Figure 5.8 Defrost - Carrier 69NT40-541-001 Operation And Service

Processing of a defrost request is controlled by the Defrost Termination Thermostat. The Defrost 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 termi-
nate 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 Sen-
sor reading is at or below 25.6°C(78°F). The unit will terminate defrost when the Defrost Temperature Sensor read-
ing rises above 25.6°C(78°F)
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 Tem-
perature 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
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 Termination Thermostat "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 con-
troller 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.

Figure 5.8 Defrost

ENERGIZED
DE-ENERGIZED
FOR FULL DIAGRAM AND
LEGEND, SEE SECTION 7
POWER TO
CONTROLLER
SIGNAL TO
CONTROLLER
SIGNAL TO
CONTROLLER
Figure 5.8.) The COOL light is also de-energized.
5–13
CONTROL TRANSFORMER
SIGNAL TO
CONTROLLER
T-316