Heating Mode; Outdoor/Indoor Fan Operation In Heating Mode; Auto Mode In Heating Mode; Defrost - Carrier 38GRQB12-3 Service Manual

HEATING MODE

In this mode, the system heats the room air with the indoor fan
running at either the selected speed or on Auto. As in the cooling
mode, the indoor fan will run continuously unless interrupted by
the cold blow algorithm. This algorithm will not allow the fan to
run if the indoor coil temperature drops below a preset value.
Defrost is controlled by the on- board microprocessor.
Compressor operation in heating mode
In heating mode, the maximum operation frequency (Fmax) of the
compressor, after it starts running, depends on the outdoor ambient
temperature (T4).
Once the system starts running, the compressor will run at the
Fmax frequency for 7 minutes at a specific outdoor ambient
temperature (T4). During this time, the frequency limitation is
active, therefore the compressor will continue running even if the
set point condition is satisfied. The compressor running frequency
will then be controlled based on the difference between the room
temperature, the temperature set point, and a temperature
difference that takes a default value of 32_F (T1- Ts- ΔT).
Outdoor fan Operation in Heating Mode
When in heating mode, the outdoor fan motor cycles based on the
outdoor ambient temperature as shown below:
Fig. 21 – Heating mode
Indoor fan Operation in heating mode
In heating mode, the indoor fan runs depending on the evaporator
coil temperature (T2).
If the set point conditions are satisfied and the compressor stops,
the indoor fan will be forced to run for 127 seconds with breeze.
During this period, the anti- cold air is disabled.
If the machine is running at the rated capacity test mode, the indoor
fan will run at the rated speed and the anti- cold- wind function is
disabled.

Auto Mode in Heating Mode

In heating mode, When the fan speed is set to Auto, the fan will
run at a speed determined by the microprocessor based on the
difference between the room temperature and the temperature set
point as shown below:
2.5
2
1.5
1
Fig. 22 – Heating mode

Defrost

Defrost on heat pump units is controlled by the microprocessor and
is initiated if either of the following conditions occur:
1 If for the outdoor temperature, T4>32_F 0_C):
— The outdoor coil temperature (T3) has been lower than
37F (3 ) for about 40 minutes and during that period, the
coil temperature is lower than TCDI for more than 3
minutes.
— The outdoor coil temperature (T3) has been lower than
37F (3 ) for about 80 minutes and during that period,
the coil temperature is lower than (TCDI+2 ) for more
than 3 minutes.
2 If for the outdoor temperature, T4＜32_F 0_C):
— If the conditions described above are satisfied, then the
program judges if the evaporator coil temperature (T2)
has decreased more than 41F (5 ). When the
evaporator coil temperature has decreased more than
41F (5 ), the defrost mode starts.
3 At any value of outdoor ambient temperature (T4):
— If the machine runs with a condenser coil temperature
lower than 37F (3 ) for more than 120 minutes and the
outdoor coil temperature (T3) has been lower than
(TCDI+4 ) for more than 3 minutes, the machine will
enter into defrost mode.
TCDI = - 7_C = 19.4_F
Defrost cycle termination:
If any one of the following conditions is satisfied, the defrost cycle
will end and the system will return to normal operation:
1 The outdoor coil temperature (T3) reaches (TCDE1).
— For 9K- 12K: TCDE1 = (64_F) 18_C
— For 18K- 30K: TCDE1 = (59_F) 15_C
2 The outdoor coil temperature (T3) is kept at about 46F
(8 ) for at least 80 seconds.
3 The defrost cycle reaches 10 minutes.
The cycles of defrost algorithm are shown below:
16
Low
Medium
High