Defrost Interval; Defrost Mode - Sequence Of Operation - Carrier 69NT40-561-019 Operation And Service Manual

4.3.17

Defrost Interval

Controller function code Cd27 sets two modes for defrost initiation, either user-selected timed intervals or auto-
matic control. The user-selected values are 3, 6, 9, 12, 24 hours, AUTO, or PuLs. Some units may be configured to
allow defrost to be disabled altogether. In this case, a user-selected value of OFF will be available. The factory
default for defrost is AUTO. Refer to
In perishable mode, perishable-pulldown mode, or frozen-pulldown mode, automatic defrost starts with an initial
defrost set to three hours and then adjusts the interval to the next defrost based on the accumulation of ice on the
evaporator coil. In this way, defrosts are scheduled to occur only when necessary.
Once set point has been reached in frozen operation, the automatic selection will set the time interval to 12 hours
for the first two defrosts once the return probe is reading below the frozen set point and then adjust to 24 hours
thereafter.
All defrost interval times reflect the number of compressor runtime hours since the last defrost de-ice cycle. The
minimum defrost interval under the automatic setting is three hours while the maximum is 24. In frozen mode the
amount of wall-clock time necessary to accumulate a given amount of defrost interval time will exceed the defrost
interval time by a factor of two to three depending on the compressor duty-cycle. Defrost inter val time is not accu-
mulated in any mode until the defrost termination sensor reads less than 10°C (50°F).
If defrost does not terminate correctly and temperature reaches set point of the heat termination thermostat (HTT),
the thermostat will open to de-energize the heaters. If termination does not occur within two hours, the controller
will terminate defrost. An alarm will be activated to inform of a possible DTS failure.
If probe check (controller function code CnF31) is con figured to SPECIAL, the unit will proceed to the next opera-
tion (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 in the sensed temperature. If probe check fails, the sys-
tem will run for eight minutes to validate. At the end of 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 ensures that 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 4 minutes
at 100% capacity. When the snap freeze is completed, defrost is formally terminated.
4.3.18

Defrost Mode - Sequence of Operation

The defrost cycle may consist of up to three distinct operations. The first is de-icing of the coil, the second is a
probe check cycle and the third is snap freeze. Defrost may be initiated by any one of the following methods:
1. The manual defrost function (also manual defrost switch function, if equipped) is initiated by the user through
the use of the keypad or manual defrost switch. The manual defrost function is ended by use of the DTS.
The Manual Defrost / Interval key can be used to initiate a manual defrost.
Manual Defrost/Interval key operation:
Depressing and holding the Manual Defrost / Interval key for five seconds will initiate defrost. If the Man-
ual Defrost / Interval key is released in less than five seconds, defrost interval (code 27) shall be dis-
played.
2. The user sends a defrost command by communications.
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 temperature
values currently reported by the supply and return probes.
5. If the controller is programmed with the Demand Defrost option and the option is set to "IN" the unit will enter
defrost if it has been in operation for more than 2.5 hours without reaching set point.
6. The system is actively in a compressor suction pressure or high pressure ratio protection mode and reduced
the average system capacity below a predetermined threshold value.
Table 4–5.
NOTE
4–11 T-359