Table of Contents
Advertisement
Units with the factory installed low ambient option are
equipped with a solid-state head pressure control which regu-
lates fan speed. A temperature sensor, mounted on circuit 1 of
the outdoor coil (see Fig. H), controls the speed of approved
outdoor fan motors in order to maintain a constant head pres-
sure 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
Table A and Fig. I).
OPERATION
Fan on/off control in heat pump units (575J) 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 tempera-
ture sensor for a minimum coil condensing temperature of
approximately 100°F (37.8°C) at higher outdoor ambient
temperature and 80°F (26.7°C) at lower ambient.
For heat pump units, the speed control bypass relay (SCB) or
temperature bypass relay (TBR) 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. remove sensor and place in hot water >120°F (>49°C)
or
b. rewire to bypass control by connecting speed control
input and output power wires.
APPENDIX C — LOW AMBIENT OPTION —
FACTORY INSTALLED
45
TROUBLESHOOTING
OBSERVATION
Fans won't start
Cooling — Slow fan speed at
start or during low outdoor
ambient
Cooling — Slow fan speed
above 85°F outdoor ambient
(should be full speed)
Cooling — motor current into
speed control is greater than
motor nameplate FLA
Heating — Full fan speed
Heating — Slow fan speed
(should be full speed)
SPEED CONTROL SENSOR RESISTANCE
TEMPERATURE
°F ± 2°F
–22
–4
14
32
50
68
77
86
104
122
140
158
POSSIBLE REMEDY
Check power and wiring.
Check sensor location.
Check sensor resistance.
Check OFR.
Normal operation.
Check sensor location.
Check sensor resistance.
Check fan motor capacitor.
Normal operation.
Up to 30% higher amps at partial
speed at low ambient.
Normal operation.
Check wiring.
Check SCB relay for speed control
bypass.
RESISTANCE
°C ± 1°C
Ohms, nominal
–30
88,350
–20
48,485
–10
27,650
0
16,325
10
9,950
20
6,245
25
5,000
30
4,028
40
2,663
50
1,801
60
1,244
70
876
Advertisement
Table of Contents
