1133391-YIM-A-1014
Johnson Controls Unitary Products
47
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 and initiate the ASCD.
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 and initiate the ASCD.
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 sensor detects a
temperature below 26 Deg. F (default), the UCB will de-
energize the associated compressor, initiate the ASCD, and, if
the other compressor is idle, 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 temperature sensor falling below 26 Deg.
F (default) three times within two hours of operation, the UCB
will lock-out the associated compressor. If the other compressor
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 temperature-activated
switch set at 45ºF. When the low ambient 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 minimum run time.
When operating in low ambient mode, the UCB will not lockout
the compressor due to a freezestat trip. However, a freezestat
trip will de-energize the 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 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
safety switches are independently monitored.
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 compressor
also has inherent (internal) protection. If there is an abnormal
temperature rise in the 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 the compressor
too soon after its previous run. Additionally, a minimum run time
is imposed any time the 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 and dehumidification, by raising
thermostat setting higher than the conditioned space
temperature and raising the dehumidistat setting higher than
the conditioned space humidity.
Reheat Mode Sequence Of Operation
When the Unit Control Board (UCB) detects a need for
dehumidification at RAH via the field supplied dehumidistat
connected to RAH, and there is not a call for cooling, it
energizes Solenoids SOL 3, SOL 2 and the Reheat Relay
(RHR), which de-energizes Solenoid SOL 1. The unit then
operates with refrigerant flow in the evaporator reheat coil and
condenser coil #1. See Figures 29 and 32.
When the room thermostat calls for first stage cooling while
there is still a call for dehumidification, no operational change is
made. The call for cooling is ignored and the unit continues to
operate with refrigerant flow in reheat coil and condenser coil
#1. See Figures 30 and 32.
When the room thermostat calls for second stage cooling, the
RCB senses a signal through "Y1" and "Y2" and de-energizes
SOL 3, SOL 2 and the RHR, which energizes SOL 1. The unit
operates in the cooling mode. See Figures 29, 30 and 31.
