507635-05
Page 15 of 22
Issue 2245
In the cooling mode, the blower control delays blower
operation for 5 seconds after the compressor starts. The
blower continues to operate for 90 seconds after the
compressor is de-energized. The delay is 5 minutes on the
first start.
HEAT
The HEAT jumper is used to determine CFM during electric
heat operation only. These jumper selections are activated
only when W1/W2 is energized.
CONTINUOUS FAN
When the thermostat is set for “Continuous Fan” operation
and there is no demand for heating or cooling, the blower
control will provide 50 percent of the COOL CFM selected.
DEHUMIDIFICATION
The blower control includes an HUM terminal, which
provides for connection of a humidistat. The JV1 resistor
on the blower control must be cut to activate the HUM
terminal. The humidistat must be wired to open on humidity
rise. When the dehumidification circuit is used, the variable
speed motor will reduce the selected air flow rate by 25
percent when humidity levels are high. An LED (D1) lights
when the blower is operating in the dehumidification mode.
Cooling System
The cooling system is factory-charged with HFC-R-410A.
The compressor is hermetically sealed and base-mounted
with rubber-insulated bolts.
Cooling
When the thermostat calls for cooling, R is closed to Y1
and O (see the wiring diagrams starting on Page 21).
This action completes the low voltage control circuit,
energizing the compressor, condenser fan motor, and
blower motor. Second-stage cooling is initiated by the
thermostat energizing Y2 in AC units and Y2 and O in heat
pumps.
Unit compressors have internal protection. In the event there
is an abnormal rise in the temperature of the compressor,
the protector will open and cause the compressor to stop.
The thermostat automatically closes the R to G circuit,
which brings on the indoor blower. 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
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
Defrost System
Demand Defrost System
The demand defrost system measures differential
temperatures to detect when the system is performing
poorly because of ice build-up on the outdoor coil. The
system “self-calibrates” when the defrost system starts
and after each system defrost cycle. The demand defrost
components on the control board are listed below.
NOTE:
The demand defrost system accurately measures
the performance of the system as frost accumulates on the
outdoor coil. This typically will translate into longer running
time between defrost cycles as more frost accumulates on
the outdoor coil before the board initiates defrost cycles.
Defrost System Sensors
Sensors connect to the defrost board through a field–
replaceable harness assembly that plugs into the board.
Through the sensors, the board detects outdoor ambient
and coil fault conditions. As the detected temperature
changes, the resistance across the sensor changes.
Sensor resistance values can be checked by ohming
across pins.
NOTE:
When checking the ohms across a sensor, be
aware that a sensor showing a resistance value that is not
