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Heating - Heat Pump Stage
Upon heating demand, the thermostat closes circuit R to
Y1, which closes the unit contactor, starting the compressor
and outdoor fan. Second-stage heating is initiated when
the thermostat energizes Y2, or when the outdoor ambient
temperature is below the lock-in temperature (see Second-
Stage Lock-In section). 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.
NOTE:
O is de-energized in heating mode.
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; the
blower continues to operate until all heating elements have
turned off.
Auxiliary electric heat can be staged using W1, W2 on 10,
15 and 20 kW models. Staged wiring diagrams are included
with the installation instructions of electric heater kits.
Heating - Emergency Mode
When the thermostat calls for emergency heat, the R to W
circuit is closed. Upon satisfying heat demand, the circuit
is open and the blower continues to operate through an off
delay period. The primary function of emergency mode is
to provide emergency heat should the heat pump operation
fail.
Defrost System
The defrost system includes two components: the defrost
thermostat and the defrost control.
Figure 13. Defrost Control Board
24V TERMINAL
STRIP
CONNECTIONS
DIAGNOSTIC
LEDS
HIGH PRESSURE
SWITCH
TEST
PINS
DEFROST TIMING
PINS (P1)
REVERSING
VALVE
DEFROST
THERMOSTAT (S6)
LOW PRESSURE
SWITCH
COMPRESSOR
DELAY PINS
S4
S87
SERVICE LIGHT
CONNECTIONS
Defrost Thermostat
The defrost thermostat is located on the evaporator 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.
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 13).
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, and 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.
1. An on-board outdoor ambient temperature sensor on
the defrost control bypasses the low pressure switch
during low ambient temperature below 15
°
F in heating
mode to eliminate nuisance low pressure trips.
NOTE:
15°F is an approximate temperature,
depending upon installation and geographic location.
2. A defrost cycle will initiate when there has been a low
pressure switch trip; the defrost sensor must be closed
and the defrost time interval must not have expired.
3. At the end of the defrost cycle, when the unit goes
back to heating mode, the low pressure switch is
checked to see if it has reset. If so, the strikeout is not
counted. This prevents lockout during extreme winter
conditions.
