Bryant CHS072 - Low Ambient Option - Factory Installed

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Specifications are subject to change without notice.
APPENDIX C
Low Ambient Option — Factory Installed
Units with the factory installed low ambient option are
equippe d with a Motorm aster
R
solid--state head pressure
control which regulates fan speed. A temperature sensor,
mounted on circuit 1 of the outdoor coil (se e 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 requi red to prevent wind cross currents
from causing abnormal ly 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 operati on
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 out put 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 a t
start or du rin g low ou tdoor
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
pa rtia l spe ed at low amb ie n t
H e a t i n g --- F u l l f a n s p e e d Normal operation
H e a t i n g --- S l o w f a n s p e e d
(should be full speed)
Check wiring
Check SCB relay for speed
control bypass
Speed Control Sensor Resistance —
TEMPERATURE RESISTANCE
_F + / --- 2 _F_ _C + / --- 1 C Ohms, nominal
--- 2 2 --- 3 0 88350
--- 4 --- 2 0 48485
14 --- 1 0 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