Carrier 50LJ - Manual (Page 24)

cooling requirements.) If outdoor air alone will not meet
the cooling requirements, Y2 on the thermostat will close,
energizing Compressor no. 1 to work in conjunction with
the modulating economizer to meet the cooling require­
ment. While the unit is operating using outdoor-air. Com­
pressor no. 2 cannot be energized. If enthalpy is above
setting on enthalpy switch, the economizer outdoor-air damp­
ers move to the minimum (ventilation) position, and con­
denser fans nos. 1 and 2 cycle on and off as described in
Cooling, Units Without Economizer section on page 23.
NOTE: If fan switch is in ON position, and the room ther­
mostat is satisfied, the outdoor-air dampers move to the min­
imum position.
HEATING, UNITS WITH ECONOMIZER - Operation
is the same as described in Heating, Units Without Econo­
mizer section on page 23, except that the outdoor-air
dampers move to the minimum position.
BUILDING PRESSURIZATION MODE - Building pres­
surization is used to pressurize conditioned space in the event
of a fire or smoke condition. For building pressurization
mode to work effectively, unit must be equipped with econ­
omizer option. On a large building with multiple zones, it
may be desirable to pressurize a zone that does not have
smoke in it to keep smoke from entering the zone from other
zones that may be filled with smoke.
In building pressurization mode, supply-air fan operates
with outdoor-air dampers wide open (on units with econo­
mizer) and, if unit is equipped with power exhaust option,
the power exhaust fan off. This pumps outdoor air into the
zone but does not exhaust it, resulting in the zone becom­
ing positively pressurized.
All switches and wiring required for building pressuriza­
tion must be field supplied and wired.
To go into pressurization mode, power to the control
circuit must be interrupted. This can be done using alarm
relay contacts described in 115-V Connections section on
page 17. These contacts can be energized by a relay in a
smoke detector, a firestat, or by a relay that is manually
energized in a central control room. Switches must be field
installed to energize supply-air fan and drive economizer
dampers wide open. After power to control circuit is inter­
rupted, these switches must be manually closed to put unit
into pressurization mode; switches would probably be lo­
cated in central control room. If unit is equipped with return/
exhaust fan option, return-air fan should be off.
SMOKE PURGE MODE — In order to use smoke purge,
unit must be equipped with power exhaust option.
Smoke purge is used to exhaust smoke from a zone in
the event of a fire or heavy smoke condition. In this mode,
power exhaust fan runs, supply fan is shut off, and econo­
mizer dampers are wide open. With power exhaust fan run­
ning and exhaust damper wide open, smoke-filled air is
exhausted out of the conditioned space to the outdoors.
With smoke purge mode, it is necessary to interrupt power
to unit control circuit as described above for building pres­
surization. All switches and wiring for putting unit into smoke
purge mode must be field supplied and installed. Terminals
have been provided in unit control box to facilitate field
hookups.
The field-installed switches must energize return-air fan
and drive economizer dampers wide open. After power is
interrupted to unit control circuit, these switches must be
manually closed to place unit into smoke purge mode. As
with building pressurization switches, these switches would
probably be located in a central control room.
NOTE: With both building pressurization and smoke purge
modes, do not overpressurize the zone; unit will keep pump­
ing air in or exhausting air out until told to do otherwise.
Ventilation Air Circulation (Continuous Fan) —
Set unit power at ON position, system selector switch at
OFF, HEAT, or COOL position and fan switch at ON
position.
Evaporator-fan contactor is energized through the switch
on the thermostat and the evaporator fan runs continuously.
Automatic Changeover Using Automatic
Changeover Thermostat — Set unit power at ON
position, and system selector switch at AUTO position.
When the temperature of the conditioned space rises to
the cooling selector lever setting, unit automatically switches
from the heating mode to the cooling mode. When the tem­
perature of the conditioned space falls to the heating selec­
tor switch setting, unit automatically changes from cooling
mode to heating mode. The thermostat is interlocked so that
cooling and heating systems do not operate at the same time.
Head Pressure Control — All units have a fan
cycling thermostat which cycles the no. 2 condenser fan.
(The no. 2 condenser fan is located over the control box.)
This switch opens at 60 F ± 3° F and closes at 70 F ±
3 ° F. This allows the unit to operate down to 20 F outdoor
ambient temperature.
NOTE: Accessory Motormaster® head pressure control is
available which allows mechanical cooling down to
-20 F.
SERVICE
Service Access — All unit components can be reached
through clearly labeled removable access panels, with the
exception of the unit control box. Access to the unit control
box is through a hinged access door.
Access panels are held in place by 2 latches (one on each
side) and 4 screws in the bottom flange. To remove panels:
1. Loosen the latch bolt using a Yis-in. wrench and pivot
the latches so they are not in contact with the panel.
2. Using a Vs-in. wrench, remove the 4 screws in the bot­
tom flange.
3. Pull the bottom of the panel out and down to remove.
To replace panel:
1. Place top edge of panel under drip lip at top.
2. Push panel up and in.
3. Replace 4 screws at the bottom.
4. Tighten latches.
The control box access door is not equipped with tie-
backs, so if heavy-duty servicing is needed, either remove
the door or prop it open to prevent accidental closure.
The door is held closed with 3 latches. The latches are
secured to the unit with a single l/i-in. — 20 x Vi-in. long
bolt. To open, loosen the latch bolt using a Vie-in. wrench.
Pivot the latch so that it is not in contact with the door, and
open the door. To shut, reverse the above procedure.
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