5803641-JIM-B-0620
Johnson Controls Ducted Systems
45
(Anti-short cycle delay), and stop the condenser fans. If the call
for cooling is still present at the conclusion of the ASCD, the
UCB will re-energize the halted compressor.
Should a high-pressure switch open three times within two
hours of operation, the UCB will lock-out the compressor.
Low-Pressure Limit Switch
The low-pressure limit switch is not monitored during the initial
30 seconds of a cooling system's operation. For the following
30 seconds, the UCB will monitor the low-pressure switch to
ensure it closes. If the low-pressure switch fails to close after
the 30-second monitoring phase, the UCB will de-energize the
compressor, initiate the ASCD, and stop the condenser fans.
Once the low-pressure switch has been proven (closed during
the 30-second monitor period described above), the UCB will
monitor the low-pressure limit switch for any openings. If the
low-pressure switch opens for greater than 5 seconds, the UCB
will de-energize the compressor, initiate the ASCD, and stop
the condenser fans.
If the call for cooling is still present at the conclusion of the
ASCD, the UCB will re-energize the halted compressor.
Should a low-pressure switch open three times within one hour
of operation, the UCB will lock-out the =compressor.
Evaporator Low Limit
During cooling operation, if the
Evaporator Low Limit Sensor
(EC1)
(Located on the Suction Line at the Evaporator Coil.)
detects a temperature below 26 Deg. F (default), the UCB will
de-energize the compressor, initiate the ASCD, and stop the
condenser fans. If the call for cooling is still present at the
conclusion of the ASCD, the UCB will re-energize the halted
compressor. Should the UCB detect the evaporator low limit
sensor
(EC1)
falling below 26 Deg. F (default) three times
within two hours of operation, the UCB will lock-out the
compressor.
Low Ambient Cooling
To determine when to operate in low ambient mode, the UCB
has an
Outdoor Air Temperature Sensor (OAT)
with a low
ambient setpoint at 45ºF (default). When the
OAT Sensor
senses a temperature below the low ambient setpoint 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 off cycle will begin immediately
following the elapse of the minimum run time.
When operating in low ambient mode, an evaporator low limit
sensor
(EC1)
temperature below 26ºF will de-energize the
compressor. If the call for cooling is still present at the end of
the ASCD and the and the evaporator temperature sensor
(EC1)
temperature is above 26°F, the unit will resume
operation.
Safety Controls
The unit control board monitors the following inputs for each
cooling system:
1.
An evaporator low limit sensor
(EC1)
(Located on the
Suction Line at the Evaporator Coil.) to protect against low
evaporator temperatures due to a low airflow or a low
return air temperature, set at 26°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 625 ± 25 psig).
3.
A low-pressure switch to protect against loss of refrigerant
charge, (opens at 50 ± 5 psig).
The above pressure switches are hard-soldered to the unit. The
refrigeration systems are independently monitored 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 the external pressure switches, the compressors
also have inherent (internal) protection. If there is an abnormal
temperature rise in a compressor, the protector 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 compressor
reset or lock-out.
Reset
Remove the call for cooling, by raising thermostat setting higher
than the conditioned space temperature.
Electric Heating Sequence Of Operations
The following sequence describes the operation of the electric
heat section.
Two-stage heating:
a. Upon a call for first stage heat by the thermostat, the
heater relay (RA) 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 relay (RB) will be energized. After completing the
