Bryant 551C - Manual (Page 49)

49
Cooling
Set the space thermostat to the OFF position. Set the system
selector switch at COOL position and the fan switch at AUTO
position. Adjust the thermostat to a setting below room
temperature. The compressor starts when contactor closes.
Check cooling effects at a setting below room temperature.
Check the unit charge. Refer to Refrigerant Charge section.
Reset the thermostat at a position above room temperature. The
compressor will shut off.
TO SHUT OFF UNIT -- Set the system selector switch at OFF
position. Resetting the thermostat at a position above room
temperature shuts off the unit temporarily until the space
temperature exceeds the thermostat setting. Units are equipped
with a Cycle-LOC™ protection device. The unit shuts down on
any safety trip and remains off; an indicator light on the
thermostat comes on. Check the reason for the safety trip.
Compressor restart is accomplished by manual reset at the
thermostat by turning the selector switch to OFF position and
then to ON position.
Heating
To start unit, turn on main power supply.
Set system selector switch at HEAT position and set thermostat at
a setting above room temperature. Set fan at AUTO position.
First stage of thermostat energizes the first--stage electric heater
elements; second stage energizes second--stage electric heater
elements, if installed. Check heating effects at air supply grille(s).
If 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 thermostat at a position below room
temperature temporarily shuts unit off until space temperature
falls below thermostat setting.
Safety Relief
A soft solder joint at the suction line fitting provides pressure
relief under abnormal temperature and pressure conditions.
Ventilation (Continuous Fan)
Set fan and system selector switches at ON and OFF positions,
respectively. Evaporator fan operates continuously to provide
constant air circulation.
Operating Sequence
Cooling -- Units Without Economizer
When thermostat calls for cooling, terminals G and Y1 are
energized. The indoor-fan contactor (IFC), reversing valve
solenoid (RVS) and compressor contactor are energized and
indoor-fan motor, compressor, and outdoor fan starts. The
outdoor fan motor runs continuously while unit is cooling.
Heating -- Units Without Economizer
When the thermostat calls for heating, terminal W1 will be
energized with 24v. The IFC and heater contactor no. 1 (HC1)
are energized.
Cooling -- Units With EconoMi$er IV
When free cooling is not available, the compressors will be
controlled by the zone thermostat. When free cooling is available,
the outdoor-air damper is modulated by the EconoMi$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 modulated (open or close) to
bring the supply-air temperature back within the set point limits.
Integrated EconoMi$er IV operation on single-stage units
requires a 2-stage thermostat (Y1 and Y2).
For EconoMi$er IV operation, there must be a thermostat call for
the fan (G). This will move the damper to its minimum position
during the occupied mode.
If the increase in cooling capacity causes the supply--air
temperature to drop below 45_F, then the outdoor--air damper
position will be fully closed. If the supply--air temperature
continues to fall, the outdoor--air damper will close. Control
returns to normal once the supply--air temperature rises above
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$er IV control, a demand controlled ventilation strategy
will begin to operate. As the CO
2
level in the zone increases
above the CO
2
set point, the minimum position of the damper
will be increased proportionally. As the CO
2
level decreases
because of the increase in fresh air, the outdoor--air damper will
be proportionally closed. Damper position will follow the higher
demand condition from DCV mode or free cooling mode.
Damper movement from full closed to full open (or vice versa)
will take between 1--1/2 and 2--1/2 minutes.
If free cooling can be used as determined from the appropriate
changeover command (switch, dry bulb, enthalpy curve,
differential dry bulb, or differential enthalpy), a call for cooling
(Y1 closes at the thermostat) will cause the control to modulate
the dampers open to maintain the supply air temperature set point
at 50_ to 55_F.
As the supply air temperature drops below the set point range of
50_ to 55_F, the control will modulate the outdoor--air dampers
closed to maintain the proper supply--air temperature.
Heating -- Units With EconoMi$er IV
When the room temperature calls for heat, the heating controls are
energized as described in the Heating, Units Without Economizer
section. During the occupied mode, whenever the indoor fan
contactor is energized, the economizer damper moves to the
minimum position.
Units With Perfect Humidity™ Adaptive Dehumidification
System
Normal Design Operation
When the rooftop operates under the normal sequence of
operation, the compressors will cycle to maintain indoor
conditions. (See Fig. 42.)
The Perfect Humidity adaptive 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.
Subcooling Mode
When subcooling mode is initiated, this will energize (close) the
liquid line solenoid valve (LLSV) forcing the hot liquid
refrigerant to enter into the subcooling coil. (See Fig. 43.)
As the hot liquid refrigerant passes through the subcooling/ reheat
dehumidification coil, it is exposed to the cold supply airflow
coming through the evaporator coil. The liquid is further
subcooled to a temperature approaching the evaporator
leaving-air temperature. The liquid then enters a thermostatic
expansion valve (TXV) where the liquid drops to a lower
pressure. The TXV does not have a pressure drop great enough to
change the liquid to a 2-phase fluid, so the liquid then enters the
Acutrol™ device at the evaporator coil.
551B
,C