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Sequence of Operation
General
Operation of the unit heating and cooling cycles is automatic when
the system is set to HEAT or COOL (the optional automatic
changeover thermostat, when set to AUTO, automatically changes
to heat or cool with an appropriate room temperature change). The
fan can be set to ON, causing continuous evaporator (indoor) fan
operation or set to AUTO causing fan operation to coincide with
heating or cooling run cycles. Continuous fan mode during cooling
operation may not be appropriate in humid climates. If the indoor
air exceeds 60% relative humidity or simply feels uncomfortably
humid, it is recommended that the fan only be used in the AUTO
mode. With the thermostat set to ON current is supplied to the
control transformer (on 460v models the outdoor fan relay (ODF)
is energized through normally closed contacts on the defrost
control (DFC).
COOLING MODE
Thermostat call for cooling (2-stage thermostat)
st
Call for 1
stage cooling only:
With the room thermostat set to COOL and the fan set to AUTO,
power is supplied from the room thermostat "O" terminal to the
unit switchover valve coil (SOV) and the "O" terminal on the
ECMC board. This energizes the switch-over valve (SOV)
placing it in the position for cooling (it is in the position for
heating when de-energized). On a call for cooling, power is
supplied to the unit from the room thermostat (Y1) and (G)
terminal. (Y1) provides power to the compressor contactor
(CC), the defrost control (DFC) and the electronically
commutated motor control (ECMC). (G) provides power to the
(ECMC) for low speed (IDM) indoor fan motor operation.
The energized compressor contactor (CC) completes the
circuit to the compressor for 1
outdoor single speed fan motor (ODM). The (G) signal
energizes the (ECMC) for the indoor fan motor (ECM) to
operate on low speed. The thermostat will continue to cycle the
compressor and fans to maintain the desired temperature.
nd
Call for 2
stage after 1st stage
On a call for 2nd stage cooling, power is supplied from the
room thermostat (Y2) terminal to the A/C rectifier (ACR) and
the (ECMC). This energizes the (ACR) switching the
compressor to 2
nd
stage (High) operation. The (ECMC) is
energized for indoor fan motor (ECM) high speed fan
operation. The room thermostat will continue to cycle the
st
nd
system between 1
and 2
temperature.
HEATING MODE
Thermostat call for heat (2-stage thermostat)
Call for 1
st
stage heating only:
With the room thermostat set to HEAT and the fan set to AUTO,
no power is supplied from the room thermostat "O" terminal to
the unit switchover valve coil (SOV) and the "O" terminal on the
ECMC board. This leaves the switch-over valve (SOV) in the
normal position for heating and is the signal for the (ECMC) to
run at indoor fan speeds designed for heating. On a call for
heating, power is supplied to the unit from the room thermostat
(Y1) and (G) terminal. (Y1) provides power to the compressor
contactor (CC), the defrost control (DFC) and the electronically
commutated motor control (ECMC). (G) provides power to the
(ECMC) for low speed (ECM) indoor fan motor operation.
st
stage (Low) operation and the
stage to maintain the desired
Installer's Guide
The energized compressor contactor (CC) completes the
circuit to the compressor for 1
the outdoor single speed fan motor (ODM). The indoor fan
motor (ECM) will operate on low speed. The room thermostat
will continue to cycle the compressor and fans to maintain
the desired temperature.
nd
Call for 2
stage after 1st stage
On a call for 2nd stage heating, power is supplied from the
room thermostat (Y2) terminal to the compressor rectifier
(ACR) and the (ECMC). This energizes the (ACR) switching
nd
the compressor to 2
stage (High) operation. The (ECMC) is
energized for indoor fan motor (ECM) high speed fan
operation. The room thermostat will continue to cycle the
st
nd
system between 1
and 2
temperature.
Supplementary Heat
The supplementary electric heat is brought on when the indoor
temperature drops below the thermostat setting. The thermostat
provides power from the "W" terminal to the supplementary
heater control circuit. Note that the fan relay (F) must have been
energized. An outdoor thermostat may have been added to
disallow the second stage (if provided) of electric heat above a
selected outdoor temperature. If the outdoor temperature falls
below the setting on the outdoor thermostat, this additional
heater stage will come on. When the outdoor air temperature
rises, and the outdoor T-stat setpoint is reached, the system will
revert back to first stage electric heating.
When the indoor ambient is satisfied, "W" contacts will open and
the unit will revert back to the compressor only heating mode and
then off. For emergency heat (use of supplementary electric heat
only), an emergency (EMERG) heat switch is provided within the
thermostat. When placed in the emergency heat position, it will
disable the compressor, bypass the outdoor thermostats, if
provided, and engage the supplementary electric heaters and
indoor fan.
Demand Defrost Operation
During the heating cycle, the outdoor coil may require a defrost
cycle which is determined by the demand defrost control (DFC).
This control continuously measures the outdoor coil temperature
(CBS) and the outdoor ambient temperature (ODS-B) and calcu-
lates the difference or delta-T measurement. When the calculated
delta-T is met, the demand defrost control (DFC) opens the circuit
to the outdoor fan motor (ODM) and energizes the switch-over valve
(SOV), placing the unit in the cooling mode to defrost the outdoor
coil (on SCROLL bearing units only, the control will stop the
compressor for a minimum of thirty (30) seconds). The outdoor coil
temperature sensor (CBS) terminates the defrost cycle, or it times
off after twelve (12) minutes in defrost, the (DFC) energizes the
outdoor fan motor (ODM) and twelve seconds later de-energizes
the (SOV), which returns the unit to the heating mode. Supple-
mentary electric heat, if provided, is brought on to control indoor
temperature during the defrost cycle. During this defrost cycle the
indoor fan will run at the speed designated for 2nd stage cooling.
Defrost Control
The demand defrost control measures heat pump outdoor
ambient temperature with a sensor located outside the outdoor
coil. A second sensor located on the outdoor coil is used to
measure the coil temperature. The difference between the
ambient and the colder coil temperature is the difference or
delta-T
measurement.
representative of the operating state and relative capacity of the
heat
pump
system.
Measuring
determines the need for defrost. The coil sensor also senses
outdoor coil temperature for termination of the defrost cycle.
st
stage (Low) operation and
stage to maintain the desired
This
delta-T
measurement
the
change
in
Page 19
is
delta-T
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