10
Heating Cycle
The expansion devices are flowrator distributors and perform the
same function on the heating cycle as on the cooling cycle. The
flowrator distributors 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 temperature and the outdoor coil
temperature, the greater 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
(7ºC). 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 the freezing point, frost forms on
the surfaces of the outdoor coil under certain weather conditions
related to 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 feeder tube entering
the outdoor coil. Defrost timing periods of 30, 60 and 90 minutes
may be selected by connecting the circuit board jumper to 30,
60 and 90 respectively. Accumulation of time for the timing
period selected starts when the sensor closes (approximately
31°F[0ºC]), and when the wall thermostat calls for heat. At the
end of the timing period, the unit’s defrost cycle will be initiated
provided the sensor remains closed. When the sensor opens
(approximately 75°F [24ºC]), the defrost cycle is terminated and
the timing period is reset. If the defrost cycle is not terminated
due to the sensor temperature, a 10-minute override interrupts
the unit’s defrost period.
Circuit Board
Suggested Field Testing/Troubleshooting
1. Run the unit in the heating mode (room thermostat calling for
heat).
2. Check the unit for proper charge.
NOTE: Bands of frost on the condenser coil indicate low
refrigerant charge.
3. Turn off power to the unit.
4. Disconnect the outdoor fan by removing the purple lead from
“DF2” on the defrost control.
5. Restart the unit and allow frost to accumulate.
6. After a few minutes of operation, the defrost thermostat
should close. To verify this, check for 24 volts between “DFT”
and “C” on the control board. If the temperature at the
thermostat is less than 28°F (-2ºC) and the thermostat is
open, replace the defrost thermostat, since it is defective.
7. When the unit’s defrost thermostat has closed, short the test
pins on the circuit board until the reversing valve shifts,
indicating defrost. This should take up to 21 seconds
depending on what timing period the control is set.
NOTE: After defrost initiation, the short must instantly be
removed, or the unit’s defrost period will last only 2.3 seconds.
8. After the unit’s defrost thermostat has terminated, check the
defrost thermostat for 24 volts between “DFT” and “C.” The
reading should indicate 0 volts (open sensor).
9. Turn off power to the unit.
10. Replace the outdoor fan motor lead to terminal “DF2” on the
circuit board and turn on power.
Airflow Measurement and Adjustment
After reviewing the “Ductwork” section in “Circulating Air and
Filters,” proceed with the airflow measurements and
adjustments. The unit blower curves (see Specification Sheets)
are based on the external static pressure (ESP per in./W.C.). The
duct openings on the unit are considered internal static pressure.
As long as ESP is maintained, the unit will deliver the proper air
up to the maximum static pressure listed for the CFM required by
the application (for example, home, building, etc.).
In general, 400 CFM per ton of cooling capacity is a rule of
thumb. Some applications depending on the sensible and latent
capacity requirements may need only 350 CFM or up to 425 CFM
per ton. Check condition space load requirements (from load
calculations) and equipment expanded ratings data to match
CFM and capacity.
After the unit is set and the ductwork completed, verify the ESP
with a 1" (2.5 cm) inclined manometer with pitot tubes or a
Magnahelic gauge and confirm CFM to blower curves in the
Specification Sheets. All units have 3-speed blower motors. If the
low speed is not utilized, the speed tap can be changed to
medium or high speed.
NOTE: Never run CFM below 350 CFM per ton. Evaporator
freezing or poor unit performance is possible.
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
A
F
D
C
I
E
C
B
H
C Y
W2
R
R DFT
TEST
DF1
DF2
Jumper Wire
90
60
30
A
