035-20401-001 Rev. A (0904)
18
Unitary Products Group
Auxiliary Heat Defrost Operation – Fossil Fuel Mode with
Bonnet Sensor
With a bonnet sensor present the control will energize W1 OUT and W2
OUT 45 seconds prior to the initiation of the defrost cycle.
During defrost operation, if the bonnet sensor input reaches 109F, the
control will de-energize W1 OUT and W2 OUT. The control will re-ener-
gize W1 OUT and W2 OUT when the bonnet sensor input drops below
90F again and the defrost cycle is still in process.
If a call for heating (Y1) is still present after the defrost cycle has termi-
nated, the control will continue to energize W1 OUT and W2 OUT after
the defrost cycle has been terminated until the bonnet sensor reaches
109F. When the bonnet sensor reaches 109F after the defrost cycle has
terminated, the control will de-energize W1 OUT and W2 OUT and will
not re-energize them during this call for heat. That is, once the defrost
cycle has terminated, the control will not cycle W1 OUT and W2 OUT
with the bonnet sensor.
The control will begin normal heat pump heating mode operation upon
termination of the defrost cycle.
Auxiliary Heat Defrost Operation – Fossil Fuel Mode with-
out Bonnet Sensor
If the control is in fossil fuel mode and senses that no bonnet sensor is
present, it will energize W1 OUT and W2 OUT immediately when the
defrost cycle is initiated. If a call for heating (Y1) is still present after the
defrost cycle has terminated, the control will de-energize W1 OUT and
W2 OUT immediately and return to normal heat pump mode operation.
In this mode the control will energize the fossil fuel furnace only during
defrost and not provide any comfort enhancements during the transi-
tion. The heat pump and furnace will not operate at the same time if the
bonnet sensor is not in place.
Pipe Freeze Protection Timer – Fossil Fuel Mode Operation
The control starts a sixty-minute timer when a call for compressor oper-
ation and auxiliary heat (Y1 + W) is received. If the outdoor temperature
is above the balance point, the control will energize the compressor
instead of the auxiliary heat outputs. If the call for compressor operation
and auxiliary heat is still present after the timer expires, the control will
energize W1 OUT and W2 OUT and de-energize the compressor
regardless of the balance point setting. The control will keep the W1
OUT and W2 OUT signals energized until the Y1 signal is removed.
That is, the control will lock into auxiliary heat furnace operation until the
room thermostat is satisfied. The purpose of this feature is to prevent
the pipes in a home from freezing if the balance point is set too low and
the heat pump cannot heat the home using compressor operation only.
This will be a benefit if a home is not occupied and a compressor prob-
lem occurs. The control will also store and display a fault flash code
when the pipe freeze timer has expired.
Y2 LOCK
The control includes a Y2 LOCK feature which allows the unit to antici-
pate the need for second-stage cooling during high-load conditions.
Refer to the “Second-Stage Cooling Anticipation Mode” section of this
document for detailed information.
The Y2 Lock jumper on the control is shown in figures 11 & 14. The fac-
tory places the Y2 Lock jumper in the ON position. If the jumper is
removed, the control will behave as if the jumper is in the ON position.
SWITCH POINT
The control includes a switch point feature which determines the liquid
temperatures at which the compressor will be forced to operate in sec-
ond-stage. Refer to the “Switch Point Operation” section of this docu-
ment for detailed information.
Setting
The switch point jumper on the control is shown in Figure 11 & 14. The
control allows for switch point settings to be 35, 40 or 45. The valves
shown are in degrees F.
The factory places the switch point jumper in the 35°F position. If the
jumper is removed, the control will behave as if the jumper is in the
35°F position. The switch point feature is used exclusivly for the HP
heating operation mode. Cooling operation is not effected by switch
point setting.
HIGH-PRESSURE SWITCH FAULT
The heat pump is equipped with a high-pressure switch that is con-
nected to the control at the HPS terminals. If the high-pressure switch
opens for more than 40 milliseconds, the control will de-energize the
compressor and store and display the appropriate fault code. If the
pressure switch closes and a thermostat call for compressor operation
is present, the control will apply the five-minute anti-short cycle delay
timer and start the compressor when the timer expires.
When the compressor is started following a high-pressure switch fault,
the control will start a six-hour timer based on accumulated compressor
run time. If the control senses another opening of the high-pressure
switch before the timer expires, it will cause a soft lockout condition.
The second opening of the high-pressure switch must be greater than
160 milliseconds for the lockout to occur. If the second opening is
between 40 and 160 milliseconds, the control will de-energize the com-
pressor but not cause a soft lockout condition. If the control does not
sense a second high-pressure switch opening before the six-hour timer
expires, the timer and counter will be reset.
LOW-PRESSURE SWITCH
The heat pump is equipped with a low-pressure switch which is con-
nected to the control at the LPS terminals. If the low-pressure switch
opens for more than five seconds, the control will cause a soft lockout
condition and display the appropriate fault codes. However, the control
will ignore the low pressure switch input and not cause a soft lockout
condition if it opens during the following conditions.
•
Defrost operation
•
First thirty seconds of compressor operation
•
Two minutes following the completion of a defrost cycle
•
TEST input shorted with Y1 input energized
DISCHARGE LINE TEMPERATURE SENSOR
The heat pump may be equipped with an optional discharge line tem-
perature sensor. If a discharge sensor is present, the control will pro-
vide the following features.
High Discharge Line Temperature
If the control senses a discharge line temperature reading of 263F for
30 seconds continually, it will cause a soft lockout condition. If the dis-
charge line temperature drops below 263F during the 30-second timer,
the control will reset the 30-second timer and restart the timer if the dis-
charge line temperature again exceeds 263F.
