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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 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. 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 tem-
perature of approximately 100°F (37.8°C) at higher outdoor ambi-
ent 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 re-
lay (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 control
input and output power wires.
APPENDIX C — LOW AMBIENT OPTION —
FACTORY INSTALLED
42
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
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