035-17311-000 REV A (1200)
30
Unitary Products Group
The unit control board monitors the temperature limit switch
of electric heat units.
PROTECTION
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.
CONTINUOUS BLOWER
By setting the room thermostat to “ON,” the supply air blower
will operate continuously.
INTERMITTENT BLOWER
With the room thermostat fan switch set to “AUTO” and the
system switch set to either the “AUTO” or “HEAT” settings,
the blower is energized whenever a cooling or heating opera-
tion is requested. The blower is energized after any specified
delay associated with the operation.
When energized, the indoor blower has a minimum run time
of 30 seconds. Additionally, the indoor blower has a delay of
10 seconds between operations.
COOLING SEQUENCE OF OPERATION
When the thermostat calls for the first stage of cooling, the
low-voltage control circuit from “R” to “Y1” and “G” is com-
pleted. The UCB energizes the economizer (if installed and
free cooling is available) or the first available compressor
*
and the condenser fans. For first stage cooling, compressor
#1 is energized. If compressor #1 is unavailable, compressor
#2 is energized. After completing the specified fan on delay
for cooling, the UCB will energize the blower motor.
When the thermostat calls for the second stage of cooling,
the low-voltage control circuit from “R” to “Y2” is completed.
The control board energizes the first available compressor. If
free cooling is being used for the first stage of cooling, com-
pressor #1 is energized. If compressor #1 is active for first
stage cooling or the first compressor is locked-out, compres-
sor #2 is energized. In free-cooling mode, if the call for the
second stage of cooling continues for 20 minutes, compres-
sor #2 is energized, provided it has not been locked-out.
If there is an initial call for both stages of cooling, the UCB will
delay energizing compressor #2 by 30 seconds in order to
avoid a power rush.
Once the thermostat has been satisfied, the it will de-ener-
gize Y1 and Y2. If the compressors have satisfied their mini-
mum run times, the compressors and condenser fans are de-
energized. Otherwise, the unit operates each cooling system
until the minimum run times for the compressors have been
completed. Upon the final compressor de-energizing, the
blower is stopped following the elapse of the fan off delay for
cooling.
* To be available, a compressor must not be locked-out due
to a high or low-pressure switch or freezestat trip and the
anti-short cycle delay (ASCD) must have elapsed.
COOLING OPERATION ERRORS
Each cooling system is monitored for operation outside of the
intended parameters. Errors are handled as described
below. All system errors override minimum run times for
compressors.
HIGH-PRESSURE LIMIT SWITCH
During cooling operation, if a high-pressure limit switch
opens, the UCB will de-energize the associated compressor,
initiate the ASCD, and, if the other compressor is idle, stop
the condenser fans. If the call for cooling is still present at the
conclusion of the ASCD, the UCB will re-energize the halted
compressor.
Should a high-pressure switch open three times within two
hours of operation, the UCB will lock-out the associated com-
pressor and flash a code (see Table 33). If the other com-
pressor is inactive, the condenser fans will be de-energized.
LOW-PRESSURE LIMIT SWITCH
The low-pressure limit switch is not monitored during the ini-
tial 30 seconds of a cooling system’s operation. For the fol-
lowing 30 seconds, the UCB will monitor the low-pressure
switch to ensure it closes. If the low-pressure switch fails to
close after the 30-second monitoring phase, the UCB will de-
energize the associated compressor, initiate the ASCD, and,
if the other compressor is idle, stop the condenser fans.
Once the low-pressure switch has been proven (closed dur-
ing the 30-second monitor period described above), the UCB
will monitor the low-pressure limit switch for any openings. If
the low-pressure switch opens for greater than 5 seconds,
the UCB will de-energize the associated compressor, initiate
the ASCD, and, if the other compressor is idle, stop the con-
denser fans.
If the call for cooling is still present at the conclusion of the
ASCD, the UCB will re-energize the halted compressor.
