Low-Pressure Limit Switch; Freezestat; Low Ambient Cooling; Safety Controls - York DW-15 Installation Manual

LOW-PRESSURE LIMIT SWITCH

The low-pressure limit switch is not monitored during
the initial 30 seconds of a cooling system's operation.
For the following 30 seconds, the UCB will monitor the
low-pressure switch to ensure it closes. If the low-pres-
sure switch fails to close after the 30-second monitor-
ing phase, the UCB will de-energize the associated
compressor, initiate the ASCD, and, if the other com-
pressor is idle, stop the condenser fans.
Once the low-pressure switch has been proven (closed
during 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 condenser fans.
If the call for cooling is still present at the conclusion of
the ASCD, the UCB will re-energize the halted com-
pressor.
Should a low-pressure switch open three times within
one hour of operation, the UCB will lock-out the associ-
ated compressor and flash a code (Table 27). If the
other compressor is inactive, the condenser fans will
be de-energized.

FREEZESTAT

During cooling operation, if a freezestat opens, the
UCB will de-energize the associated compressor, ini-
tiate 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 freezestat open three times within two hours of
operation, the UCB will lock-out the associated compres-
sor and flash a code (Table 27). If the other compressor is
inactive, the condenser fans will be de-energized.

LOW AMBIENT COOLING

To determine when to operate in low ambient mode,
the UCB has a pair of terminals connected to a temper-
ature-activated switch set at 30ºF. When the low ambi-
ent switch is closed and the 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 defrost cycle
48
will begin immediately following the elapse of the mini-
mum run time.
When operating in low ambient mode, the UCB will not
lockout the compressors due to a freezestat trip. How-
ever, a freezestat trip will de-energize the associated
compressor. If the call for cooling is still present at the
end of the ASCD and the freezestat has closed, the
unit will resume operation.

SAFETY CONTROLS

The unit control board monitors the following inputs for
each cooling system:
1. A suction line freezestat to protect against low
evaporator temperatures due to a low airflow or a
low return air temperature, (opens at 26 ± 5 °F and
resets at 38 ± 5°F).
2. A high-pressure switch to protect against excessive
discharge pressures due to a blocked condenser
coil or a condenser motor failure. For 15-ton units,
the switch opens at 405 ± 7 psig and resets at 265 ±
20 psig. For 17.5, 20 and 25 ton units, the switch
opens at 380 ± 10 psig and resets at 300 ± 10 psig.
3. A low-pressure switch to protect against loss of
refrigerant charge, (opens at 22 ± 5 psig and resets
at 45 ± 5 psig).
The above pressure switches are hard-soldered to the
unit. The refrigeration systems are independently mon-
itored and controlled. On any fault, only the associated
system will be affected by any safety/preventive action.
The other refrigerant system will continue 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

The compressors also have inherent (internal) protec-
tion. If there is an abnormal temperature rise in a com-
pressor, 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 com-
pressor reset or lock-out.

FLASH CODES

The UCB will initiate a flash code associated with
errors within the system. Refer to UNIT CONTROL
BOARD FLASH CODES Table 27.
284816-BIM-B-0707
Unitary Products Group