41 setting.
When the unit enters defrost, the controller opens
contacts TC, TN and TE (or TV) to de-energize the
compressor, condenser fan and evaporator fans. (See
Figure 4-8.) The COOL light is also de--energized.
The controller then closes TH to supply power to the
heaters. The defrost light is illuminated.
When the defrost temperature sensor reading rises to
the applicable Defrost
"opening" point the de--icing operation is terminated.
If defrost does not terminate correctly and temperature
reaches the set point of the heat termination thermostat
(HTT) the thermostat will open to de--energize the
heaters. If termination does not occur within 2.0 hours,
the controller will terminate defrost. An alarm will be
given of a possible DTS failure.
If probe check (controller function code CnF31) is
configured to special, the unit will proceed to the next
operation (snap freeze or terminate defrost). If the code
is configured to standard, the unit will perform a probe
check. The purpose of the probe check is to detect
malfunctions or drift in the sensed temperature that is
too small to be detected by the normal sensor out of
range tests. The system will run for eight minutes in this
condition. At the end of the eight minutes, probe alarms
will be set or cleared based on the conditions seen.
When the return air falls to 7_C (45_F), the controller
checks to ensure the defrost temperature sensor (DTS)
reading has dropped to 10_C or below. If it has not, a
DTS failure alarm is given and the defrost mode is
operated by the return temperature sensor (RTS).
If controller function code CnF33 is configured to snap
freeze, the controller will sequence to this operation.
The snap freeze consists of running the compressor
without the evaporator fans in operation for a period of
four minutes with the suction modulation valve at
maximum allowed opening. When the snap freeze is
completed, defrost is formally terminated.
4-8
Termination Thermostat