Demand Limit Control — Demand limit is a method
to limit the capacity of the VAV unit. Two options are avail-
able for this method. Single-step demand limit control is a
field-supplied 5 to 20 Kohm potentiometer, which provides
a single-step capacity reduction based on the setting of the
potentiometer. The Carrier accessory two-step demand limit
control module utilizes 2 potentiometers and provides 2 steps
of capacity reduction.
22Once either method is signaled to begin, code will
appear on the display screen. The unit capacity will then be
reduced by the amount set for the demand limit.
Building Pressurization Mode — Building pressur-
ization is used to pressurize conditioned space in the event
of a fire or smoke condition. On a large building with mul-
tiple 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 return or exhaust fan op-
tion, exhaust damper opens, and the fan is off. This pumps
outdoor air into the zone but does not exhaust it; resulting in
the zone becoming positively pressurized.
All switches and wiring required for building pressuriza-
tion must be field supplied and wired. Terminals are pro-
vided to make this job easier.
To go into pressurization mode, power to the control cir-
cuit must be interrupted. This can be done using alarm relay
contacts described in Fig. 25. These contacts can be ener-
gized 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 and supply-air fan inlet guide vanes wide
open.After power to control circuit is interrupted, these switches
must be manually closed to put unit into pressurization mode;
switches would probably be located in a central control room.
If unit is equipped with return or exhaust fan option, return-
air fan should be off.
Smoke Purge Mode — In order to use smoke purge,
unit must be equipped with return or exhaust fan 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 economizer damp-
ers are wide open. With power exhaust fan running and ex-
haust damper wide open, smoke-filled air is exhausted out
of the conditioned space to the outside.
With smoke purge mode, it is necessary to interrupt power
to unit control circuit as described in Building Pressuriza-
tion Mode section on this page. 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-supplied switches must energize exhaust fans and
drive power exhaust damper (if so equipped) and econo-
mizer damper 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 pres-
surization switches, these switches would probably be lo-
cated in a central control room.
Units Equipped with Static Pressure Control
Option —
If unit is equipped with static pressure control
option, differential pressure switch will modulate fan inlet
guide vanes to vary airflow to maintain set point of the switch.
See Differential Pressure Switch Set Point and Null Span
sections on pages 23 and 23 for details 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
minimum 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 re-
sult in a smaller pressure fluctuation than if it is set at maxi-
mum 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.
Quick Test Program — Turn on power to unit.
IMPORTANT: The field-supplied switch (or time-
clock) must be closed to put unit into the occupied mode.
The quick test program utilizes the 2-digit LED display
(see Fig. 6) on the set point board to show status of all input
and output signals to microprocessor. Display action and quick
test procedures are described below.
The quick test program is a 33-step program that provides
a means of checking all input and output signals of controls
prior to unit start-up. This check ensures that all control op-
tions, thermistors, and control switches are in proper work-
ing order.
When unit control circuit is switched to occupied mode,
20
a will appear on the display. Immediately press display
88
button once. An will appear on the display and alarm
light on display board will be energized. This indicates that
control system is ready to run quick test program.
IMPORTANT: Do not allow unit control circuit to
20
remain energized with showing on display for more
than 2 minutes. If display button is not pressed within
this time period, control will attempt to start unit.
For each step of the 33-step program, display button must
be pressed twice. On first press, step number is displayed;
second press initiates required action and appropriate code
is displayed.
NOTE: The step number is a numeral followed by a decimal
point (a 2-digit number has a decimal point after each nu-
meral). The action code number is one or 2 digits with no
decimal point(s).
17
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