Low Ambient Cooling; Safety Controls; Compressor Protection; Flash Codes - York SUNLINE MagnaDRY WR300 Installation Manual

284813-YIM-C-1008
Should a freezestat open three times within two hours
of operation, the UCB will lock-out the associated com-
pressor and flash a code (Table 36). If the other com-
pressor 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 45ºF. 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
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 exces-
sive discharge pressures due to a blocked con-
denser coil or a condenser motor failure, (opens at
405 ± 7 psig and resets at 265 ± 20 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.
54
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 mini-
mize compressor wear and damage. An anti-short
cycle delay (ASCD) is utilized to prevent operation of a
compressor too soon after its previous run. Addition-
ally, a minimum run time is imposed any time a com-
pressor 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 36.

RESET

Remove the call for cooling, by raising thermostat set-
ting higher than the conditioned space temperature.
This resets any pressure or freezestat flash codes.

REHEAT MODE SEQUENCE OF OPERATION

The reheat control board allows the user to select two
different modes of operation via a jumper connection
on the board. (See Figure 21.) Each mode is described
below. Refer to Figures 21 - 23 when reading this sec-
tion.

"NORMAL" MODE

When the reheat control board (RCB) detects a need
for dehumidification (24VAC) at "HUM" via the field
supplied dehumidistat connected to RHTB-1 and
RHTB-2 and there is not a call for cooling, it energizes
the hot gas relay (HGR), which energizes the 3-way
valve (SOL 3), the condenser coil valve (SOL 2), and
de-energizes the reheat coil bleed valve (SOL 1). The
Y1 signal is passed to the unit control board (UCB),
which engages circuit # 1, resulting in circuit #1 reheat
mode operation.
When the room thermostat calls for first stage cooling,
with or without a call for dehumidification, the RCB
senses a signal through "Y1", de-energizing the HGR,
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