5566020-JIM-C-1019
58
Johnson Controls Ducted Systems
ambient setpoint at 45ºF (default). When the
OAT Sensor
senses a temperature below the low ambient setpoint and the
thermostat is calling for cooling, the UCB operates 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 begins immediately
following the elapse of the minimum run time.
When operating in low ambient mode, an evaporator low limit
sensor
(EC1, 2)
temperature below 26ºF will de-energize the
associated compressor. If the call for cooling is still present at
the end of the ASCD and the and the evaporator temperature
sensor
(EC1, 2)
temperature is above 26°F, 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
(EC1, 2)
(Located on the
Suction Line at the Evaporator Coil.) 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 is affected
by any safety/preventive action. The other refrigerant system
continues 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 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 27,
Electric heat limit setting 50 in. cabinet,
Electric heat limit setting 42 in. cabinet,
on page 59. 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.
For units with VFD and electric heat, the speed of the
indoor blower motor continues to be controlled by duct
static pressure via the VAV 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.
