9
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 figures 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.
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. 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 liquid 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 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), 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), the
defrost cycle is terminated and the timing period is reset. If
the defrost cycle is not terminated due to the sensor
temperature, a ten minute override interrupts the unit’s
defrost period.
C
Y
W2
R
R DFT
TEST
DF1
DF2
JUMPER WIRE
90
60
30
A
FIGURE 7
SUGGESTED FIELD TESTING/TROUBLE SHOOTING
1.
Run unit in the heating mode (room thermostat calling for
heat).
2.
Check unit for proper charge.
Note: Bands of frost on the
condenser coil indicate low refrigerant charge.
3.
Shut off power to unit.
4.
Disconnect outdoor fan by removing the purple lead from
“DF2” on defrost control.
5.
Restart unit and allow frost to accumulate.
6.
After a few minutes of operation, the unit’s defrost
thermostat should close. To verify this, check for 24 volts
between “DFT” and “C” on board. If the temperature at the
thermostat is less than 28° F and the thermostat is open,
replace the unit’s defrost thermostat, as it is defective.
7.
When the unit’s defrost thermostat has closed, short the
test pins on the defrost board until the reversing valve shifts,
indicating defrost. This should take up to 21 seconds
depending on what timing period the control is set on.
After defrost initiation, the short must instantly be removed
or the unit’s defrost period will only last 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.
Shut off power to unit.
10. Replace outdoor fan motor lead to terminal “DF2” on defrost
board and turn on power.
AIR FLOW MEASUREMENT AND ADJUSTMENT
After reviewing section on DUCTING, proceed with airflow
measurements and adjustments. Unit’s blower curves (in
Specification Sheets) are based on external static pressure
(ESP, in. of W.C.). The duct openings on the unit are
considered internal static pressure, so 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 (i.e. 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 unit is set and ducted, verify ESP with a 1" inclined
manometer with pitot tubes or a Magnahelic gauge and
confirm CFM to blower curves in the specification sheets.
All units have three speed blower motors. If low speed is
not utilized, the speed tap can be changed to medium or
high speed. Never run CFM below 350 CFM per ton,
evaporator freezing or poor unit performance is possible.
