66396-YIM-C-1004
Unitary Products Group
29
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 -3°C (26°F)
and resets at 3°C (38°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 2660
kPa (380 psig) and resets at 2100 kPa (300 psig)).
3.
A low-pressure switch to protect against loss of
refrigerant charge (opens at 49 kPa (7 psig) and
resets at 154 kPa (22 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 21.
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 and #2
contactor M1. After a two second delay, it then ener-
gizes compressor #3 and #4 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 ener-
gizes the M4 and M5 contactor which brings on both
condenser fans.
If the compressors alone cannot satisfy the heating
requirements a second stage call from the thermostat
completes the circuit between “R” and “W2”. This
24vac signal is passed through the UCB to the RW2
relay. Contacts RW2-1 close, completing the circuit to
the electric heat section (if available). For the 18 kW
heater 18 kW will be energized, for the 36 kW heater
18 kW will be energized, for the 54 kW heater 18 kW
will be energized, and for 72 kW heater 36 kW will be
energized. In the event that all compressors are locked
out by the UCB, the total available kW of electric heat
will be energized on a call for “W2”. Take note that the
MV terminal on the UCB is constantly monitored while
there is a demand for heat. If the UCB does not see
24vac at terminal MV after six minutes, it initiates a
fault code 9, indicating a heating problem.
WITHOUT ELECTRIC HEAT
When the thermostat calls for heating, the low voltage
control circuit is completed between “R” and “W1”. This
24vac signal is passed through the UCB to the RW1
Relay and the MV terminal on the UCB. Contacts RW1-
2 open, assuring the reversing valve cannot be ener-
gized, except during defrost. Contacts RW1-1 close,
completing the circuit to Y on the
Defrost Control
