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XII. SYSTEM FUNCTIONS AND SEQUENCE OF
OPERATION
The outdoor unit control system has special functions. The
following is an overview of the 2-speed control functions:
A. Cooling Operation
This product utilizes a 2-stage cooling indoor thermostat. With a
call for first stage cooling (Y1), the outdoor fan and low-speed
compressor are energized. If low speed cannot satisfy cooling
demand, high speed is energized (Y1 and Y2) by the second stage
of indoor thermostat. After second stage is satisfied, the unit
returns to low-speed operation until first stage is satisfied or until
second stage is required again.
B. LED Function Lights
System control function LED indicator lights are available at the
outdoor unit 2-speed control board. The indoor thermostat pro-
vides indicator signals for high- and low-speed operation and
system malfunction (if equipped to do so). The 2-speed control
board has an LED which provides signals for several system
operations. See Table 7 for LED functions and definitions. Table
7 also provides the order of signal importance if more than 1 signal
should occur. If equipped and properly wired, a signal to indoor
thermostat is supplied by the low-voltage L lead.
NOTE: A signal (code) is sent through the L lead to thermostat
unless a failure has occurred.
TABLE 7—CONTROL FUNCTION LED CODE
CODE
DEFINITION
Constant flash
No demand
No pause
Stand by
1 flash
Low-speed operation
w/pause
2 flashes
High-speed operation
w/pause
3 flashes
Outdoor thermistor failure
w/pause
4 flashes
Used for heat pump only
w/pause
Outdoor coil thermistor failure
3 flashes
Used for heat pump only
pause
Thermistor out of range
4 flashes
5 flashes
Pressure switch trip (LM1/LM2)
w/pause
6 flashes
Compressor PTC's out of limit
w/pause
Constant light
No Pause
Board failure
No Flash
* Function light signal order of importance in case of multiple signal request; 1
is most important.
C. 3-Sec Time Delay
Any time the control receives a 24-v input, such as Y1 or Y2, there
is a 3-sec time delay before control function is initiated. This helps
prevent nuisance trips from thermostat jiggling.
D. 1-Minute Speed Change Time Delay
When compressor changes speeds from high to low or low to high,
there is a 1-minute time delay before compressor restarts. The
outdoor fan motor remains running.
E. 5-Minute Time Delay
The 2-speed control logic contains a 5-minute time delay that
prevents unit from short cycling after a thermostat off cycle or
power interruption. The unit can be forced to operate immediately
by momentarily touching a jumper between speed-up terminals of
control board. (See Fig. 8.) The speed-up feature does not bypass
any other function or time delay.
F. 2-Minute Low-Speed Minimum
If unit has not operated within the past 30 minutes, upon the next
thermostat high- or low-speed demand, unit operates for a mini-
mum of 2 minutes in low speed.
G. Crankcase Heater Operation
The 2-speed control energizes crankcase heater during unit off
cycle when outdoor ambient is below 75°F.
SIGNAL
IMPORTANCE*
H. Outdoor Fan Motor Operation
9
The 2-speed control energizes outdoor fan anytime compressor is
operating. The outdoor fan remains energized during the 1-minute
8
speed change time delay and if a pressure switch or compressor
PTC overload should trip.
7
I. Second-Stage Latching
6
Mechanical thermostats operate with a staging differential of 2°F
(droop). Bryant electronic thermostats are droopless, meaning that
5
they maintain room temperature at set point without latching
second stage. If system is installed with a mechanical thermostat or
second stage latching is desired, the following information applies.
4
During normal operation, the compressor operates at low speed to
satisfy first stage of indoor thermostat. If indoor thermostat
3
temperature increases 2 degrees, the compressor shifts into high-
speed operation. When indoor thermostat temperature is satisfied,
2
the compressor returns to low-speed, first-stage operation.
The installing technician can select high-speed compressor opera-
1
tion until first stage of indoor thermostat is satisfied. This
eliminates the temperature droop of indoor thermostat between
first- and second-stage operation and holds room temperature
closer to set point when load requirements are high. To select this
option, rotate the STAGE 2 LATCH potentiometer (pot) to desired
temperature. (See Fig. 9). The pot is factory set at OFF; however,
a temperature of 85°, 90°, 95°, 100°, or 105°F can be selected. The
selected temperature is the outdoor temperature at which the
structure's cooling load requires high-speed operation.
The unit's capacities should be plotted versus cooling load (heat
gain) of structure to accurately determine the STAGE 2 LATCH
temperature to be selected. The unit's capacities versus outdoor
temperature can be found in presale literature. The cooling load
must be taken from structure's heat gain/loss calculations. The
selected temperature is the point at which high-speed capacity is
needed and is just above the low-speed balance point. (See Fig.
10.)
After the temperature is selected, unit operates in high speed
during first-stage demand at any temperature at or above the
setting.
—14—
LM1 LM2
DFT1 DFT2 T1
1
K7
SPEED-UP
Fig. 8—Two-Speed Control Board
T2
S2
S1 PW2 PW1
P1
18
STAGE 2
DEFROST
BALANCE
LATCH
TIME
POINT
A91504
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