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blower is operated throughout the cycle. The 5-minute
off period is necessary to defrost the indoor coil.
Low ambient mode always begins with compressor
operation. Compressor minimum run time may extend
the minutes of compressor operation. The defrost cycle
will begin immediately following the elapse of the mini-
mum run time.
When operating in low ambient mode, the UCB will not
lockout the compressors due to a freezestat trip. How-
ever, a freezestat trip will de-energize the associated
compressor. If the call for cooling is still present at the
end of the ASCD and the freezestat has closed, the
unit will resume operation.
SAFETY CONTROLS
The unit control board monitors the following inputs for
each cooling system:
1. A suction line freezestat to protect against low
evaporator temperatures due to a low airflow or a
low return air temperature (opens at 26 ± 5 °F and
resets at 38 ± 5°F).
2. A high-pressure switch to protect against excessive
discharge pressures due to a blocked condenser
coil or a condenser motor failure (opens at 405 ± 7
psig and resets at 265 ± 20 psig).
3. A low-pressure switch to protect against loss of
refrigerant charge (opens at 22 ± 5 psig and resets
at 45 ± 5 psig).
The above pressure switches are hard-soldered to the
unit. The refrigeration systems are independently mon-
itored and controlled. On any fault, only the associated
system will be affected by any safety/preventive action.
The other refrigerant system will continue in operation
unless it is affected by the fault as well.
The unit control board monitors the temperature limit
switch of electric heat units and the temperature limit
switch and the gas valve of gas furnace units.
COMPRESSOR PROTECTION
The compressors also have inherent (internal) protec-
tion. If there is an abnormal temperature rise in a com-
pressor, the protector will open to shut down the
compressor. The UCB incorporates features to mini-
mize compressor wear and damage. An anti-short
cycle delay (ASCD) is utilized to prevent operation of a
compressor too soon after its previous run. Addition-
ally, a minimum run time is imposed any time a com-
pressor is energized.
Johnson Controls Unitary Products
The ASCD is initiated on unit start-up and on any com-
pressor reset or lock-out.
FLASH CODES
The UCB will initiate a flash code associated with
errors within the system. Refer to UNIT CONTROL
BOARD FLASH CODES Table 36.
RESET
Remove the call for cooling, by raising thermostat set-
ting higher than the conditioned space temperature.
This resets any pressure or freezestat flash codes.
ELECTRIC HEATING SEQUENCE OF OPERATIONS
The following sequence describes the operation of the
electric heat section.
For units with VFD and electric heat, the speed
of the indoor blower motor continues to be con-
trolled by duct static pressure via the VAV con-
trol board.
If there are VAV boxes present in the duct sys-
tem, the boxes must be driven to the full-open
position using a customer-supplied power
source to assure adequate airflow across the
heating elements.
Single-stage heating: (applies only to 18 KW heater, all
other heaters MUST use a two-stage thermostat:).
a.
Upon a call for heat by the thermostat, the heater
contactor (6M) will be energized. After completing
the specified fan on delay for heating, the UCB will
energize the blower motor.
b.
The thermostat will cycle the electric heat to satisfy
the heating requirements of the conditioned space.
Two-stage heating: (applies to all heaters except 18
KW):
a.
Upon a call for first-stage heat by the thermostat, the
heater contactor (6M) (6M & 7M on 72 KW, 240V)
will be energized. After completing the specified fan
on delay for heating, the UCB will energize the
blower motor.
If the second stage of heat is required, heater con-
tactor (7M) will be energized. Note that on the 54
KW, 240V heater, heater contactors (7M & 8M) will
be energized and on the 72 KW, 240V heater,
heater contactors (8M & 9M) will be energized. After
completing the specified fan on delay for heating,
the UCB will energize the blower motor.
284812-YIM-D-1008
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