Carrier 38AUQ - APPENDIX C Low Ambient Option - Factory; Installed

APPENDIX C
Low Ambient Option m Factory Installed
Units with the factory installed low ambient option are
,_)
equipped with a Motormaster sohd-state head pressure
control which regulates fan speed. A temperature sensor,
mounted on circuit 1 of the outdoor coil (see Fig. 34)
controls the speed of approved outdoor fan motors in
order to maintain a constant head pressure in the outdoor
coil. The control maintains the appropriate head pressure
at low ambient temperatures down to -20°F (-28 °C).
Wind baffles are required to prevent wind cross currents
from causing abnormally low condensing temperatures.
Use 20-gauge sheet metal to fabricate wind baffles (see
Fig. 35 and Table 14).
Operation --
Fan on/off control in heat pump units (38AUQ) is
provided by an outdoor fan relay (OFR). In heat pumps,
the OFR is controlled by defrost control board (DFB).
Fan motor speed in cooling mode is regulated by the
temperature sensor for a minimum coil condensing
temperature of approximately 100 ° at higher outdoor
ambient temperature and 80°F at lower ambient.
For heat pump units, the speed control bypass relay (SCB)
is used to bypass the speed control during the heating
mode, resulting in full fan speed at all times.
To override the speed control for full fan speed operation
during service or maintenance, either:
a)
or
b)
remove sensor and place in hot water
>120°F (>49°C)
rewire to bypass control by connecting speed
control input and output power wires.
Troubleshooting --
OBSERVATION POSSIBLE REMEDY
Check power & wiring
Check sensor location
Fanswon't start
Check sensor resistance
Check OFR
Cooling - Slow fan speed at
start or during low outdoor Normaloperation
ambient
Cooling - Slow fan speed Check sensor location
above 85°F outdoor ambient Check sensor resistance
(should be full speed) Check fan motor capacitor
Cooling - motor current into Normal operation
speed control is greater than Up to 30% higher amps at
motor nameplate FLA partial speed at low ambient
Heating - Fullfan speed Normaloperation
Check wiring
Heating - Slow fan speed Check SCB relayfor speed
(should be full speed) control bypass
Speed Control Sensor Resistance --
TEMPERATURE RESISTANCE
°F +/-2°F ° °C +/-1C Ohms, nominal
- 22 - 30 88350
- 4 - 20 48485
14 - 10 27650
32 0 16325
50 10 9950
68 20 6245
77 25 5000
86 30 4028
104 40 2663
122 50 1801
140 60 1244
158 70 876
38