175245-YIM-C-1008
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Johnson Controls Unitary Products
Should a freezestat open three times within two hours
of operation, the UCB will lock-out the associated com-
pressor and flash a code (Table 20). If the other com-
pressor is inactive, the condenser fans will be de-
energized.
LOW AMBIENT COOLING
To determine when to operate in low ambient mode,
the UCB has a pair of terminals connected to a temper-
ature-activated switch set at 45°F. When the low ambi-
ent switch is closed and the thermostat is calling for
cooling, the UCB will operate in the low ambient mode.
Low ambient mode operates the compressors in this
manner: 10 minutes on, 5 minutes off. The indoor
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 exces-
sive discharge pressures due to a blocked con-
denser coil or a condenser motor failure, (opens at
380 ± 10 psig and resets at 300 ± 10 psig).
3. A low-pressure switch to protect against loss of
refrigerant charge, (opens at 7 ± 3 psig and resets
at 22 ± 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
In addition to external pressure switches, the compres-
sors also have inherent (internal) protection. If there is
an abnormal temperature rise in a compressor, the pro-
tector will open to shut down the compressor. The UCB
incorporates features to minimize compressor wear
and damage. An
Anti-Short Cycle Delay (ASCD)
is
utilized to prevent operation of a compressor too soon
after its previous run. Additionally, a minimum run time
is imposed any time a compressor is energized.
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 20.
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.
HEATING SEQUENCE OF OPERATIONS
WITH ELECTRIC HEAT
When the thermostat calls for the first stage of heating,
the low voltage control circuit is completed between “R”
and “W1”. This 24vac signal is passed through the UCB
to the RW1 Relay. Contacts RW1-2 open, assuring the
reversing valve cannot be energized, except during
defrost. Contacts RW1-1 close, completing the circuit to
Y on the
Defrost Control (DC)
. After its five minute
ASCD timer is satisfied, the DC closes its internal com-
pressor relay contacts, sending a 24vac signal to the
MV1 relay. Contacts MV1-1 close sending a 24vac sig-
nal to the MV terminal of the UCB. If its ASCD timer is
satisfied the UCB will energize compressor #1 (contac-
tor M1). After a two second delay, it then energizes
compressor #2 (contactor M2). Therefore, on a call for
heat from W1, both sets of compressors are always
energized, unless one or the other is locked out by the
UCB. Also on the call for heat, the DC energizes the M4
and M5 contactor which brings on both condenser fans.
