5188238-YIM-D-0518
Johnson Controls Ducted Systems
45
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 a error message associated with errors
within the system.
Reset
Remove the call for cooling, by raising thermostat setting higher
than the conditioned space temperature. This resets any
pressure or evaporator low limit error messages.
Heating Sequence Of Operations
With or Without Electric Heat
When the thermostat calls for the first stage of heating, the low
voltage control circuit is completed between "R" and "W1". The
24vac signal is passed through the UCB to the Fan, C1 and
CN-Fan output Terminals and withholds a 24vac output to the
H1 terminal assuring the reversing valve cannot be energized,
except during defrost. If the ASCD timer is satisfied the UCB will
energize compressor contactor M1.
If the compressor 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 H2 output terminal to the electric
heat section (if available). The total available kW of electric heat
will be energized on a call for "W2".
Defrost Initiation
Defrost control implements a temperature differential, demand
defrost algorithm. The heat pump is allowed to operate in the
heating mode until the combination of outdoor ambient
temperature and outdoor coil temperature indicate that
defrosting is necessary. When the coil temperature is
maintained below the initiate point for a given ambient
temperature, continuously for 4-1/2 minutes, the heat pump is
put into a defrost cycle. This 4-1/2 minute timer eliminates
unnecessary defrost cycles caused by refrigeration surges such
as those that occur at the start of a heating cycle.
For defrost, the UCB will signal the energizing of the reversing
valve and de-energizing the systems condenser fan
motor(s).The unit's optional electric first-stage heater is also
energized via a 24-volt VAC output terminal labeled "H2".
Defrost Termination
The UCB terminates the defrost mode when either of the
following two conditions are met;
1.
The outdoor coil temperature sensor reaches 50°F, or
2.
The maximum allowable defrost run time of 8 minutes.
Interval between Defrosts
A timed inhibit feature prevents the system from responding to
a call for defrost less than 40 minutes after the initiation of the
previous defrost. After this inhibit time has expired, temperature
conditions must call for defrost continuously for 4- 1/2 minutes
before another defrost cycle is initiated. A temperature inhibit
feature prohibits defrost if the coil temperature is above 40°F.
All defrost timing occurs only while the compressor is on.
Forced Defrost
A forced-defrost feature puts the system into a defrost cycle
every 6 hours and 4 minutes to recirculate lubricants, unless the
coil temperature is above 40°F. All defrost timing occurs only
while the compressor is on.
For trouble shooting purposes, the defrost cycle can be
manually initiated by selecting "Test Defrost" in the UCB menu.
Safety Controls
The electric heat 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 18. 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.
