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10. Indoor Blower Motor - Units with ECM Motors.
The ECM model indoor blower motor is activated by
the room thermostat by cooling/heating or fan "ON"
position. ECM motors are constant torque motors with
the very low power consumption.
(See Air Flow Measurement and Adjustment for speed
adjustment instructions).
11. Blower Interlock Relay – 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 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 Air Conditioner unit. See Figure 9 - Heat Pump
Cooling Schematic.
Typical Heat Pump System in Cooling
Indoor
Coil
Accumulator
Heat Pump Cooling Schematic
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 with 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 figure, Figure 10 -
Heat Pump Heating Schematic, show a schematic of a
heat pump in the heating cycle.
Reversing Valve
(Energized)
Figure 9
TYPICAL HEAT PUMP SYSTEM IN HEATING
Indoor
Coil
Accumulator
For Heat Pump units, the expansion devices are Thermal
Expansion Devices (TXV) and perform the same function
on the heating cycle as on the cooling cycle. The TXVs
also act as check valves to allow for the reverse of
refrigerant flow.
When the heat pump is on the heating cycle, the outdoor
coil is functioning as an evaporator. The temperature
of the refrigerant in the outdoor coil must be below the
temperature of the outdoor air in order to extract heat from
the air. Thus, the greater the difference in the outdoor
Outdoor
temperature and the outdoor coil temperature, the greater
Coil
the heating capacity of the heat pump. This phenomenon
is a characteristic of a heat pump. It is a good practice to
provide supplementary heat for all heat pump installations
in areas where the temperature drops below 45°F. It is also
a good practice to provide sufficient supplementary heat
to handle the entire heating requirement should there be a
component failure of the heat pump, such as a compressor,
or refrigerant leak, etc.
Since the temperature of the refrigerant in the outdoor
coil on the heating cycle is generally below freezing point,
frost forms on the surfaces of the outdoor coil under
certain weather conditions of temperature and relative
humidity. Therefore, it is necessary to reverse the flow of
the refrigerant to provide hot gas in the outdoor coil to melt
the frost accumulation. This is accomplished by reversing
the heat pump to the cooling cycle. At the same time, the
outdoor fan stops to hasten the temperature rise of the
outdoor coil and lessen the time required for defrosting.
The indoor blower continues to run and the supplementary
heaters are energized.
Defrost Control
During operation the power to the circuit board is controlled
by a temperature sensor, which is clamped to a return bend
entering the outdoor coil. Defrost timing periods of 30,
60 and 90 minutes may be selected by setting the circuit
board jumper to 30, 60 and 90 respectively. Accumulation
of time for the timing period selected starts when the
sensor closes (approximately 30 ± 5°F), and when the wall
11
Reversing Valve
(De-Energized)
Heat Pump Heating Schematic
Figure 10
Outdoor
Coil
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