175245-YIM-C-1008
Johnson Controls Unitary Products
29
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
(DC)
. If its ASCD timer is satisfied the UCB will ener-
gize compressor #1 (contactor M1). After a two second
delay, it then energizes compressor #2 (contactor M2).
Therefore, on a call for heat from W1, both sets of com-
pressors 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.
Take note that the MV terminal on the UCB is con-
stantly 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 a DC will enter a defrost
mode:
1. The DC must first satisfy its accumulated minimum
run time. This is factory set at 90 minutes, but is
field adjustable to 30, 60 or 90 minutes.
2. Defrost thermostats (DF1 or DF2) must be closed.
These normally open thermostats are mounted on
their respective outdoor coil (return bend) and are
set to close at 28 ± 4°F.
If the defrost thermostat is open 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 thermostat. This
action is repeated until, at the completion of a run time
cycle, the defrost thermostat is found to be closed and
the DC enters defrost mode.
When a DC enters the defrost mode, it’s on-board
defrost relay is powered. This energizes the reversing
valve, de-energizes the condenser fan motor and ener-
gizes the unit’s optional electric heater. The DC
remains in defrost mode until either of the following two
conditions is met:
1. The defrost thermostat is open. It is set to open at
55 ± 4°F.
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 this unit, 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.
HEATING 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 sta-
tus, i.e. the limit is monitored at all times.
