508196-01
Issue 2120
Page 11 of 18
Operation
Sequence of Operation
Upon initial “power up” to unit, there is a 3-minute time delay
to the compressor contactor (R to Y). Any 24V interrupt (R,
C) to the defrost control will initiate the 3-minute delay to
the contactor.
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 to Y and
G. Closing R to Y closes the unit contactor, starting the
compressor and outdoor fan, and signaling the indoor
blower to run at cooling speed. Upon satisfying cooling
demand, the thermostat will open the above circuits and
open the main contactor, stopping the compressor and
outdoor fan. The unit is equipped with a blower OFF delay;
the blower will continue to operate for a fixed 90 seconds
after the thermostat is satisfied.
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 Y signal from the thermostat brings
the indoor blower on at heat pump speed. The second
stage of the thermostat closes circuit R to W, which closes
the unit sequencers, bringing the auxiliary electric heat on.
The W signal from the thermostat brings the indoor blower
on at electric heat speed. Upon satisfying heating demand,
the thermostat opens the above circuits and stops unit
operation.
The unit is equipped with a blower OFF delay; the blower
will continue to operate for a fixed 90 seconds (heat pump)
or 120 seconds (electric heat) after the thermostat is
satisfied.
NOTE:
The 7.2 and 10 kW heats strips offer a W1 and
W2 thermostat connection. Taking advantage of the two
heat strip circuits requires a 3-stage thermostat (Y for heat
pump, W1 for electric heat stage 1, and W2 for electric
heat stage 2). If a 2-stage thermostat is used, “pigtail” W1
and W2 wires together when connecting the thermostat
wires at the unit.
Defrost Cycle
If the outdoor ambient conditions are such that frost forms
on the outdoor coil, the defrost control monitors the need
for, initiates, and terminates defrost cycles as necessary to
maintain system performance.
The defrost control is time/temperature initiated and
temperature terminated with a maximum defrost time
(time-out) of 14 minutes. The time between defrost cycles
is preset at 90-minute intervals at the factory, but can be
field adjusted to 30 or 60 minutes. To field adjust time
between defrost cycles, place defrost time plug in the
proper position (see Figure 11).
Figure 10. Defrost Control
24V TERMINAL
STRIP
CONNECTIONS
DIAGNOSTIC
LEDS
HIGH PRESSURE
SWITCH
TEST
PINS
REVERSING
VALVE
DEFROST
THERMOSTAT
LOW PRESSURE
SWITCH
SERVICE LIGHT
CONNECTIONS
FIELD SELECT
DEFROST INTERVAL
TIMING PINS
30
TEST
90
60
P1
Figure 11. Defrost Interval Timing Jumper
Defrost control will initiate a defrost cycle if the time period
has elapsed and the defrost sensor sees a temperature
below freezing. At the start of a defrost cycle, the defrost
control will energize the reversing valve solenoid, shifting
the reversing valve and de-energizing the outdoor fan.
The defrost control will also energize auxiliary heat for
increased comfort during defrost. The unit will remain in
defrost until the defrost sensor has determined that the
frost has been removed from the coil or a 14-minute time
period has elapsed.
