Specifications

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,
drive economizer dampers wide open and, on VAV units, drive
return-air fan inlet guide vanes wide open. After power is
interrupted to unit control circuit, these switches must be manu-
ally closed to place unit into smoke purge mode. As with
building pressurization switches, these switches would prob-
ably be located in a central control room.
VARIABLE VOLUME UNITS EQUIPPED WITH THE
STATIC PRESSURE CONTROL OPTION — If unit is
equipped with static pressure control option, differential pres-
sure switch will modulate fan inlet guide vanes to vary air-
flow to maintain set point of the switch. See Differential Pressure
Switch Set Point and Null Span sections on page 49 for de-
tails on setting switch set point and null span.
For example, assume that set point on supply fan differ-
ential switch is 1.9 in. wg. If pressure in supply duct goes
above 1.9 in. wg, switch will make to the normally-open con-
tact and energize inlet guide vane motor to drive inlet guide
vanes to a more closed position, thus reducing airflow and
lowering duct pressure. Once set point pressure is reached,
switch will open and deenergize inlet guide vane motor. If
pressure in supply duct is below 1.9 in. wg, the switch will
make to the normally-closed contact and energize inlet guide
vane motor to drive inlet guide vane to a more open posi-
tion; increasing airflow and raising duct pressure. Once again,
once desired pressure has been reached, switch will open and
deenergize inlet guide vane motor. How far above or below
the set point setting the switch goes before energizing de-
pends on setting of null span (null span is pressure change
that can be made without contacts opening or closing). If
null span is at maximum position, pressure will vary from
0.17 in. wg to 0.31 in. wg depending on set point (if set point
is at minimum setting, null span will be 0.17 in. wg, while
if it is at maximum position, the null span will be 0.31 in.
wg) before switch acts. If null span is adjusted to a mini-
mum setting, duct pressure will vary from 0.06 in. wg to
0.11 in. wg (again depending on switch set point) before switch
acts. Setting null span to minimum position will result in a
smaller pressure fluctuation than if it is set at maximum
position.
The modulating power exhaust option switch operates in
same manner as supply switch except that it has a different
set point range and null span.
VENTILATIONAIR CIRCULATION (CONTINUOUS FAN)
— Turn unit power to ON position, and set system selector
switch at OFF, HEAT, or COOL position. Set fan switch at
ON position:
Indoor (evaporator) fan contactor (IFC) is energized through
the switch on the thermostat and the evaporator fan runs con-
tinuously.
AUTOMATIC CHANGEOVER USING AUTOMATIC
CHANGEOVER THERMOSTAT — Turn unit power to ON
position, and set 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 selector
switch setting, unit automatically changes from cooling mode
to heating mode. The thermostat is interlocked so that cool-
ing and heating systems do not operate at the same time.
Head Pressure Control — All units have a fan cy-
cling 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 6 3° F and closes at 70 F 6 3° F. This
allows the unit to operate down to 45 F outdoor ambient
temperature.
SERVICE
Service Access —
All unit components can be reached
through clearly labeled hinged access doors. These doors are
not equipped with tiebacks, so if heavy duty servicing is needed,
either remove them or prop them open to prevent accidental
closure.
Each door is held closed with 3 latches. The latches are
secured to the unit with a single
1
⁄ 4 -in.—20 x
1
⁄ 2 -in. long bolt.
See Fig. 32.
To open, loosen the latch bolt using a
7
⁄ 16 -in. wrench. Pivot
the latch so it is not in contact with the door. Open the door.
To shut, reverse the above procedure.
NOTE: Disassembly of the top cover may be required under
special service circumstances. It is very important that the
orientation and position of the top cover be marked on the
unit prior to disassembly. This will allow proper replace-
ment of the top cover onto the unit and prevent rain water
from leaking into the unit.
IMPORTANT:After servicing is completed, make sure
door is closed and relatched properly, and that the latches
are tight. Failure to do this can result in water leakage
into the indoor-air section of the unit.
COMPRESSORS — Access to the compressors is through
the door on the right side of the unit (when facing the con-
denser coil). This door also provides access to the discharge
and liquid line service valves, the crankcase heaters, the fil-
ter driers, the sight glasses, and the high- and low-pressure
switches. Compressor no. 1 is compressor closest to the con-
denser coil.
Fig. 32 — Door Latch
44