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XIX. DEMAND DEFROST CONTROL AND
HIGH/LOW PRESSURE CONTROLS
The demand defrost control monitors the outdoor ambient
temperature, outdoor coil temperature and the compressor run
time to determine when a defrost cycle is required.
Enhanced Feature Demand Defrost Control:
This defrost
control has high and low pressure control inputs with unique
pressure switch logic built into the microprocessor to provide
compressor and system protection without nuisance lockouts.
The control cycles the compressor off for 30 seconds at the
beginning and the end of the defrost cycle to eliminate the
increased compressor noise caused by rapidly changing sys-
tem pressures when the reversing valve switches. See next
page for diagnostic flash codes and sensor resistance values
at various temperatures.
DEFROST INITIATION
A defrost will be initiated when the three conditions below are
satisfied:
1. The outdoor coil temperature is below 35°F as measured by
a good coil sensor,
2. The compressor has operated for at least 34 minutes with
the outdoor coil temperature below 35°F and
3. The measured difference between the ambient temperature
and the outdoor coil temperature is greater than the calcu-
lated difference determined by the defrost control micro-
processor.
DEFROST TERMINATION
Once a defrost is initiated, the defrost will continue until four-
teen minutes has elapsed or the coil temperature has reached
the selected termination temperature. The factory setting is
70°F but can be changed to 50°F, 60°F, or 80°F by relocating
the jumper on the control board.
TEMPERATURE SENSORS
The coil sensor is located on the outdoor coil near the point fed
by the distribution tubes from the expansion device, on the top
most cross-over tube. The ambient air sensor is located out-
side the control box so it can sense outdoor temperatures.
If the ambient sensor fails, the defrost control will initiate a
defrost every 34 minutes of compressor run time with the coil
temperature below 35°F.
If the coil sensor fails, the defrost control will not initiate a
defrost.
TEST MODE
The test mode is initiated by shorting the TEST pins. The
unit must have an active heat pump heating call to enter the
test mode. In this mode of operation, the enable temperature
is ignored and all timers are sped up. To initiate a manual
defrost, short and hold the TEST pins. Remove the short when
the system switches to defrost mode after the compressor
noise abatement delay. The defrost will terminate on time
(14 minutes) or when the termination temperature has been
reached.
Test Sequence of Operation:
1) Provide a heating call to the heat pump.
2) Short test pins to bypass anti-short cycle timer. (If unit is run
-
ning, this step is not necessary.)
3) Short test pins and hold them shorted to enter defrost mode.
4) Release test pins once control exits noise abatement delay.
5) Monitor coil temperature when control exits defrost.
6) Unit should return to heating mode.
TROUBLESHOOTING DEMAND DEFROST
During the test mode the coil temperature should be moni-
tored. If the system exits defrost at approximately the termina-
tion temperature, the control is operating normlly. If not, check
the coil and ambient temperature sensor resistances, using
the sensor temperature vs. resistance table at the end of this
section.
Immerse the sensor in water and measure the resistance of
the sensor. At 35°F the resistance of the sensor should be
approximately 30,000 ohms.
Ensure that the coil sensor is properly installed that is not loose
or touching the cabinet.
HIGH/LOW PRESSURE CONTROL MONITORING -
ENHANCED DEFROST CONTROL
Status of high and low pressure controls is monitored by the
enhanced feature demand defrost control and the following
actions are taken.
High Pressure Control –
Provides active protection in both
cooling and heating modes at all outdoor ambient tempera-
tures. The high pressure control is an automatic reset type and
opens at approximately 610 psig and closes at approximately
420 psig. The compressor and fan motor will stop when the
high pressure control opens and will start again if the high side
pressure drops to approximately 420 psig where the automatic
reset high pressure control resets. If the high pressure control
opens 3 times within a particular call for heating or cooling
operation, the defrost control will lock out compressor and out-
door fan operation.
Low Pressure Control –
Provides active protection in both
heating and cooling modes at all outdoor ambient tempera-
tures. The low pressure control is an automatic reset type
and opens at approximately 15 psig and closes at approxi-
mately 40 psig. Operation is slightly different between cool-
ing and heating modes.
Cooling Mode:
The compressor and fan motor will
stop when the low pressure control opens and will start
again when the low side pressure rises to approximate-
ly 40 psig after the low pressure control automatically
resets. If the low pressure switch opens 3 times within
a particular call for cooling operation, the defrost control
will lock out compressor and outdoor fan operation.
Heating Mode:
The compressor and outdoor fan motor
will stop when the low pressure control opens and
will start again when the low side pressure rises to
approximately 40 psig when the low pressure control
automatically resets. If the low pressure switch trips
3 times within 120 minutes of operation during a par-
ticular call for heating operation, the defrost control
will lock out compressor and outdoor fan operation. If
the lock-out due to low pressure occurs at an outdoor
ambient temperature below 5°F, the defrost control will
automatically exit the lock-out mode when the outdoor
ambient temperature rises to 5°F. This feature is nec-
essary since the low pressure control could possibly
have opened due to the outdoor ambient being very
low rather than an actual system fault.
Exiting Lock-Out Mode:
To exit the lock-out mode, remove
24 volts to the defrost control by removing power to the unit
or by shorting the two defrost control pins together.
ENHANCED FEATURE DEFROST CONTROL
Содержание RHPD Series
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