Anti-short Cycle Timer
This unit has an anti-short cycle timer built in to the defrost con-
trol. This timer will not permit the compressor to start within five
minutes after the completion of the last cycle or power interrup-
tion. To bypass the anti-short cycle feature, short the “TEST”
pins together for 2 seconds.
The following sequences of operation are based on using a
standard single-stage heat pump thermostat.
COOLING OPERATION
1. When the fan switch on the thermostat is in the “ON” posi-
tion, the 24 volts at “G” will bring on the indoor blower motor
at the cooling airflow. When the fan switch on the thermo-
stat is in the “AUTO” position, the blower operates only
when there is a call for cooling by the thermostat.
2. On a call for cooling, the thermostat sends 24 volts to “Y”
and “O” on the fan control and defrost control boards. The
reversing valve solenoid is energized, and after the anti-
short cycle period is complete contactor coil M1 is ener-
gized. Power is supplied to the compressor and outdoor fan
motor, and the reversing valve switched to the cooling posi-
tion. When the fan switch on the thermostat is in the “AUTO”
position the indoor blower motor is energized at the cooling
airflow.
3. When the demand for cooling has been satisfied, the 24
volt “Y” signal is removed, and the M1 contactor is de-
energized. When the fan switch on the thermostat is in the
“ON” position, the indoor blower motor continues to run. If
the fan switch is in the “AUTO” position. the indoor motor
ramps down over a 30-second period.
HEATING OPERATION
1. When the fan switch on the thermostat is in the “ON” posi-
tion, the 24 volts at “G” brings on the indoor blower motor at
the heating flow. 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 fan control board. After the anti-short cycle period is
complete, the 24 volt signal energizes contactor coil M1
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 thermostat is in the “AUTO” po-
sition, the indoor blower is energized at the heating airflow.
3. For units equipped with supplementary electric heat, when
the heat pump cannot meet the demand, the thermostat
“W” sends 24 volts to “W2” on the fan control board. This
signal also is sent through the defrost control terminals “W”
and “W6” and back to the fan control “W1”. The 24 volt sig-
nal energizes all stages of electric heat.
4
When the heating demand is satisfied, the electric heat is
de-energized when the 24 volt “W” signal is removed, and
the M1 contactor is de-energized when the 24 volt “Y” sig-
nal is removed. When the fan switch on the thermostat is in
the “ON” position, the indoor blower continues to run. When
the fan switch is in the “AUTO” position, the indoor blower
motor ramps down over a 15-second period.
Please refer to Tables 8 and 9 for more information.
Defrost Operation
The demand defrost control implements a temperature differ-
ential (”delta-T”) demand defrost algorithm. The heat pump is
allowed to operate 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 ini-
tiate 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.
A timed inhibit feature prevents the system from responding to
a call for defrost less than 20 minutes after the initiation of the
previous defrost. After the 20 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 in-
hibit 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 to recirculate lubricants, unless
the coil temperature is above 40
°
F. All defrost timing occurs
only while the compressor is on.
During the defrost mode, the defrost control will provide a 24
volt signal from terminal “W1/66” to the fan control terminal
“W1”. This signal will energize electric heat stage 1, if the unit is
so equipped.
For trouble shooting purposes, the defrost cycle can be manu-
ally initiated by shorting the “TEST” pins together for 5 sec-
onds. Defrost will terminate normally during the “TEST” mode.
Heat Pump Safety Switch Operation
The unit is equipped with a safety package. The refrigeration
system will be protected against high or low refrigerant pres-
sure. If either of these safety switches opens, the unit will be
shut off for the 5 minute anti-short cycle time. Once this has ex-
pired, a six hour elapsed run timer begins. If a second opening
of a safety switch occurs during this six hour period, the com-
pressor will be locked out.
Resetting the lockout function is accomplished by;
1. Removing power from the control's thermostat 1st stage
(Y) input for a time not to exceed 5 seconds (ON-OFF-ON).
2. Removing power from “R” for more than 2 seconds.
3. Shorting the “TEST” pins together for more than 2 seconds.
Electric Heat Limit Switch Operation
The limit switch responds to over temperature conditions in the
air duct. Opening of the device results in dropping power to the
relays. The control logic will also respond by turning off the re-
lays. After four limit cycle trips the unit goes into a 1 hour soft
lockout period. If during this period the control “sees” another
limit cycle, the unit will go into a hard lockout condition. Once in
a hard lockout state, the fan is locked on and the heaters are
disabled. Only a power cycle will clear this state.
During the soft lockout period, the fan responds to thermostat
input but the heaters are enabled. This is to sense a failed
heater relay. The limit cycle count is reset at the start of a heat
request. If the limit remains open for period of 80 seconds or
more, the control is immediately put into a hard lockout condi-
tion. Only a power cycle will clear this state.
Unitary Products Group
9
035-20188-000-A-0704
SEQUENCE OF OPERATION
SECURE OWNER'S APPROVAL
: When the system is
functioning properly, secure the owner's approval. Show him
the location of all disconnect switches and the thermostat.
Teach him how to start and stop the unit and how to adjust tem-
perature settings within the limitations of the system.
