998659-YIM-C-0614
Johnson Controls Unitary Products
45
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.
Flash Codes
The UCB will initiate a flash code associated with errors within
the system. Refer to UNIT CONTROL BOARD FLASH CODES
Table 23.
Reset
Remove the call for cooling, by raising thermostat setting higher
than the conditioned space temperature. This resets any
pressure or freezestat flash codes.
Heating Sequence of Operation
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 RY1 Relay.
Contacts RY1-2 open, assuring the reversing valve cannot be
energized, except during defrost. Contacts RY1-1 close,
completing the circuit to Y on the defrost control (DC). After its
five minute ASCD timer is satisfied, the DC closes it’s internal
compressor relay contacts, sending a 24vac signal to the MV
terminal on the UCB. If its ASCD timer is satisfied the UCB will
energize compressor #1 relay. After a two second delay, it then
energizes compressor #2 relay (if applicable). Therefore, on a
call for heat from W1, both 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 contactor which brings
on both condenser fans.
A second stage call from the thermostat completes the circuit
between R and W2. This 24vac signal is passed through the
UCB to the defrost control board. If the unit is equipped with an
optional electric heater it would be energized through a set of
normally closed contacts on the defrost board. 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.
As mentioned earlier, the defrost control (DC) utilizes a time/
temperature defrost scheme. The following two conditions must
be met before the DC will enter a defrost mode:
1.
The DC must first satisfy its accumulated minimum run
time. This is factory set at 60 minutes, but is field
adjustable to 30, 60 or 90 minutes.
2.
Either of the two defrost thermostats (DF1 or DF2) must be
closed. These normally open thermostats are mounted on
the respective liquid lines and are set to close at 31
degrees (+/-3) (28° F on XP090).
If neither defrost thermostat is closed at the completion of it’s
minimum accumulated run time cycle, the DC initiates another
run time cycle, which it must complete before it looks at the
position of the defrost thermostats. This action is repeated until,
at the completion of a run time cycle, one of the defrost
thermostats is found to be closed and the DC enters defrost
mode.
When the DC enters the defrost mode, it’s on-board defrost
relay is powered. This energizes both reversing valves, de-
energizes both condenser fan motors and energizes the unit’s
optional electric heater. The DC remains in defrost mode until
either of the following two conditions is met:
1.
Both of the liquid line thermostats are open. Each is set to
open at 55 degrees (+/- 3).
2.
The maximum defrost run time of 10 minutes is met.
The DC also contains a set of test pins. Placing a jumper across
these pins will result in the following actions:
• If the ASCD timer is active, it is now by-passed, allowing
the compressor to run.
• If the DC is in a lockout condition, the lockout is reset.
• If the compressor is running, the DC is forced into defrost
mode. The control will remain in defrost mode for as long
as the jumper is in place. When the jumper is removed,
the control will terminate the defrost mode in the normal
manner as described above.
NOTE:
The DC has two flashing codes which are only initiated
if the two pressure switch terminals are open. As used
in the Predator
®
there is a jumper across the pressure
switch terminals. Therefore the field should never
experience a DC lockout mode unless that jumper is
removed or broken.
Electric Heat Operation Errors
Temperature Limit
If the UCB senses zero volts from the high temperature limit,
the indoor blower motor is immediately energized.
This limit is monitored regardless of unit operation status, i.e.
the limit is monitored at all times.
If the temperature limit opens three times within one hour, it will
lock-on the indoor blower motor and a flash code is initiated
(See Table 23).
Safety Controls
The UCB monitors the temperature limit switch of electric heat
units.
The control circuit includes the following safety controls:
