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Units with the factory installed low ambient option are
equipped with a low ambient head pressure control which
regulates fan speed. Two temperature sensors, mounted on
circuits 1 and 2 of the outdoor coil (see Fig. H and I), control
the speed of outdoor fan motors in order to maintain a con-
stant head pressure in the outdoor coil. The control main-
tains the appropriate head pressure at low ambient tem-
peratures 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. J and Table A) and mounting brackets (see
Fig. K).
NOTE: Mounting brackets are for use on 15 ton model units
only.
•
Install the wind baffles as show in Fig. L, for 15 ton
units and Fig. M, for 20 ton units.
Operation
Fan on/off control is provided by an outdoor fan relay
(OFR).
In cooling mode, fan motor speed of outdoor motors OFM1
and OFM3 is regulated by the speed control temperature
sensors on outdoor coil circuits 1 and 2 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. Additionally, outdoor fan motor OFM2 and
OFM4 are turned on/off by the low ambient temperature
switch, LAS, operating the low ambient relay (LAR). The
LAS control temperatures are open 55°F ± 2.16°F, close
60°F ± 2.16°F (open 12.8°C ± 1.2°C, close 15.6°C ± 1.2°C).
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 con-
trol input and output power wires.
42
APPENDIX C — LOW AMBIENT OPTION —
FACTORY INSTALLED
Specifications subject to change without notice.
Troubleshooting
OBSERVATION
Fans won't start
Cooling— Center outdoor fans
(OFM2, OFM4) off below
approximately 55°F (12.8°C)
outdoor ambient.
Cooling — Center outdoor fans
(OFM2, OFM4) not on above
approximately 60°F (16°C)
outdoor ambient
Cooling — Slow fan speed for
outer fans (OFM1, OFM3) at
start or during low outdoor
ambient
Cooling — Slow fan speed for
outer fans (OFM1, OFM3) above
85°F (29°F) outdoor ambient
(should be full speed)
Cooling— motor current into
speed control is greater than
motor nameplate FLA
Speed Control Sensor Resistance
TEMPERATURE
°F ± 2°F
–22
–4
14
32
50
68
77
86
104
122
140
158
POSSIBLE REMEDY
All fans:
Check power and wiring
Check outdoor fan relay (OFR)
OFM1, OFM3 only:
Check speed control sensor
location
Check speed sensor resistance
OFM2, OFM4 only:
Check low ambient switch (LAS)
Check low ambient relay (LAR)
Normal operation
Check low ambient switch (LAS)
Check low ambient relay (LAR)
Normal operation
Check speed control sensor
location
Check speed control sensor
resistance
Check fan motor capacitor
Normal operation
Up to 30% higher amps at partial
speed at low ambient
RESISTANCE
°C ± 1C
Ohms, nominal
–30
88350
–20
48485
–10
27650
0
16325
10
9950
20
6245
25
5000
30
4028
40
2663
50
1801
60
1244
70
876
CHS-180-240-01SI Rev. A
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