46
506 01 2303 01
Specifications are subject to change without notice.
APPENDIX C
Low Ambient Option — Factory Installed
Units with the factory installed low ambient option are
equipped with a Motormaster
R
solid--state head pressure
control which regulates fan speed. A temperature sensor,
mounted on circuit 1 of the outdoor coil (see Fig. 38)
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. 39 and Table 14).
Operation —
Fan on/off control in heat pump units (CHS) 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) 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.
Troubleshooting —
OBSERVATION
POSSIBLE REMEDY
Fans won’t start
Check power & wiring
Check sensor location
Check sensor resistance
Check OFR
Cooling --- Slow fan speed at
start or during low outdoor
ambient
Normal operation
Cooling --- Slow fan speed
above 85
_
F outdoor ambient
(should be full speed)
Check sensor location
Check sensor resistance
Check fan motor capacitor
Cooling --- motor current into
speed control is greater than
motor nameplate FLA
Normal operation
Up to 30% higher amps at
partial speed at low ambient
Heating --- Full fan speed
Normal operation
Heating --- Slow fan speed
(should be full speed)
Check wiring
Check SCB relay for 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
