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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.
Electric Heating Sequence Of Operations
The following sequence describes the operation of the electric
heat section.
For units with VFD and electric heat, the speed of the
indoor blower motor continues to be controlled by duct
static pressure via the Smart Equipment™ control
board.
If there are VAV boxes present in the duct system, the
boxes must be driven to the full-open position using a
customer-supplied power source to assure adequate
airflow across the heating elements.
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
specified fan on delay for heating, the UCB will energize
the blower motor.
b The thermostat will cycle the electric heat to satisfy the
heating requirements of the conditioned space.
Johnson Controls Ducted Systems
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.
Safety Controls
The UCB monitors the temperature limit switch of electric heat
units.
The control circuit includes the following safety controls:
Limit Switch (LS)
This control is located inside the heater compartment and is set
to open at the temperature indicated in the Electric Heat Limit
Setting Tables 24. It resets automatically. The limit switch
operates when a high temperature condition, caused by
inadequate supply air flow occurs, thus shutting down the
heater and energizing the blower.
Table 24: Electric Heat Limit Setting
UNIT (TONS)
ZWT06, 07
(6.5, 7.5)
ZWT08, T10, T12
(8.5,10, 12.5)
ZWT06, 07
(6.5, 7.5)
ZWT08, T10, T12
(8.5, 10, 12.5)
ZWT06, 07
(6.5, 7.5)
ZWT08, T10, T12
(8.5, 10, 12.5)
Reset
Remove the call for heating by lowering the thermostat setting
lower than the conditioned space temperature.
5565606-TIM-B-1018
LIMIT
HEATER
VOLTAGE
SWITCH
kW
OPENS °F
9
150
18
150
24
150
34
150
208/230
18
150
24
150
34
150
54
130
9
150
18
150
24
150
34
150
480
18
150
24
150
34
150
54
130
9
150
18
150
24
150
34
150
600
18
150
24
150
34
150
54
130
55
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