9
Crankcase Heater
This item is “ON” whenever power is supplied to the unit. It
warms the compressor crankcase thereby preventing liquid
migration and subsequent compressor damage. The insert type
heater is self-regulating. It is connected electrically to the
contactor L1 and L2 terminals.
Condenser Motor
This item is activated by the contactor during heating and
cooling, except during defrost and emergency heat operation.
Compressor
This item is activated by the contactor for heating and cooling,
except during emergency heat. It is protected by an internal
overload.
Contactor Relay
This control is activated by the thermostat (24V coil) and supplies
power to the contactor.
Defrost Control
The Defrost control provides time/temperature initiation and
termination of the defrost cycle. When a defrost cycle is initiated,
the defrost control shifts the reversing valve to “COOLING”
mode, stops the outdoor fan and brings on supplemental heat.
Normally, a defrost cycle will take only 2 to 3 minutes unless the
system is low on charge or outdoor conditions are severe (windy
and cold).
Outdoor Thermostat
These optional controls are used to prevent full electric heater
operation at varying outdoor ambient (0°F to 45°F [-18ºC to 7ºC]).
They are normally open above their set points and closed below
to permit staging of the indoor supplement heater operation. If
the outdoor ambient temperature is below 0° F (-18° C) with 50%
or higher relative humidity (RH), an outdoor thermostat (OT) must
be installed and set at (0°) on the dial. Failure to comply with this
requirement may result in damage to the product which may not
be covered by the manufacturer’s warranty.
Reversing Valve Coil
This coil is activated by the thermostat, in the cooling mode and
during defrost. It positions the reversing valve pilot valve for
cooling operation.
Indoor Blower Motor
This is activated by setting the room thermostat to COOLING or
FAN ON position. The motor is energized through the EBTDR for
PSC motors and directly by the room thermostat for X-13 motors
(4 or 5 Ton units). X-13 motors are constant torque motors with
very low power consumption. This motor is energized by a 24V
signal from the thermostat. See “Airflow Measurement and
Adjustment” for speed adjustment instructions.
Blower Interlock Relay—Single-Phase PSC Models Only
This relay is used to energize the blower during the electric heat
operation. Some room thermostats do not energize the motor
during electric heat. This relay ensures blower operation when
the room thermostat energizes heat. This relay has a 240-volt coil
and an 80-amp contact relay. This relay is energized by the
electric heat kit sequencer.
HEAT PUMP OPERATION
Cooling Cycle
When the heat pump is in the cooling cycle, it operates exactly as
a summer air conditioner unit. In this mode, all the charts and
data for service that apply to summer air conditioning apply to
the heat pump. Most apply on the heating cycle except that the
condenser becomes the evaporator, the evaporator becomes the
condenser and cooling becomes heating.
Heating Cycle
The heat pump operates in the heating cycle by redirecting
refrigerant flow through the refrigerant circuit external to the
compressor. This is accomplished through the reversing valve.
Hot discharge vapor from the compressor is directed to the
indoor coil (evaporator on the cooling cycle) where the heat is
removed, and the vapor condenses to liquid. It then goes through
the expansion device to the outdoor coil (condenser on the
cooling cycle) where the liquid is evaporated, and the vapor goes
to the compressor.
When the solenoid valve coil is operated either from heating to
cooling or vice versa, the piston in the reversing valve to the low
pressure (high pressure) reverse positions in the reversing valve.
The following illustrations show a schematic of a heat pump on
the cooling cycle and the heating cycle. In addition to a reversing
valve, a heat pump is equipped with an expansion device and
check valve for the indoor coil, and similar equipment for the
outdoor coil. It is also provided with a defrost control system.
Cooling Cycle
A. Indoor coil
B. Service valves
C. Service ports
D. Reversing valve
E. Outdoor coil
F. Accumulator
G. Compressor
H. Distributors
I. Check valve orifices.
G
H
I
B
E
F
D
C
I
A
C
C
B
H
