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HEAT PUMP HEATING SEQUENCE OF OPERATION
1. When the fan switch on the thermostat is in the "ON" position, the 24
volts at "G" brings on the indoor blower motor at the GSPD. When
the fan switch on the thermostat is in the "AUTO" position, the blower
operates when there is a call for heating by the thermostat.
2. On a call for heating, the thermostat sends 24 volts to "Y" on the
defrost control board. After the anti-short cycle period is complete,
the 24 volt signal energizes contactor coil M and power is supplied
to the compressor and outdoor fan motor. The reversing valve
remains in the heating position. When the fan switch on the thermo-
stat is in the "AUTO" position, the indoor blower is energized at the
YSPD.
3. When the heating demand is satisfied, the M contactor is de-ener-
gized when the 24 volt "Y" signal is removed. When the fan switch
on the thermostat is in the "ON" position, the indoor blower contin-
ues to run. When the fan switch is in the "AUTO" position, the
indoor blower motor ramps down after a 60-second delay.
DEFROST OPERATION
The demand defrost control implements a temperature differential
("delta-T") demand defrost algorithm. The heat pump is allowed to oper-
ate in the heating mode until the combination of outdoor ambient and
outdoor coil temperatures indicate that defrosting is necessary. When
coil temperature is below the initiate point for the ambient temperature
continuously for 4-1/2 minutes, the heat pump is put into a defrost cycle.
This 4-1/2 minute timer eliminates unnecessary defrost cycles caused
by refrigeration surges such as those that occur at the start of a heating
cycle.
TEST PINS
STATUS LED
FIGURE 9: Demand Defrost Control
Heat Pump Safety Switch Operation
The unit is equipped with a safety package. The refrigeration system
will be protected against high refrigerant pressure and a loss of charge
switch. If either of these safety switches open, the unit will be shut off
for the 5 minute anti-short cycle time. Once this has expired, a six hour
elapsed run timer begins. If a second opening of a safety switch occurs
during this six hour period, the compressor will be locked out.
Resetting the lockout function is accomplished by:
1. Removing power from the control's thermostat 1st stage (Y) input for
longer than 2 seconds.
Johnson Controls Unitary Products
A timed inhibit feature prevents the system from responding to a call for
defrost less than 40 minutes after the initiation of the previous defrost.
After the 40 minute inhibit time has expired, temperature conditions
must call for defrost continuously for 4-1/2 minutes before a defrost
cycle is initiated. A temperature inhibit feature prohibits defrost if the coil
temperature is above 40°F.
A forced-defrost feature puts the system into a defrost period every 6
hours and 4 minutes of accumulated compressor run-time to recirculate
lubricants, unless the coil temperature is above 40°F and the ambient
temperature is above 50°F. All defrost timing occurs only while the com-
pressor is on. Defrost curve selection maybe switched to pin 4 if a more
aggressive defrost cycle is necessary.
During the defrost mode, the reversing valve is energized, the outdoor
fan is de-energized, the compressor is energized, and the defrost con-
trol will provide a 24 volt signal from terminal "W out" to energize gas
heat.
For trouble shooting purposes, the defrost cycle can be manually initi-
ated by shorting the "TEST" pins together for 5 seconds while "Y" is
energized. After removing the short, defrost will terminate normally
during the "TEST" mode.
Table 15: Demand Defrost Selection
Unit
048, 060
024 — 042
024 — 060
*
For extreme environments as necessary only.
2. Removing power from "R" for more than 2 seconds.
3. Shorting the "TEST" pins together for more than 2 seconds while
"Y" is energized.
4. Shorting the "TEST" pins together for more than 5 seconds while
"Y" is de-energized.
Table 16: Test Pins
Test Pin Shorted
With Y Call
greater than 2 sec
Bypass ASCD
greater than 5 sec
Forced defrost
5132210-UIM-H-0916
Pin Position
2
3
4*
DEFROST CURVE
SELECTION JUMPER
A0349-001
Without Y Call
Display error codes
Clear error codes
15
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