507296-01B
Page 7 of 13
Issue 1650
Sequence of Operation
Cooling
When the thermostat is in the cooling mode, the O circuit
is powered, which energizes the reversing valve. Upon
cooling demand, the thermostat closes circuit R and Y.
Closing R and Y closes the unit contactor, starting the
compressor and outdoor fan. The thermostat automatically
closes the R to G circuit, which brings on the indoor blower
at the same time. Upon satisfying cooling demand, the
thermostat will open the above circuits and open the main
contactor, stopping the compressor and outdoor fan. If the
unit is equipped with a delay timer, the blower will continue
to operate for 60 to 90 seconds, which improves system
efficiency.
Heating - Heat Pump Stage
Upon heating demand, the thermostat closes circuit R to
Y, which closes the unit contactor, starting the compressor
and outdoor fan. The reversing valve is not energized in
the heating mode. The thermostat again automatically
brings on the indoor fan at the same time. Upon satisfying
heating demand, the thermostat opens above circuits and
stops unit operation.
Heating - Auxiliary Electric Heat
Upon heating demand for auxiliary electric heat, the
thermostat closes circuit R to W, which energizes the
heater sequencers as well as the indoor blower. Upon
satisfying auxiliary heat demand, the thermostat opens
above circuits and heating elements sequence off; blower
continues to operate until all heating elements have turned
off.
Defrost System
The defrost system includes two components: the defrost
thermostat and the defrost control.
Defrost Thermostat
The defrost thermostat is located on the outdoor coil.
When the defrost thermostat senses 35°F or cooler, the
thermostat contacts close and send a signal to the defrost
control board to start the defrost timing. It also terminates
defrost when the liquid line warms up to 60°F.
Figure 7. Defrost Control Board
Diagnostic LEDs
24V Terminal Strip
Connections
Defrost Interval
Timing Pins
Low Pressure Switch
(optional)
Test Pins
Compressor
Delay Pins
High Pressure
Switch
Reversing Valve
Defrost Thermostat
FAN
K1 RELAY
C2
U1
U2
K2 RELAY
K3 RELAY
P1
P5
P2
C5
P6
O-OUT
DS1
DS2
LO-PS
DF
Y1-OUT
COMMON
L
24V
W1
C
L
R
O
Y1
HI-PS
Y1
O
R
C
DF
PS
TST
30
60
90
TEST
Defrost Control
The defrost control board includes the combined functions
of time/temperature defrost control, defrost relay, diagnostic
LEDs and terminal strip for field wiring connections (see
The control provides automatic switching from normal
heating operation to defrost mode and back. During the
compressor cycle (call for defrost), the control accumulates
compressor run time at 30, 60, 90 minute field-adjustable
intervals. If the defrost thermostat is closed when the
selected compressor run time interval ends, the defrost
relay is energized and the defrost begins.
Defrost Control Timing Pins
Each timing pin selection provides a different accumulated
compressor run time period during one thermostat run
cycle. This time period must occur before a defrost cycle
is initiated. The defrost interval can be adjusted to 30
(T1), 60 (T2), or 90 (T3) minutes.
It is intended that this
product should be set at the 60-minute time interval at
initial installation.
If the timing selector jumper is not in
place, the control defaults to a 90-minute defrost interval.
The maximum defrost period is 14 minutes and cannot be
adjusted.
For geographic areas that experience low temperature
and high humidity conditions (below 35°F and above
80% RH), the defrost timer pin must be field set at
installation to a 60 or 30 minute defrost interval to
ensure reliable system operation while in heating mode.
NOTE: