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IX. COOLING
To start unit, turn on main power supply. Set system selector
switch at COOL position and fan switch at AUTO position.
Adjust thermostat to a setting below room temperature.
Compressor starts on closure of contactor.
Check unit charge. Refer to Refrigerant Charge section on
page 42.
Reset thermostat at a position above room temperature.
Compressor will shut off.
To Shut Off Unit
Set system selector switch at OFF position. Resetting ther-
mostat at a position above room temperature shuts unit off
temporarily until space temperature exceeds thermostat set-
ting. Units are equipped with Cycle-LOC™ protection
device. Unit shuts down on any safety trip and remains off;
an indicator light on the thermostat comes on. Check reason
for safety trip.
Compressor restart is accomplished by manual reset at the
thermostat by turning the selector switch to OFF position
and then ON position.
X. HEATING (IF ACCESSORY HEATER IS INSTALLED)
To start unit, turn on main power supply.
Set thermostat at HEAT position and a setting above room
temperature, and set fan at AUTO position.
First stage of thermostat energizes the first-stage electric
heater elements; second stage energizes second-stage electric
heater elements. Check heating effects at air supply grille(s).
If accessory electric heaters do not energize, reset limit
switch (located on evaporator-fan scroll) by pressing button
located between terminals on the switch.
To Shut Off Unit
Set system selector switch at OFF position. Resetting heating
selector lever below room temperature temporarily shuts unit
off until space temperature falls below thermostat setting.
XI. SAFETY RELIEF
A soft solder joint in the suction line at the loss-of-charge/
low-pressure switch fitting provides pressure relief under
abnormal temperature and pressure conditions.
XII. VENTILATION (CONTINUOUS FAN)
Set fan and system selector switches at ON and OFF posi-
tions, respectively. Evaporator fan operates continuously to
provide constant air circulation.
XIII. OPERATING SEQUENCE
A. Cooling, Units Without EconoMi$er IV
When the thermostat calls for cooling, terminals G and Y1
are energized. The indoor (evaporator) fan contactor (IFC),
outdoor (condenser) fan contactor (OFC), and compressor
contactor no. 1 (C1) are energized and the evaporator-fan
motor, compressor no. 1, and condenser fans start. The
condenser-fan motors run continuously while unit is in cool-
ing. If the thermostat calls for a second stage of cooling by
energizing Y2, compressor contactor no. 2 (C2) is energized
and compressor no. 2 starts.
B. Heating, Units Without EconoMi$er IV (If Accessory
Heater is Installed)
Upon a call for heating through terminal W1, IFC and
heater contactor no. 1 (HC1) are energized. On units
equipped for 2 stages of heat, when additional heat is
needed, HC2 is energized through W2.
C. Cooling, Units With EconoMi$er IV
When free cooling is not available, the compressors will be
controlled by the zone thermostat. When free cooling is avail-
able, the outdoor-air damper is modulated by the
EconoMi$er IV control to provide a 50 to 55 F supply-air
temperature into the zone. As the supply-air temperature
fluctuates above 55 or below 50 F, the dampers will be modu-
lated (open or close) to bring the supply-air temperature
back within the set point limits.
For EconoMi$er IV operation, there must be a thermostat
call for the fan (G). This will move the damper to its mini-
mum position during the occupied mode.
Above 50 F supply-air temperature, the dampers will modu-
late from 100% open to the minimum open position. From
50 F to 45 F supply-air temperature, the dampers will main-
tain at the minimum open position. Below 45 F the dampers
will be completely shut. As the supply-air temperature rises,
the dampers will come back open to the minimum open posi-
tion once the supply air temperature rises to 48 F.
If optional power exhaust is installed, as the outdoor-air
damper opens and closes, the power exhaust fans will be
energized and deenergized.
If field-installed accessory CO
2
sensors are connected to the
EconoMi$er IV control, a demand controlled ventilation
strategy will begin to operate. As the CO
2
level in the zone
increases above the CO
2
set point, the minimum position of
the damper will be increased proportionally. As the CO
2
level
decreases because of the increase in fresh air, the outdoor-air
damper will be proportionally closed. Damper position will
follow the higher demand condition from DCV mode or free
cooling mode.
Damper movement from full closed to full open (or vice
versa) will take between 1
1
/
2
and 2
1
/
2
minutes.
If free cooling can be used as determined from the appropri-
ate changeover command (switch, dry bulb, enthalpy curve,
differential dry bulb, or differential enthalpy), a call for cool-
ing (Y1 closes at the thermostat) will cause the control to
modulate the dampers open to maintain the supply air tem-
perature set point at 50 to 55 F.
As the supply-air temperature drops below the set point
range of 50 to 55 F, the control will modulate the outdoor-
air dampers closed to maintain the proper supply-air
temperature.
D. Heating, Units with EconoMi$er IV
Upon a call for heating through terminal W1, IFC and
heater contactor no. 1 (HC1) are energized. On units
equipped for 2 stages of heat, when additional heat is
needed, HC2 is energized through W2. The economizer
damper moves to the minimum position. When the thermo-
stat is satisfied, the damper moves to the fully closed
position.
E. Units With Perfect Humidity™ Dehumidification System
Normal Design Cooling Operation
When the rooftop operates under the normal sequence of
operation, the compressors will cycle to maintain indoor con-
ditions. See Fig. 40.
The Perfect Humidity dehumidification system includes a
factory-installed Motormaster® low ambient control to keep
the head and suction pressure high, allowing normal design
cooling mode operation down to 0° F.