5808011-JIM-A-0320
62
Johnson Controls Ducted Systems
Safety controls
The unit control board monitors the following inputs for each
cooling system:
1.
An evaporator low limit sensor (EC1) to protect against low
evaporator temperatures due to a low airflow or a low
return air temperature, set at 26°F. The evaporator low limit
sensor is located on the suction line at the evaporator coil.
2.
A high-pressure switch to protect against excessive
discharge pressures due to a blocked condenser coil or a
condenser motor failure. The switch opens at 625 ± 25
psig.
3.
A low-pressure switch to protect against loss of refrigerant
charge. the switch opens at 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 is affected
by any safety or preventive action.
The UCB 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 opens to shut
down the compressor. The UCB incorporates features to
minimize compressor wear and damage. An
anti-short cycle
delay (ASCD) is used 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.
Two-stage heating:
a. When there is a call for first stage heat by the thermostat,
the heater relay (RA) is energized. After completing the
specified fan on delay for heating, the UCB energizes the
blower motor. If the second stage of heat is required,
heater relay (RB) is energized. After completing the
specified fan on delay for heating, the UCB energizes the
blower motor.
b The thermostat cycles the electric heat to satisfy the
heating requirements of the conditioned space.
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, that
is, the limit is monitored at all times.
If the temperature limit opens three times within one hour, it
locks on the indoor blower.
Safety controls
The UCB monitors the temperature limit switch of electric heat
units.
The control circuit includes the following safety controls:
Limit switch
The limit switch (LS) is located inside the heater compartment
and is set to open at the temperature indicated in Table 26,
Electric heat limit setting,
on page 64. It resets automatically.
The limit switch operates when a high temperature condition
caused by inadequate supply air flow occurs. This shuts down
the heater and energizes the blower.
Reset
Reset removes the call for heating by lowering the thermostat
setting lower than the conditioned space temperature.
Electric heat anticipator setpoints
The anticipator setpoint must be correct. Too high of a setting
results in longer heat cycles and a greater temperature swing in
the conditioned space. Reducing the value below the correct
setpoint gives shorter ON cycles and may result in the lowering
of the temperature within the conditioned space. See Table 25
for the required electric heat anticipator setting.
Table 24: Electric Heat Limit Setting
UNIT (TONS)
VOLTAGE
HEATER
kW
LIMIT
SWITCH
OPENS °F
J06-07ZB
(6.5, 7.5)
208/230
9
150
18
150
24
150
34
150
J08, 10, 12ZB
(8.5,10, 12.5)
18
150
24
150
34
150
54
130
J06-07ZB
(6.5, 7.5)
480
9
150
18
150
24
150
34
150
J08, 10, 12ZB
(8.5, 10, 12.5)
18
150
24
150
34
150
54
130
J06-07ZB
(6.5, 7.5)
600
9
150
18
150
24
150
34
150
J08, 10, 12ZB
(8.5, 10, 12.5)
18
150
24
150
34
150
54
130
