4–13
T-340
Defrost may be initiated any time the defrost temperature sensor reading falls below the controller DTT set point.
Defrost will terminate when the defrost temperature sensor reading rises above the DTT set point. The DTT is not
a physical component. It is a controller setting that acts as a thermostat, “closing” (allowing defrost) when the
defrost temperature sensor reading is below the set point and “opening” (terminating or preventing defrost) when
the sensor temperature reading is above set point. When the unit is operating in bulb mode (see
),
special settings may be applicable.
If the controller is programmed with the Lower DTT setting option, the defrost termination thermostat set point may
be configured to the default of 25.6°C (78°F) or lowered to 18°C (64°F). When a request for defrost is made
through the manual defrost switch, communications or probe check the unit will enter defrost if the DTT reading is
at or below the DTT setting. Defrost will terminate when the DTS reading rises above the DTT setting. When a
request for defrost is made with the defrost interval timer or by demand defrost, the defrost temperature setting
must be below 10°C (50°F).
When defrost is initiated, the controller closes the EEV, opens contacts TC, TN and TE (or TV) to de-energize the
compressor, condenser fan and evaporator fans. The white COOL light is also de-energized. The controller then
closes contacts TH to supply power to the heaters, and the orange DEFROST light is illuminated. When the DTS
reading rises to the DTT setting, the de-icing operation is terminated.
Figure 4.10 Defrost
NOTE
The EEV and DUV are independently operated by the microprocessor. Complete schematics and legends
are located in
.
4.3.23
Defrost Interval
There are two modes for defrost initiation, user-selected timed intervals and automatic control.
The user-selected values are (OFF), 3, 6, 9, 12, 24 hours, AUTO, or PuLS; factory default is 3 hours.
Automatic defrost starts with an initial defrost at three hours and then adjusts the interval to the next defrost based
on the accumulation of ice on the evaporator coil. Following a start-up or after termination of defrost, the time will
not begin counting down until the DTS reading falls below set point (DTT closed). If the reading of DTS rises above
set point (DTT open) any time during the timer count down, the interval is reset and the countdown starts over.
4.3.24
Defrost Related Settings
If probe check (CnF31) is configured to SPECIAL, the unit will proceed to the next operation (snap freeze or termi
-
nate defrost). If CnF31 is configured to STANDARD, the unit will perform a probe check. The probe check is a test
that compares temperature sensor readings to determine if any sensors have failed.
If probe check fails, the system will run for eight minutes to validate. At the end of eight minutes, probe alarms will be
set or cleared based on the current conditions.
When the return air temperature falls to 7°C (45°F), the controller ensures that the defrost temperature sensor
(DTS) reading has dropped to 10°C or below. If it has not it indicates a failed DTS, a DTS failure alarm is triggered
and the defrost mode is operated by the return temperature sensor (RTS).
24 VOLT POWER
ENERGIZED
DE-ENERGIZED
TN
ST
F
HPS
PB
IP-CM
TC
CF
TE
IP-EM1
EF
IP-EM2
HTT
TH
HR
ESV
TS
TV
EF
ES
