Ducane (HVAC) (2/4)SH13 - Manual (Page 7)

# 48369K005 Page 7
OPERATION
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 R to G
circuit which also 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 90 seconds which improves system efficiency.
Heating
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.
Defrost System
The defrost system includes two components: the defrost
thermostat and the defrost control.
Defrost Thermostat
The defrost thermostat is located on the liquid line between
the check/expansion valve and the distributor. When the
defrost thermostat senses 42°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 70°F.
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 Figure 8).
The control provides automatic switching from normal
heating operation to defrost mode and back. During the
compressor cycle (call for defrost), the control accumu-
lates 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. The defrost timing jumper is
factory installed to provide a 60-minute defrost interval. If
Defrost Control Board
Figure 8
Defrost Interval
Timing Pins
Diagnostic LEDs
24V TerminalStrip
Connections
High Pressure Switch
(optional)
Defrost Thermostat
Reversing Valve
Compressor Delay Pins
Test Pins
K1 Relay
K2 Relay
FAN
DS1
L
24V
P2
P5
O-OUT
DF
Y1-OUT
HI-PS
U1
U2
DS2
K3 Relay
P6
TST PS DF C R O Y1
C5
LO-PS
C2
P1
30
60
90
TEST
W1
C
L
R
Y1
O
Low Pressure Switch
(optional)