
13
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 in-
creased compressor noise caused by rapidly changing system
pressures when the reversing valve switches. See next page
for diagnostic ash codes and sensor resistance values at var-
ious temperatures.
DEFROST INITIATION
A defrost will be initiated when the three conditions below are
satised:
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 tempera-
ture and the outdoor coil temperature is greater than the
calculated difference determined by the defrost control
microprocessor.
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
outside 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 de-
frost.
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 ig-
nored 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 min-
utes) 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
running, 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 termi-
nation temperature, the control is operating normally. 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 ap-
proximately 30,000 ohms.
Ensure that the coil sensor is properly installed, and 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 approximately
40 psig. Operation is slightly different between cooling 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 approximately 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 com-
pressor 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 approxi-
mately 40 psig when the low pressure control automati-
cally resets. If the low pressure switch trips 3 times within
120 minutes of operation during a particular call for heat-
ing operation, the defrost control will lock out compressor
and outdoor fan operation. If the lock-out due to low pres-
sure 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 necessary 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.