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5121817-TIM-A-0515
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 off cycle will begin immediately
following the elapse of the minimum run time.
When operating in low ambient mode, an evaporator
temperature below 26ºF will de-energize the compressor. If the
call for cooling is still present at the end of the ASCD and the
evaporator temperature is above 26ºF, the unit will resume
operation.
Safety Controls
The unit control board monitors the following inputs for each
cooling system:
1.
An evaporator low limit sensor to protect against low
evaporator temperatures due to a low airflow or a low
return air temperature, set at 26°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
refrigeration systems are independently monitored and
controlled. On any fault, only the associated system will be
affected by any safety/preventive action.
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 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.
Error Message
The UCB will initiate an error message associated with errors
within the system. Refer to UNIT CONTROL BOARD error
messages.
44
Reset
Remove the call for cooling, by raising thermostat setting higher
than the conditioned space temperature. This resets any
pressure or freezestat error messages.
Electric Heating Sequence Of Operations
The following sequence describes the operation of the electric
heat section.
Single-stage heating (applies only to 5, 7 & 10 kW 230V
heaters and to 7, 10, & 15 kW 460V and 575V heaters. All other
heaters MUST use a two-stage thermostat):
a. Upon a call for heat by the thermostat, the heater
sequencer (1S) 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.
Two-stage heating (applies only to 15, 20 and 30 kW 230V
heaters and 20 and 30 kW heater 460V and 575V heaters.):
a. Upon a call for first-stage heat by the thermostat, the
heater sequencer (1S) (15, 20, 30 kW 230 volt) and
contactor (2M) (20, 30 kW 460 and 575 volt) 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 sequencer
(2S) (12, 20, 30 kW 230 volt) or contactor (3M) (20, 30 kW
460 and 575 volt) 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.
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 shows an error message.
Safety Controls
The UCB monitors the temperature limit switch of electric heat
units.
The control circuit includes the following safety controls:
Temperature Limit Switch (TLs)
This control is located inside the heater compartment and is set
to open at the temperature indicated in the Electric Heat Limit
Setting Table 20. It resets automatically. The limit switch
operates when a high temperature condition, caused by
Johnson Controls Unitary Products
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